TSConfig Reference - Docs on every TSConfig option (original) (raw)

These options make up the bulk of TypeScript’s configuration and it covers how the language should work.

#Type Checking

# Allow Unreachable Code - allowUnreachableCode

When:

These warnings are only about code which is provably unreachable due to the use of JavaScript syntax, for example:

ts

`function fn(n: number) {

if (n > 5) {

return true;

} else {

return false;

}

return true;

}

`

With "allowUnreachableCode": false:

ts

`function fn(n: number) {

if (n > 5) {

return true;

} else {

return false;

}

return true;

Unreachable code detected.7027Unreachable code detected.

}

`Try

This does not affect errors on the basis of code which appears to be unreachable due to type analysis.

# Allow Unused Labels - allowUnusedLabels

When:

Labels are very rare in JavaScript and typically indicate an attempt to write an object literal:

ts

`function verifyAge(age: number) {

// Forgot 'return' statement

if (age > 18) {

verified: true;

Unused label.7028Unused label.

}

}

`Try

# Always Strict - alwaysStrict

Ensures that your files are parsed in the ECMAScript strict mode, and emit “use strict” for each source file.

ECMAScript strict mode was introduced in ES5 and provides behavior tweaks to the runtime of the JavaScript engine to improve performance, and makes a set of errors throw instead of silently ignoring them.

# Exact Optional Property Types - exactOptionalPropertyTypes

With exactOptionalPropertyTypes enabled, TypeScript applies stricter rules around how it handles properties on type or interfaces which have a ? prefix.

For example, this interface declares that there is a property which can be one of two strings: ‘dark’ or ‘light’ or it should not be in the object.

ts

`interface UserDefaults {

// The absence of a value represents 'system'

colorThemeOverride?: "dark" | "light";

}

`

Without this flag enabled, there are three values which you can set colorThemeOverride to be: “dark”, “light” and undefined.

Setting the value to undefined will allow most JavaScript runtime checks for the existence to fail, which is effectively falsy. However, this isn’t quite accurate; colorThemeOverride: undefined is not the same as colorThemeOverride not being defined. For example, "colorThemeOverride" in settings would have different behavior with undefined as the key compared to not being defined.

exactOptionalPropertyTypes makes TypeScript truly enforce the definition provided as an optional property:

ts

`const settings = getUserSettings();

settings.colorThemeOverride = "dark";

settings.colorThemeOverride = "light";

// But not:

settings.colorThemeOverride = undefined;

Type 'undefined' is not assignable to type '"dark" | "light"' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the type of the target.2412Type 'undefined' is not assignable to type '"dark" | "light"' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the type of the target.`Try

# No Fallthrough Cases In Switch - noFallthroughCasesInSwitch

Report errors for fallthrough cases in switch statements. Ensures that any non-empty case inside a switch statement includes either break, return, or throw. This means you won’t accidentally ship a case fallthrough bug.

ts

`const a: number = 6;

switch (a) {

case 0:

Fallthrough case in switch.7029Fallthrough case in switch.

console.log("even");

case 1:

console.log("odd");

break;

}

`Try

# No Implicit Any - noImplicitAny

In some cases where no type annotations are present, TypeScript will fall back to a type of any for a variable when it cannot infer the type.

This can cause some errors to be missed, for example:

ts

`function fn(s) {

// No error?

console.log(s.subtr(3));

}

fn(42);

`Try

Turning on noImplicitAny however TypeScript will issue an error whenever it would have inferred any:

ts

`function fn(s) {

Parameter 's' implicitly has an 'any' type.7006Parameter 's' implicitly has an 'any' type.

console.log(s.subtr(3));

}

`Try

# No Implicit Override - noImplicitOverride

When working with classes which use inheritance, it’s possible for a sub-class to get “out of sync” with the functions it overloads when they are renamed in the base class.

For example, imagine you are modeling a music album syncing system:

ts

`class Album {

download() {

// Default behavior

}

}

class SharedAlbum extends Album {

download() {

// Override to get info from many sources

}

}

`Try

Then when you add support for machine-learning generated playlists, you refactor the Album class to have a ‘setup’ function instead:

ts

`class Album {

setup() {

// Default behavior

}

}

class MLAlbum extends Album {

setup() {

// Override to get info from algorithm

}

}

class SharedAlbum extends Album {

download() {

// Override to get info from many sources

}

}

`Try

In this case, TypeScript has provided no warning that download on SharedAlbum expected to override a function in the base class.

Using noImplicitOverride you can ensure that the sub-classes never go out of sync, by ensuring that functions which override include the keyword override.

The following example has noImplicitOverride enabled, and you can see the error received when override is missing:

ts

`class Album {

setup() {}

}

class MLAlbum extends Album {

override setup() {}

}

class SharedAlbum extends Album {

setup() {}

This member must have an 'override' modifier because it overrides a member in the base class 'Album'.4114This member must have an 'override' modifier because it overrides a member in the base class 'Album'.

}

`Try

# No Implicit Returns - noImplicitReturns

When enabled, TypeScript will check all code paths in a function to ensure they return a value.

ts

`function lookupHeadphonesManufacturer(color: "blue" | "black"): string {

Function lacks ending return statement and return type does not include 'undefined'.2366Function lacks ending return statement and return type does not include 'undefined'.

if (color === "blue") {

return "beats";

} else {

("bose");

}

}

`Try

# No Implicit This - noImplicitThis

Raise error on ‘this’ expressions with an implied ‘any’ type.

For example, the class below returns a function which tries to access this.width and this.height – but the context for this inside the function inside getAreaFunction is not the instance of the Rectangle.

ts

`class Rectangle {

width: number;

height: number;

constructor(width: number, height: number) {

this.width = width;

this.height = height;

}

getAreaFunction() {

return function () {

  return this.width * this.height;

'this' implicitly has type 'any' because it does not have a type annotation.
'this' implicitly has type 'any' because it does not have a type annotation.2683
2683'this' implicitly has type 'any' because it does not have a type annotation.
'this' implicitly has type 'any' because it does not have a type annotation.

};

}

}

`Try

# No Property Access From Index Signature - noPropertyAccessFromIndexSignature

This setting ensures consistency between accessing a field via the “dot” (obj.key) syntax, and “indexed” (obj["key"]) and the way which the property is declared in the type.

Without this flag, TypeScript will allow you to use the dot syntax to access fields which are not defined:

ts

`` interface GameSettings {

// Known up-front properties

speed: "fast" | "medium" | "slow";

quality: "high" | "low";

// Assume anything unknown to the interface

// is a string.

}

const settings = getSettings();

settings.speed;

      (property) GameSettings.speed: "fast" | "medium" | "slow"

settings.quality;

       (property) GameSettings.quality: "high" | "low"

// Unknown key accessors are allowed on

// this object, and are string

settings.username;

        (index) GameSettings[string]: string

``Try

Turning the flag on will raise an error because the unknown field uses dot syntax instead of indexed syntax.

ts

`const settings = getSettings();

settings.speed;

settings.quality;

// This would need to be settings["username"];

settings.username;

Property 'username' comes from an index signature, so it must be accessed with ['username'].4111Property 'username' comes from an index signature, so it must be accessed with ['username'].

        (index) GameSettings[string]: string

`Try

The goal of this flag is to signal intent in your calling syntax about how certain you are this property exists.

# No Unchecked Indexed Access - noUncheckedIndexedAccess

TypeScript has a way to describe objects which have unknown keys but known values on an object, via index signatures.

ts

`interface EnvironmentVars {

NAME: string;

OS: string;

// Unknown properties are covered by this index signature.

}

declare const env: EnvironmentVars;

// Declared as existing

const sysName = env.NAME;

const os = env.OS;

  const os: string

// Not declared, but because of the index

// signature, then it is considered a string

const nodeEnv = env.NODE_ENV;

    const nodeEnv: string

`Try

Turning on noUncheckedIndexedAccess will add undefined to any un-declared field in the type.

ts

`declare const env: EnvironmentVars;

// Declared as existing

const sysName = env.NAME;

const os = env.OS;

  const os: string

// Not declared, but because of the index

// signature, then it is considered a string

const nodeEnv = env.NODE_ENV;

    const nodeEnv: string | undefined

`Try

# No Unused Locals - noUnusedLocals

Report errors on unused local variables.

ts

`const createKeyboard = (modelID: number) => {

const defaultModelID = 23;

'defaultModelID' is declared but its value is never read.6133'defaultModelID' is declared but its value is never read.

return { type: "keyboard", modelID };

};

`Try

# No Unused Parameters - noUnusedParameters

Report errors on unused parameters in functions.

ts

`const createDefaultKeyboard = (modelID: number) => {

'modelID' is declared but its value is never read.6133'modelID' is declared but its value is never read.

const defaultModelID = 23;

return { type: "keyboard", modelID: defaultModelID };

};

`Try

# Strict - strict

The strict flag enables a wide range of type checking behavior that results in stronger guarantees of program correctness. Turning this on is equivalent to enabling all of the strict mode family options, which are outlined below. You can then turn off individual strict mode family checks as needed.

Future versions of TypeScript may introduce additional stricter checking under this flag, so upgrades of TypeScript might result in new type errors in your program. When appropriate and possible, a corresponding flag will be added to disable that behavior.

# Strict Bind Call Apply - strictBindCallApply

When set, TypeScript will check that the built-in methods of functions call, bind, and apply are invoked with correct argument for the underlying function:

ts

`// With strictBindCallApply on

function fn(x: string) {

return parseInt(x);

}

const n1 = fn.call(undefined, "10");

const n2 = fn.call(undefined, false);

Argument of type 'boolean' is not assignable to parameter of type 'string'.2345Argument of type 'boolean' is not assignable to parameter of type 'string'.`Try

Otherwise, these functions accept any arguments and will return any:

ts

`// With strictBindCallApply off

function fn(x: string) {

return parseInt(x);

}

// Note: No error; return type is 'any'

const n = fn.call(undefined, false);

`Try

# strictBuiltinIteratorReturn - strictBuiltinIteratorReturn

Built-in iterators are instantiated with a `TReturn` type of undefined instead of `any`.

# Strict Function Types - strictFunctionTypes

When enabled, this flag causes functions parameters to be checked more correctly.

Here’s a basic example with strictFunctionTypes off:

ts

`function fn(x: string) {

console.log("Hello, " + x.toLowerCase());

}

type StringOrNumberFunc = (ns: string | number) => void;

// Unsafe assignment

let func: StringOrNumberFunc = fn;

// Unsafe call - will crash

func(10);

`Try

With strictFunctionTypes on, the error is correctly detected:

ts

`function fn(x: string) {

console.log("Hello, " + x.toLowerCase());

}

type StringOrNumberFunc = (ns: string | number) => void;

// Unsafe assignment is prevented

let func: StringOrNumberFunc = fn;

Type '(x: string) => void' is not assignable to type 'StringOrNumberFunc'. Types of parameters 'x' and 'ns' are incompatible. Type 'string | number' is not assignable to type 'string'. Type 'number' is not assignable to type 'string'.2322Type '(x: string) => void' is not assignable to type 'StringOrNumberFunc'. Types of parameters 'x' and 'ns' are incompatible. Type 'string | number' is not assignable to type 'string'. Type 'number' is not assignable to type 'string'.`Try

During development of this feature, we discovered a large number of inherently unsafe class hierarchies, including some in the DOM. Because of this, the setting only applies to functions written in function syntax, not to those in method syntax:

ts

`type Methodish = {

func(x: string | number): void;

};

function fn(x: string) {

console.log("Hello, " + x.toLowerCase());

}

// Ultimately an unsafe assignment, but not detected

const m: Methodish = {

func: fn,

};

m.func(10);

`Try

# Strict Null Checks - strictNullChecks

When strictNullChecks is false, null and undefined are effectively ignored by the language. This can lead to unexpected errors at runtime.

When strictNullChecks is true, null and undefined have their own distinct types and you’ll get a type error if you try to use them where a concrete value is expected.

For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can write code as though it will:

ts

`declare const loggedInUsername: string;

const users = [

{ name: "Oby", age: 12 },

{ name: "Heera", age: 32 },

];

const loggedInUser = users.find((u) => u.name === loggedInUsername);

console.log(loggedInUser.age);

`Try

Setting strictNullChecks to true will raise an error that you have not made a guarantee that the loggedInUser exists before trying to use it.

ts

`declare const loggedInUsername: string;

const users = [

{ name: "Oby", age: 12 },

{ name: "Heera", age: 32 },

];

const loggedInUser = users.find((u) => u.name === loggedInUsername);

console.log(loggedInUser.age);

'loggedInUser' is possibly 'undefined'.18048'loggedInUser' is possibly 'undefined'.`Try

The second example failed because the array’s find function looks a bit like this simplification:

ts

`// When strictNullChecks: true

type Array = {

find(predicate: (value: any, index: number) => boolean): S | undefined;

};

// When strictNullChecks: false the undefined is removed from the type system,

// allowing you to write code which assumes it always found a result

type Array = {

find(predicate: (value: any, index: number) => boolean): S;

};

`

# Strict Property Initialization - strictPropertyInitialization

When set to true, TypeScript will raise an error when a class property was declared but not set in the constructor.

ts

`class UserAccount {

name: string;

accountType = "user";

email: string;

Property 'email' has no initializer and is not definitely assigned in the constructor.2564Property 'email' has no initializer and is not definitely assigned in the constructor.

address: string | undefined;

constructor(name: string) {

this.name = name;

// Note that this.email is not set

}

}

`Try

In the above case:

# Use Unknown In Catch Variables - useUnknownInCatchVariables

In TypeScript 4.0, support was added to allow changing the type of the variable in a catch clause from any to unknown. Allowing for code like:

ts

`try {

// ...

} catch (err: unknown) {

// We have to verify err is an

// error before using it as one.

if (err instanceof Error) {

console.log(err.message);

}

}

`Try

This pattern ensures that error handling code becomes more comprehensive because you cannot guarantee that the object being thrown is a Error subclass ahead of time. With the flag useUnknownInCatchVariables enabled, then you do not need the additional syntax (: unknown) nor a linter rule to try enforce this behavior.

#Modules

# Allow Arbitrary Extensions - allowArbitraryExtensions

In TypeScript 5.0, when an import path ends in an extension that isn’t a known JavaScript or TypeScript file extension, the compiler will look for a declaration file for that path in the form of {file basename}.d.{extension}.ts. For example, if you are using a CSS loader in a bundler project, you might want to write (or generate) declaration files for those stylesheets:

css

`/* app.css */

.cookie-banner {

display: none;

}

`

ts

`// app.d.css.ts

declare const css: {

cookieBanner: string;

};

export default css;

`

ts

`// App.tsx

import styles from "./app.css";

styles.cookieBanner; // string

`

By default, this import will raise an error to let you know that TypeScript doesn’t understand this file type and your runtime might not support importing it. But if you’ve configured your runtime or bundler to handle it, you can suppress the error with the new --allowArbitraryExtensions compiler option.

Note that historically, a similar effect has often been achievable by adding a declaration file named app.css.d.ts instead of app.d.css.ts - however, this just worked through Node’s require resolution rules for CommonJS. Strictly speaking, the former is interpreted as a declaration file for a JavaScript file named app.css.js. Because relative files imports need to include extensions in Node’s ESM support, TypeScript would error on our example in an ESM file under --moduleResolution node16 or nodenext.

For more information, read up the proposal for this feature and its corresponding pull request.

# Allow Importing TS Extensions - allowImportingTsExtensions

--allowImportingTsExtensions allows TypeScript files to import each other with a TypeScript-specific extension like .ts, .mts, or .tsx.

This flag is only allowed when --noEmit or --emitDeclarationOnly is enabled, since these import paths would not be resolvable at runtime in JavaScript output files. The expectation here is that your resolver (e.g. your bundler, a runtime, or some other tool) is going to make these imports between .ts files work.

# Allow Umd Global Access - allowUmdGlobalAccess

When set to true, allowUmdGlobalAccess lets you access UMD exports as globals from inside module files. A module file is a file that has imports and/or exports. Without this flag, using an export from a UMD module requires an import declaration.

An example use case for this flag would be a web project where you know the particular library (like jQuery or Lodash) will always be available at runtime, but you can’t access it with an import.

# Base URL - baseUrl

Sets a base directory from which to resolve bare specifier module names. For example, in the directory structure:

`project

├── ex.ts

├── hello

│ └── world.ts

└── tsconfig.json

`

With "baseUrl": "./", TypeScript will look for files starting at the same folder as the tsconfig.json:

ts

`import { helloWorld } from "hello/world";

console.log(helloWorld);

`

This resolution has higher priority than lookups from node_modules.

This feature was designed for use in conjunction with AMD module loaders in the browser, and is not recommended in any other context. As of TypeScript 4.1, baseUrl is no longer required to be set when using paths.

# Custom Conditions - customConditions

--customConditions takes a list of additional conditions that should succeed when TypeScript resolves from an exports or imports field of a package.json. These conditions are added to whatever existing conditions a resolver will use by default.

For example, when this field is set in a tsconfig.json as so:

jsonc

`{

"compilerOptions": {

"target": "es2022",

"moduleResolution": "bundler",

"customConditions": ["my-condition"]

}

}

`

Any time an exports or imports field is referenced in package.json, TypeScript will consider conditions called my-condition.

So when importing from a package with the following package.json

jsonc

`{

// ...

"exports": {

".": {

  "my-condition": "./foo.mjs",

  "node": "./bar.mjs",

  "import": "./baz.mjs",

  "require": "./biz.mjs"

}

}

}

`

TypeScript will try to look for files corresponding to foo.mjs.

This field is only valid under the node16, nodenext, and bundler options for --moduleResolution.

# Module - module

Sets the module system for the program. See the theory behind TypeScript’s module option and its reference page for more information. You very likely want "nodenext" for modern Node.js projects and preserve or esnext for code that will be bundled.

Changing module affects moduleResolution which also has a reference page.

Here’s some example output for this file:

ts

`// @filename: index.ts

import { valueOfPi } from "./constants";

export const twoPi = valueOfPi * 2;

`Try

CommonJS

ts

`"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

exports.twoPi = void 0;

const constants_1 = require("./constants");

exports.twoPi = constants_1.valueOfPi * 2;

`Try

UMD

ts

`(function (factory) {

if (typeof module === "object" && typeof module.exports === "object") {

    var v = factory(require, exports);

    if (v !== undefined) module.exports = v;

}

else if (typeof define === "function" && define.amd) {

    define(["require", "exports", "./constants"], factory);

}

})(function (require, exports) {

"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

exports.twoPi = void 0;

const constants_1 = require("./constants");

exports.twoPi = constants_1.valueOfPi * 2;

});

`Try

AMD

ts

`define(["require", "exports", "./constants"], function (require, exports, constants_1) {

"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

exports.twoPi = void 0;

exports.twoPi = constants_1.valueOfPi * 2;

});

`Try

System

ts

`System.register(["./constants"], function (exports_1, context_1) {

"use strict";

var constants_1, twoPi;

var __moduleName = context_1 && context_1.id;

return {

    setters: [

        function (constants_1_1) {

            constants_1 = constants_1_1;

        }

    ],

    execute: function () {

        exports_1("twoPi", twoPi = constants_1.valueOfPi * 2);

    }

};

});

`Try

ESNext

ts

`import { valueOfPi } from "./constants";

export const twoPi = valueOfPi * 2;

`Try

ES2015/ES6/ES2020/ES2022

ts

`import { valueOfPi } from "./constants";

export const twoPi = valueOfPi * 2;

`Try

In addition to the base functionality of ES2015/ES6, ES2020 adds support for dynamic imports, and import.meta while ES2022 further adds support for top level await.

node16/node18/nodenext

The node16, node18, and nodenext modes integrate with Node’s native ECMAScript Module support. The emitted JavaScript uses either CommonJS or ES2020 output depending on the file extension and the value of the type setting in the nearest package.json. Module resolution also works differently. You can learn more in the handbook and Modules Reference.

preserve

In --module preserve (added in TypeScript 5.4), ECMAScript imports and exports written in input files are preserved in the output, and CommonJS-style import x = require("...") and export = ... statements are emitted as CommonJS require and module.exports. In other words, the format of each individual import or export statement is preserved, rather than being coerced into a single format for the whole compilation (or even a whole file).

ts

`import { valueOfPi } from "./constants";

const constants = require("./constants");

export const piSquared = valueOfPi * constants.valueOfPi;

`Try

While it’s rare to need to mix imports and require calls in the same file, this module mode best reflects the capabilities of most modern bundlers, as well as the Bun runtime.

Why care about TypeScript’s module emit with a bundler or with Bun, where you’re likely also setting noEmit? TypeScript’s type checking and module resolution behavior are affected by the module format that it would emit. Setting module gives TypeScript information about how your bundler or runtime will process imports and exports, which ensures that the types you see on imported values accurately reflect what will happen at runtime or after bundling.

None

ts

`"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

exports.twoPi = void 0;

const constants_1 = require("./constants");

exports.twoPi = constants_1.valueOfPi * 2;

`Try

# Module Resolution - moduleResolution

Specify the module resolution strategy:

There are reference pages explaining the theory behind TypeScript’s module resolution and the details of each option.

# Module Suffixes - moduleSuffixes

Provides a way to override the default list of file name suffixes to search when resolving a module.

`{

"": {

"": [".ios", ".native", ""]

}

}

`

Given the above configuration, an import like the following:

ts

`import * as foo from "./foo";

`

TypeScript will look for the relative files ./foo.ios.ts, ./foo.native.ts, and finally ./foo.ts.

Note the empty string "" in moduleSuffixes which is necessary for TypeScript to also look-up ./foo.ts.

This feature can be useful for React Native projects where each target platform can use a separate tsconfig.json with differing moduleSuffixes.

# No Resolve - noResolve

By default, TypeScript will examine the initial set of files for import and <reference directives and add these resolved files to your program.

If noResolve is set, this process doesn’t happen. However, import statements are still checked to see if they resolve to a valid module, so you’ll need to make sure this is satisfied by some other means.

# noUncheckedSideEffectImports - noUncheckedSideEffectImports

In JavaScript it’s possible to import a module without actually importing any values from it.

ts

`import "some-module";

`

These imports are often called side effect imports because the only useful behavior they can provide is by executing some side effect (like registering a global variable, or adding a polyfill to a prototype).

By default, TypeScript will not check these imports for validity. If the import resolves to a valid source file, TypeScript will load and check the file. If no source file is found, TypeScript will silently ignore the import.

This is surprising behavior, but it partially stems from modeling patterns in the JavaScript ecosystem. For example, this syntax has also been used with special loaders in bundlers to load CSS or other assets. Your bundler might be configured in such a way where you can include specific .css files by writing something like the following:

tsx

`import "./button-component.css";

export function Button() {

// ...

}

`

Still, this masks potential typos on side effect imports.

When --noUncheckedSideEffectImports is enabled, TypeScript will error if it can’t find a source file for a side effect import.

ts

`import "oops-this-module-does-not-exist";

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

// error: Cannot find module 'oops-this-module-does-not-exist' or its corresponding

// type declarations.

`

When enabling this option, some working code may now receive an error, like in the CSS example above. To work around this, users who want to just write side effect imports for assets might be better served by writing what’s called an ambient module declaration with a wildcard specifier. It would go in a global file and look something like the following:

ts

`// ./src/globals.d.ts

// Recognize all CSS files as module imports.

declare module "*.css" {}

`

In fact, you might already have a file like this in your project! For example, running something like vite init might create a similar vite-env.d.ts.

# Paths - paths

A series of entries which re-map imports to lookup locations relative to the baseUrl if set, or to the tsconfig file itself otherwise. There is a larger coverage of paths in the moduleResolution reference page.

paths lets you declare how TypeScript should resolve an import in your require/imports.

`{

"": {

"": {

  "jquery": ["./vendor/jquery/dist/jquery"]

}

}

}

`

This would allow you to be able to write import "jquery", and get all of the correct typing locally.

`{

"": {

"": {

  "app/*": ["./src/app/*"],

  "config/*": ["./src/app/_config/*"],

  "environment/*": ["./src/environments/*"],

  "shared/*": ["./src/app/_shared/*"],

  "helpers/*": ["./src/helpers/*"],

  "tests/*": ["./src/tests/*"]

}

}

}

`

In this case, you can tell the TypeScript file resolver to support a number of custom prefixes to find code.

Note that this feature does not change how import paths are emitted by tsc, so paths should only be used to inform TypeScript that another tool has this mapping and will use it at runtime or when bundling.

# Resolve JSON Module - resolveJsonModule

Allows importing modules with a .json extension, which is a common practice in node projects. This includes generating a type for the import based on the static JSON shape.

TypeScript does not support resolving JSON files by default:

ts

`// @filename: settings.json

{

"repo": "TypeScript",

"dry": false,

"debug": false

}

// @filename: index.ts

import settings from "./settings.json";

Cannot find module './settings.json'. Consider using '--resolveJsonModule' to import module with '.json' extension.2732Cannot find module './settings.json'. Consider using '--resolveJsonModule' to import module with '.json' extension.

settings.debug === true;

settings.dry === 2;

`Try

Enabling the option allows importing JSON, and validating the types in that JSON file.

ts

`// @filename: settings.json

{

"repo": "TypeScript",

"dry": false,

"debug": false

}

// @filename: index.ts

import settings from "./settings.json";

settings.debug === true;

settings.dry === 2;

This comparison appears to be unintentional because the types 'boolean' and 'number' have no overlap.2367This comparison appears to be unintentional because the types 'boolean' and 'number' have no overlap.`Try

# Resolve package.json Exports - resolvePackageJsonExports

--resolvePackageJsonExports forces TypeScript to consult the exports field of package.json files if it ever reads from a package in node_modules.

This option defaults to true under the node16, nodenext, and bundler options for --moduleResolution.

# Resolve package.json Imports - resolvePackageJsonImports

--resolvePackageJsonImports forces TypeScript to consult the imports field of package.json files when performing a lookup that starts with # from a file whose ancestor directory contains a package.json.

This option defaults to true under the node16, nodenext, and bundler options for --moduleResolution.

# rewriteRelativeImportExtensions - rewriteRelativeImportExtensions

Rewrite .ts, .tsx, .mts, and .cts file extensions in relative import paths to their JavaScript equivalent in output files.

For more information, see the TypeScript 5.7 release notes.

# Root Dir - rootDir

Default: The longest common path of all non-declaration input files. If composite is set, the default is instead the directory containing the tsconfig.json file.

When TypeScript compiles files, it keeps the same directory structure in the output directory as exists in the input directory.

For example, let’s say you have some input files:

`MyProj

├── tsconfig.json

├── core

│ ├── a.ts

│ ├── b.ts

│ ├── sub

│ │ ├── c.ts

├── types.d.ts

`

The inferred value for rootDir is the longest common path of all non-declaration input files, which in this case is core/.

If your outDir was dist, TypeScript would write this tree:

`MyProj

├── dist

│ ├── a.js

│ ├── b.js

│ ├── sub

│ │ ├── c.js

`

However, you may have intended for core to be part of the output directory structure. By setting rootDir: "." in tsconfig.json, TypeScript would write this tree:

`MyProj

├── dist

│ ├── core

│ │ ├── a.js

│ │ ├── b.js

│ │ ├── sub

│ │ │ ├── c.js

`

Importantly, rootDir does not affect which files become part of the compilation. It has no interaction with the include, exclude, or files tsconfig.json settings.

Note that TypeScript will never write an output file to a directory outside of outDir, and will never skip emitting a file. For this reason, rootDir also enforces that all files which need to be emitted are underneath the rootDir path.

For example, let’s say you had this tree:

`MyProj

├── tsconfig.json

├── core

│ ├── a.ts

│ ├── b.ts

├── helpers.ts

`

It would be an error to specify rootDir as core and include as * because it creates a file (helpers.ts) that would need to be emitted outside the outDir (i.e. ../helpers.js).

# Root Dirs - rootDirs

Using rootDirs, you can inform the compiler that there are many “virtual” directories acting as a single root. This allows the compiler to resolve relative module imports within these “virtual” directories, as if they were merged in to one directory.

For example:

`` src

└── views

 └── view1.ts (can import "./template1", "./view2`)

 └── view2.ts (can import "./template1", "./view1`)

generated

└── templates

     └── views

         └── template1.ts (can import "./view1", "./view2")

``

`{

"": {

"": ["src/views", "generated/templates/views"]

}

}

`

This does not affect how TypeScript emits JavaScript, it only emulates the assumption that they will be able to work via those relative paths at runtime.

rootDirs can be used to provide a separate “type layer” to files that are not TypeScript or JavaScript by providing a home for generated .d.ts files in another folder. This technique is useful for bundled applications where you use import of files that aren’t necessarily code:

sh

` src

└── index.ts

└── css

 └── main.css

 └── navigation.css

generated

└── css

 └── main.css.d.ts

 └── navigation.css.d.ts

`

`{

"": {

"": ["src", "generated"]

}

}

`

This technique lets you generate types ahead of time for the non-code source files. Imports then work naturally based off the source file’s location. For example ./src/index.ts can import the file ./src/css/main.css and TypeScript will be aware of the bundler’s behavior for that filetype via the corresponding generated declaration file.

ts

`// @filename: index.ts

import { appClass } from "./main.css";

`Try

# Type Roots - typeRoots

By default all visible ”@types” packages are included in your compilation. Packages in node_modules/@types of any enclosing folder are considered visible. For example, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on.

If typeRoots is specified, only packages under typeRoots will be included. For example:

`{

"": {

"": ["./typings", "./vendor/types"]

}

}

`

This config file will include all packages under ./typings and ./vendor/types, and no packages from ./node_modules/@types. All paths are relative to the tsconfig.json.

# Types - types

By default all visible ”@types” packages are included in your compilation. Packages in node_modules/@types of any enclosing folder are considered visible. For example, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on.

If types is specified, only packages listed will be included in the global scope. For instance:

`{

"": {

"": ["node", "jest", "express"]

}

}

`

This tsconfig.json file will only include ./node_modules/@types/node, ./node_modules/@types/jest and ./node_modules/@types/express. Other packages under node_modules/@types/* will not be included.

What does this affect?

This option does not affect how @types/* are included in your application code, for example if you had the above compilerOptions example with code like:

ts

`import * as moment from "moment";

moment().format("MMMM Do YYYY, h:mm:ss a");

`

The moment import would be fully typed.

When you have this option set, by not including a module in the types array it:

This feature differs from typeRoots in that it is about specifying only the exact types you want included, whereas typeRoots supports saying you want particular folders.

#Emit

# Declaration - declaration

Generate .d.ts files for every TypeScript or JavaScript file inside your project. These .d.ts files are type definition files which describe the external API of your module. With .d.ts files, tools like TypeScript can provide intellisense and accurate types for un-typed code.

When declaration is set to true, running the compiler with this TypeScript code:

ts

`export let helloWorld = "hi";

`Try

Will generate an index.js file like this:

ts

`export let helloWorld = "hi";

`Try

With a corresponding helloWorld.d.ts:

ts

`export declare let helloWorld: string;

`Try

When working with .d.ts files for JavaScript files you may want to use emitDeclarationOnly or use outDir to ensure that the JavaScript files are not overwritten.

# Declaration Dir - declarationDir

Offers a way to configure the root directory for where declaration files are emitted.

`example

├── index.ts

├── package.json

└── tsconfig.json

`

with this tsconfig.json:

`{

"": {

"": true,

"": "./types"

}

}

`

Would place the d.ts for the index.ts in a types folder:

`example

├── index.js

├── index.ts

├── package.json

├── tsconfig.json

└── types

└── index.d.ts

`

# Declaration Map - declarationMap

Generates a source map for .d.ts files which map back to the original .ts source file. This will allow editors such as VS Code to go to the original .ts file when using features like Go to Definition.

You should strongly consider turning this on if you’re using project references.

# Downlevel Iteration - downlevelIteration

Downleveling is TypeScript’s term for transpiling to an older version of JavaScript. This flag is to enable support for a more accurate implementation of how modern JavaScript iterates through new concepts in older JavaScript runtimes.

ECMAScript 6 added several new iteration primitives: the for / of loop (for (el of arr)), Array spread ([a, ...b]), argument spread (fn(...args)), and Symbol.iterator.downlevelIteration allows for these iteration primitives to be used more accurately in ES5 environments if a Symbol.iterator implementation is present.

Example: Effects on for / of

With this TypeScript code:

ts

`const str = "Hello!";

for (const s of str) {

console.log(s);

}

`Try

Without downlevelIteration enabled, a for / of loop on any object is downleveled to a traditional for loop:

ts

`"use strict";

var str = "Hello!";

for (var _i = 0, str_1 = str; _i < str_1.length; _i++) {

var s = str_1[_i];

console.log(s);

}

`Try

This is often what people expect, but it’s not 100% compliant with ECMAScript iteration protocol. Certain strings, such as emoji (😜), have a .length of 2 (or even more!), but should iterate as 1 unit in a for-of loop. See this blog post by Jonathan New for a longer explanation.

When downlevelIteration is enabled, TypeScript will use a helper function that checks for a Symbol.iterator implementation (either native or polyfill). If this implementation is missing, you’ll fall back to index-based iteration.

ts

`"use strict";

var __values = (this && this.__values) || function(o) {

var s = typeof Symbol === "function" && Symbol.iterator, m = s && o[s], i = 0;

if (m) return m.call(o);

if (o && typeof o.length === "number") return {

    next: function () {

        if (o && i >= o.length) o = void 0;

        return { value: o && o[i++], done: !o };

    }

};

throw new TypeError(s ? "Object is not iterable." : "Symbol.iterator is not defined.");

};

var e_1, _a;

var str = "Hello!";

try {

for (var str_1 = __values(str), str_1_1 = str_1.next(); !str_1_1.done; str_1_1 = str_1.next()) {

    var s = str_1_1.value;

    console.log(s);

}

}

catch (e_1_1) { e_1 = { error: e_1_1 }; }

finally {

try {

    if (str_1_1 && !str_1_1.done && (_a = str_1.return)) _a.call(str_1);

}

finally { if (e_1) throw e_1.error; }

}

`Try

You can use tslib via importHelpers to reduce the amount of inline JavaScript too:

ts

`"use strict";

var __values = (this && this.__values) || function(o) {

var s = typeof Symbol === "function" && Symbol.iterator, m = s && o[s], i = 0;

if (m) return m.call(o);

if (o && typeof o.length === "number") return {

    next: function () {

        if (o && i >= o.length) o = void 0;

        return { value: o && o[i++], done: !o };

    }

};

throw new TypeError(s ? "Object is not iterable." : "Symbol.iterator is not defined.");

};

var e_1, _a;

var str = "Hello!";

try {

for (var str_1 = __values(str), str_1_1 = str_1.next(); !str_1_1.done; str_1_1 = str_1.next()) {

    var s = str_1_1.value;

    console.log(s);

}

}

catch (e_1_1) { e_1 = { error: e_1_1 }; }

finally {

try {

    if (str_1_1 && !str_1_1.done && (_a = str_1.return)) _a.call(str_1);

}

finally { if (e_1) throw e_1.error; }

}

`Try

Note: enabling downlevelIteration does not improve compliance if Symbol.iterator is not present in the runtime.

Example: Effects on Array Spreads

This is an array spread:

js

`// Make a new array whose elements are 1 followed by the elements of arr2

const arr = [1, ...arr2];

`

Based on the description, it sounds easy to downlevel to ES5:

js

`// The same, right?

const arr = [1].concat(arr2);

`

However, this is observably different in certain rare cases.

For example, if a source array is missing one or more items (contains a hole), the spread syntax will replace each empty item with undefined, whereas .concat will leave them intact.

js

`// Make an array where the element at index 1 is missing

let arrayWithHole = ["a", , "c"];

let spread = [...arrayWithHole];

let concatenated = [].concat(arrayWithHole);

console.log(arrayWithHole);

// [ 'a', <1 empty item>, 'c' ]

console.log(spread);

// [ 'a', undefined, 'c' ]

console.log(concatenated);

// [ 'a', <1 empty item>, 'c' ]

`

Just as with for / of, downlevelIteration will use Symbol.iterator (if present) to more accurately emulate ES 6 behavior.

# Emit BOM - emitBOM

Controls whether TypeScript will emit a byte order mark (BOM) when writing output files. Some runtime environments require a BOM to correctly interpret a JavaScript files; others require that it is not present. The default value of false is generally best unless you have a reason to change it.

# Emit Declaration Only - emitDeclarationOnly

Only emit .d.ts files; do not emit .js files.

This setting is useful in two cases:

# Import Helpers - importHelpers

For certain downleveling operations, TypeScript uses some helper code for operations like extending class, spreading arrays or objects, and async operations. By default, these helpers are inserted into files which use them. This can result in code duplication if the same helper is used in many different modules.

If the importHelpers flag is on, these helper functions are instead imported from the tslib module. You will need to ensure that the tslib module is able to be imported at runtime. This only affects modules; global script files will not attempt to import modules.

For example, with this TypeScript:

ts

`export function fn(arr: number[]) {

const arr2 = [1, ...arr];

}

`

Turning on downlevelIteration and importHelpers is still false:

ts

`var __read = (this && this.__read) || function (o, n) {

var m = typeof Symbol === "function" && o[Symbol.iterator];

if (!m) return o;

var i = m.call(o), r, ar = [], e;

try {

    while ((n === void 0 || n-- > 0) && !(r = i.next()).done) ar.push(r.value);

}

catch (error) { e = { error: error }; }

finally {

    try {

        if (r && !r.done && (m = i["return"])) m.call(i);

    }

    finally { if (e) throw e.error; }

}

return ar;

};

var __spreadArray = (this && this.__spreadArray) || function (to, from, pack) {

if (pack || arguments.length === 2) for (var i = 0, l = from.length, ar; i < l; i++) {

    if (ar || !(i in from)) {

        if (!ar) ar = Array.prototype.slice.call(from, 0, i);

        ar[i] = from[i];

    }

}

return to.concat(ar || Array.prototype.slice.call(from));

};

export function fn(arr) {

var arr2 = __spreadArray([1], __read(arr), false);

}

`Try

Then turning on both downlevelIteration and importHelpers:

ts

`import { __read, __spreadArray } from "tslib";

export function fn(arr) {

var arr2 = __spreadArray([1], __read(arr), false);

}

`Try

You can use noEmitHelpers when you provide your own implementations of these functions.

# Inline Source Map - inlineSourceMap

When set, instead of writing out a .js.map file to provide source maps, TypeScript will embed the source map content in the .js files. Although this results in larger JS files, it can be convenient in some scenarios. For example, you might want to debug JS files on a webserver that doesn’t allow .map files to be served.

Mutually exclusive with sourceMap.

For example, with this TypeScript:

ts

`const helloWorld = "hi";

console.log(helloWorld);

`

Converts to this JavaScript:

ts

`"use strict";

const helloWorld = "hi";

console.log(helloWorld);

`Try

Then enable building it with inlineSourceMap enabled there is a comment at the bottom of the file which includes a source-map for the file.

ts

`"use strict";

const helloWorld = "hi";

console.log(helloWorld);

//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiaW5kZXguanMiLCJzb3VyY2VSb290IjoiIiwic291cmNlcyI6WyJpbmRleC50cyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiO0FBQUEsTUFBTSxVQUFVLEdBQUcsSUFBSSxDQUFDO0FBQ3hCLE9BQU8sQ0FBQyxHQUFHLENBQUMsVUFBVSxDQUFDLENBQUMifQ==

`Try

# Inline Sources - inlineSources

When set, TypeScript will include the original content of the .ts file as an embedded string in the source map (using the source map’s sourcesContent property). This is often useful in the same cases as inlineSourceMap.

Requires either sourceMap or inlineSourceMap to be set.

For example, with this TypeScript:

ts

`const helloWorld = "hi";

console.log(helloWorld);

`Try

By default converts to this JavaScript:

ts

`"use strict";

const helloWorld = "hi";

console.log(helloWorld);

`Try

Then enable building it with inlineSources and inlineSourceMap enabled there is a comment at the bottom of the file which includes a source-map for the file. Note that the end is different from the example in inlineSourceMap because the source-map now contains the original source code also.

ts

`"use strict";

const helloWorld = "hi";

console.log(helloWorld);

//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiaW5kZXguanMiLCJzb3VyY2VSb290IjoiIiwic291cmNlcyI6WyJpbmRleC50cyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiO0FBQUEsTUFBTSxVQUFVLEdBQUcsSUFBSSxDQUFDO0FBQ3hCLE9BQU8sQ0FBQyxHQUFHLENBQUMsVUFBVSxDQUFDLENBQUMiLCJzb3VyY2VzQ29udGVudCI6WyJjb25zdCBoZWxsb1dvcmxkID0gXCJoaVwiO1xuY29uc29sZS5sb2coaGVsbG9Xb3JsZCk7Il19

`Try

# Map Root - mapRoot

Specify the location where debugger should locate map files instead of generated locations. This string is treated verbatim inside the source-map, for example:

`{

"": {

"": true,

"": "https://my-website.com/debug/sourcemaps/"

}

}

`

Would declare that index.js will have sourcemaps at https://my-website.com/debug/sourcemaps/index.js.map.

# New Line - newLine

Specify the end of line sequence to be used when emitting files: ‘CRLF’ (dos) or ‘LF’ (unix).

# No Emit - noEmit

Do not emit compiler output files like JavaScript source code, source-maps or declarations.

This makes room for another tool like Babel, or swc to handle converting the TypeScript file to a file which can run inside a JavaScript environment.

You can then use TypeScript as a tool for providing editor integration, and as a source code type-checker.

# No Emit Helpers - noEmitHelpers

Instead of importing helpers with importHelpers, you can provide implementations in the global scope for the helpers you use and completely turn off emitting of helper functions.

For example, using this async function in ES5 requires a await-like function and generator-like function to run:

ts

`const getAPI = async (url: string) => {

// Get API

return {};

};

`Try

Which creates quite a lot of JavaScript:

ts

`"use strict";

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {

function adopt(value) { return value instanceof P ? value : new P(function (resolve) { resolve(value); }); }

return new (P || (P = Promise))(function (resolve, reject) {

    function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }

    function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }

    function step(result) { result.done ? resolve(result.value) : adopt(result.value).then(fulfilled, rejected); }

    step((generator = generator.apply(thisArg, _arguments || [])).next());

});

};

var __generator = (this && this.__generator) || function (thisArg, body) {

var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g = Object.create((typeof Iterator === "function" ? Iterator : Object).prototype);

return g.next = verb(0), g["throw"] = verb(1), g["return"] = verb(2), typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;

function verb(n) { return function (v) { return step([n, v]); }; }

function step(op) {

    if (f) throw new TypeError("Generator is already executing.");

    while (g && (g = 0, op[0] && (_ = 0)), _) try {

        if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;

        if (y = 0, t) op = [op[0] & 2, t.value];

        switch (op[0]) {

            case 0: case 1: t = op; break;

            case 4: _.label++; return { value: op[1], done: false };

            case 5: _.label++; y = op[1]; op = [0]; continue;

            case 7: op = _.ops.pop(); _.trys.pop(); continue;

            default:

                if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }

                if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }

                if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }

                if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }

                if (t[2]) _.ops.pop();

                _.trys.pop(); continue;

        }

        op = body.call(thisArg, _);

    } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }

    if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };

}

};

var getAPI = function (url) { return __awaiter(void 0, void 0, void 0, function () {

return __generator(this, function (_a) {

    // Get API

    return [2 /*return*/, {}];

});

}); };

`Try

Which can be switched out with your own globals via this flag:

ts

`"use strict";

var getAPI = function (url) { return __awaiter(void 0, void 0, void 0, function () {

return __generator(this, function (_a) {

    // Get API

    return [2 /*return*/, {}];

});

}); };

`Try

# No Emit On Error - noEmitOnError

Do not emit compiler output files like JavaScript source code, source-maps or declarations if any errors were reported.

This defaults to false, making it easier to work with TypeScript in a watch-like environment where you may want to see results of changes to your code in another environment before making sure all errors are resolved.

# Out Dir - outDir

If specified, .js (as well as .d.ts, .js.map, etc.) files will be emitted into this directory. The directory structure of the original source files is preserved; see rootDir if the computed root is not what you intended.

If not specified, .js files will be emitted in the same directory as the .ts files they were generated from:

sh

`$ tsc

example

├── index.js

└── index.ts

`

With a tsconfig.json like this:

Running tsc with these settings moves the files into the specified dist folder:

sh

`$ tsc

example

├── dist

│ └── index.js

├── index.ts

└── tsconfig.json

`

# Out File - outFile

If specified, all global (non-module) files will be concatenated into the single output file specified.

If module is system or amd, all module files will also be concatenated into this file after all global content.

Note: outFile cannot be used unless module is None, System, or AMD. This option cannot be used to bundle CommonJS or ES6 modules.

# Preserve Const Enums - preserveConstEnums

Do not erase const enum declarations in generated code. const enums provide a way to reduce the overall memory footprint of your application at runtime by emitting the enum value instead of a reference.

For example with this TypeScript:

ts

`const enum Album {

JimmyEatWorldFutures = 1,

TubRingZooHypothesis = 2,

DogFashionDiscoAdultery = 3,

}

const selectedAlbum = Album.JimmyEatWorldFutures;

if (selectedAlbum === Album.JimmyEatWorldFutures) {

console.log("That is a great choice.");

}

`Try

The default const enum behavior is to convert any Album.Something to the corresponding number literal, and to remove a reference to the enum from the JavaScript completely.

ts

`"use strict";

const selectedAlbum = 1 /* Album.JimmyEatWorldFutures */;

if (selectedAlbum === 1 /* Album.JimmyEatWorldFutures */) {

console.log("That is a great choice.");

}

`Try

With preserveConstEnums set to true, the enum exists at runtime and the numbers are still emitted.

ts

`"use strict";

var Album;

(function (Album) {

Album[Album["JimmyEatWorldFutures"] = 1] = "JimmyEatWorldFutures";

Album[Album["TubRingZooHypothesis"] = 2] = "TubRingZooHypothesis";

Album[Album["DogFashionDiscoAdultery"] = 3] = "DogFashionDiscoAdultery";

})(Album || (Album = {}));

const selectedAlbum = 1 /* Album.JimmyEatWorldFutures */;

if (selectedAlbum === 1 /* Album.JimmyEatWorldFutures */) {

console.log("That is a great choice.");

}

`Try

This essentially makes such const enums a source-code feature only, with no runtime traces.

Strips all comments from TypeScript files when converting into JavaScript. Defaults to false.

For example, this is a TypeScript file which has a JSDoc comment:

ts

`/** The translation of 'Hello world' into Portuguese */

export const helloWorldPTBR = "Olá Mundo";

`

When removeComments is set to true:

ts

`export const helloWorldPTBR = "Olá Mundo";

`Try

Without setting removeComments or having it as false:

ts

`/** The translation of 'Hello world' into Portuguese */

export const helloWorldPTBR = "Olá Mundo";

`Try

This means that your comments will show up in the JavaScript code.

# Source Map - sourceMap

Enables the generation of sourcemap files. These files allow debuggers and other tools to display the original TypeScript source code when actually working with the emitted JavaScript files. Source map files are emitted as .js.map (or .jsx.map) files next to the corresponding .js output file.

The .js files will in turn contain a sourcemap comment to indicate where the files are to external tools, for example:

ts

`// helloWorld.ts

export declare const helloWorld = "hi";

`

Compiling with sourceMap set to true creates the following JavaScript file:

js

`// helloWorld.js

"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

exports.helloWorld = "hi";

//# sourceMappingURL=// helloWorld.js.map

`

And this also generates this json map:

json

`// helloWorld.js.map

{

"version": 3,

"file": "ex.js",

"sourceRoot": "",

"sources": ["../ex.ts"],

"names": [],

"mappings": ";;AAAa,QAAA,UAAU,GAAG,IAAI,CAAA"

}

`

# Source Root - sourceRoot

Specify the location where a debugger should locate TypeScript files instead of relative source locations. This string is treated verbatim inside the source-map where you can use a path or a URL:

`{

"": {

"": true,

"": "https://my-website.com/debug/source/"

}

}

`

Would declare that index.js will have a source file at https://my-website.com/debug/source/index.ts.

# Strip Internal - stripInternal

Do not emit declarations for code that has an @internal annotation in its JSDoc comment. This is an internal compiler option; use at your own risk, because the compiler does not check that the result is valid. If you are searching for a tool to handle additional levels of visibility within your d.ts files, look at api-extractor.

ts

`/**

*/

export const daysInAWeek = 7;

/** Calculate how much someone earns in a week */

export function weeklySalary(dayRate: number) {

return daysInAWeek * dayRate;

}

`Try

With the flag set to false (default):

ts

`/**

*/

export declare const daysInAWeek = 7;

/** Calculate how much someone earns in a week */

export declare function weeklySalary(dayRate: number): number;

`Try

With stripInternal set to true the d.ts emitted will be redacted.

ts

`/** Calculate how much someone earns in a week */

export declare function weeklySalary(dayRate: number): number;

`Try

The JavaScript output is still the same.

#JavaScript Support

# Allow JS - allowJs

Allow JavaScript files to be imported inside your project, instead of just .ts and .tsx files. For example, this JS file:

js

`// @filename: card.js

export const defaultCardDeck = "Heart";

`Try

When imported into a TypeScript file will raise an error:

ts

`// @filename: index.ts

import { defaultCardDeck } from "./card";

console.log(defaultCardDeck);

`Try

Imports fine with allowJs enabled:

ts

`// @filename: index.ts

import { defaultCardDeck } from "./card";

console.log(defaultCardDeck);

`Try

This flag can be used as a way to incrementally add TypeScript files into JS projects by allowing the .ts and .tsx files to live along-side existing JavaScript files.

It can also be used along-side declaration and emitDeclarationOnly to create declarations for JS files.

# Check JS - checkJs

Works in tandem with allowJs. When checkJs is enabled then errors are reported in JavaScript files. This is the equivalent of including // @ts-check at the top of all JavaScript files which are included in your project.

For example, this is incorrect JavaScript according to the parseFloat type definition which comes with TypeScript:

js

`// parseFloat only takes a string

module.exports.pi = parseFloat(3.142);

`

When imported into a TypeScript module:

ts

`// @filename: constants.js

module.exports.pi = parseFloat(3.142);

// @filename: index.ts

import { pi } from "./constants";

console.log(pi);

`Try

You will not get any errors. However, if you turn on checkJs then you will get error messages from the JavaScript file.

ts

`// @filename: constants.js

module.exports.pi = parseFloat(3.142);

Argument of type 'number' is not assignable to parameter of type 'string'.2345Argument of type 'number' is not assignable to parameter of type 'string'.

// @filename: index.ts

import { pi } from "./constants";

console.log(pi);

`Try

# Max Node Module JS Depth - maxNodeModuleJsDepth

The maximum dependency depth to search under node_modules and load JavaScript files.

This flag can only be used when allowJs is enabled, and is used if you want to have TypeScript infer types for all of the JavaScript inside your node_modules.

Ideally this should stay at 0 (the default), and d.ts files should be used to explicitly define the shape of modules. However, there are cases where you may want to turn this on at the expense of speed and potential accuracy.

#Editor Support

# Disable Size Limit - disableSizeLimit

To avoid a possible memory bloat issues when working with very large JavaScript projects, there is an upper limit to the amount of memory TypeScript will allocate. Turning this flag on will remove the limit.

# Plugins - plugins

List of language service plugins to run inside the editor.

Language service plugins are a way to provide additional information to a user based on existing TypeScript files. They can enhance existing messages between TypeScript and an editor, or to provide their own error messages.

For example:

VS Code has the ability for a extension to automatically include language service plugins, and so you may have some running in your editor without needing to define them in your tsconfig.json.

#Interop Constraints

# Allow Synthetic Default Imports - allowSyntheticDefaultImports

When set to true, allowSyntheticDefaultImports allows you to write an import like:

ts

`import React from "react";

`

instead of:

ts

`import * as React from "react";

`

When the module does not explicitly specify a default export.

For example, without allowSyntheticDefaultImports as true:

ts

`// @filename: utilFunctions.js

const getStringLength = (str) => str.length;

module.exports = {

getStringLength,

};

// @filename: index.ts

import utils from "./utilFunctions";

Module '"/home/runner/work/TypeScript-Website/TypeScript-Website/packages/typescriptlang-org/utilFunctions"' has no default export.1192Module '"/home/runner/work/TypeScript-Website/TypeScript-Website/packages/typescriptlang-org/utilFunctions"' has no default export.

const count = utils.getStringLength("Check JS");

`Try

This code raises an error because there isn’t a default object which you can import. Even though it feels like it should. For convenience, transpilers like Babel will automatically create a default if one isn’t created. Making the module look a bit more like:

js

`// @filename: utilFunctions.js

const getStringLength = (str) => str.length;

const allFunctions = {

getStringLength,

};

module.exports = allFunctions;

module.exports.default = allFunctions;

`

This flag does not affect the JavaScript emitted by TypeScript, it’s only for the type checking. This option brings the behavior of TypeScript in-line with Babel, where extra code is emitted to make using a default export of a module more ergonomic.

# Erasable Syntax Only - erasableSyntaxOnly

Node.js supports running TypeScript files directly as of v23.6; however, only TypeScript-specific syntax that does not have runtime semantics are supported under this mode. In other words, it must be possible to easily erase any TypeScript-specific syntax from a file, leaving behind a valid JavaScript file.

That means the following constructs are not supported:

ts

`` // ❌ error: An import ... = require(...) alias

import foo = require("foo");

// ❌ error: A namespace with runtime code.

namespace container {

foo.method();

export type Bar = string;

}

// ❌ error: An import = alias

import Bar = container.Bar;

class Point {

// ❌ error: Parameter properties

constructor(public x: number, public y: number) {}

}

// ❌ error: An export = assignment.

export = Point;

// ❌ error: An enum declaration.

enum Direction {

Up,

Down,

Left,

Right,

}

``

Similar tools like ts-blank-space or Amaro (the underlying library for type-stripping in Node.js) have the same limitations. These tools will provide helpful error messages if they encounter code that doesn’t meet these requirements, but you still won’t find out your code doesn’t work until you actually try to run it.

The --erasableSyntaxOnly flag will cause TypeScript to error on most TypeScript-specific constructs that have runtime behavior.

ts

`class C {

constructor(public x: number) { }

//          ~~~~~~~~~~~~~~~~

// error! This syntax is not allowed when 'erasableSyntaxOnly' is enabled.

}

}

`

Typically, you will want to combine this flag with the --verbatimModuleSyntax, which ensures that a module contains the appropriate import syntax, and that import elision does not take place.

# ES Module Interop - esModuleInterop

By default (with esModuleInterop false or not set) TypeScript treats CommonJS/AMD/UMD modules similar to ES6 modules. In doing this, there are two parts in particular which turned out to be flawed assumptions:

This mis-match causes these two issues:

Turning on esModuleInterop will fix both of these problems in the code transpiled by TypeScript. The first changes the behavior in the compiler, the second is fixed by two new helper functions which provide a shim to ensure compatibility in the emitted JavaScript:

ts

`import * as fs from "fs";

import _ from "lodash";

fs.readFileSync("file.txt", "utf8");

_.chunk(["a", "b", "c", "d"], 2);

`

With esModuleInterop disabled:

ts

`"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

const fs = require("fs");

const lodash_1 = require("lodash");

fs.readFileSync("file.txt", "utf8");

lodash_1.default.chunk(["a", "b", "c", "d"], 2);

`Try

With esModuleInterop set to true:

ts

`"use strict";

var __createBinding = (this && this.__createBinding) || (Object.create ? (function(o, m, k, k2) {

if (k2 === undefined) k2 = k;

var desc = Object.getOwnPropertyDescriptor(m, k);

if (!desc || ("get" in desc ? !m.__esModule : desc.writable || desc.configurable)) {

  desc = { enumerable: true, get: function() { return m[k]; } };

}

Object.defineProperty(o, k2, desc);

}) : (function(o, m, k, k2) {

if (k2 === undefined) k2 = k;

o[k2] = m[k];

}));

var __setModuleDefault = (this && this.__setModuleDefault) || (Object.create ? (function(o, v) {

Object.defineProperty(o, "default", { enumerable: true, value: v });

}) : function(o, v) {

o["default"] = v;

});

var __importStar = (this && this.__importStar) || (function () {

var ownKeys = function(o) {

    ownKeys = Object.getOwnPropertyNames || function (o) {

        var ar = [];

        for (var k in o) if (Object.prototype.hasOwnProperty.call(o, k)) ar[ar.length] = k;

        return ar;

    };

    return ownKeys(o);

};

return function (mod) {

    if (mod && mod.__esModule) return mod;

    var result = {};

    if (mod != null) for (var k = ownKeys(mod), i = 0; i < k.length; i++) if (k[i] !== "default") __createBinding(result, mod, k[i]);

    __setModuleDefault(result, mod);

    return result;

};

})();

var __importDefault = (this && this.__importDefault) || function (mod) {

return (mod && mod.__esModule) ? mod : { "default": mod };

};

Object.defineProperty(exports, "__esModule", { value: true });

const fs = __importStar(require("fs"));

const lodash_1 = __importDefault(require("lodash"));

fs.readFileSync("file.txt", "utf8");

lodash_1.default.chunk(["a", "b", "c", "d"], 2);

`Try

Note: The namespace import import * as fs from "fs" only accounts for properties which are owned (basically properties set on the object and not via the prototype chain) on the imported object. If the module you’re importing defines its API using inherited properties, you need to use the default import form (import fs from "fs"), or disable esModuleInterop.

Note: You can make JS emit terser by enabling importHelpers:

ts

`"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

const tslib_1 = require("tslib");

const fs = tslib_1.__importStar(require("fs"));

const lodash_1 = tslib_1.__importDefault(require("lodash"));

fs.readFileSync("file.txt", "utf8");

lodash_1.default.chunk(["a", "b", "c", "d"], 2);

`Try

Enabling esModuleInterop will also enable allowSyntheticDefaultImports.

# Force Consistent Casing In File Names - forceConsistentCasingInFileNames

TypeScript follows the case sensitivity rules of the file system it’s running on. This can be problematic if some developers are working in a case-sensitive file system and others aren’t. If a file attempts to import fileManager.ts by specifying ./FileManager.ts the file will be found in a case-insensitive file system, but not on a case-sensitive file system.

When this option is set, TypeScript will issue an error if a program tries to include a file by a casing different from the casing on disk.

# isolatedDeclarations - isolatedDeclarations

Require sufficient annotation on exports so other tools can trivially generate declaration files.

For more information, see the 5.5 release notes

# Isolated Modules - isolatedModules

While you can use TypeScript to produce JavaScript code from TypeScript code, it’s also common to use other transpilers such as Babel to do this. However, other transpilers only operate on a single file at a time, which means they can’t apply code transforms that depend on understanding the full type system. This restriction also applies to TypeScript’s ts.transpileModule API which is used by some build tools.

These limitations can cause runtime problems with some TypeScript features like const enums and namespaces. Setting the isolatedModules flag tells TypeScript to warn you if you write certain code that can’t be correctly interpreted by a single-file transpilation process.

It does not change the behavior of your code, or otherwise change the behavior of TypeScript’s checking and emitting process.

Some examples of code which does not work when isolatedModules is enabled.

Exports of Non-Value Identifiers

In TypeScript, you can import a type and then subsequently export it:

ts

`import { someType, someFunction } from "someModule";

someFunction();

export { someType, someFunction };

`Try

Because there’s no value for someType, the emitted export will not try to export it (this would be a runtime error in JavaScript):

js

`export { someFunction };

`

Single-file transpilers don’t know whether someType produces a value or not, so it’s an error to export a name that only refers to a type.

Non-Module Files

If isolatedModules is set, namespaces are only allowed in modules (which means it has some form of import/export). An error occurs if a namespace is found in a non-module file:

ts

`namespace Instantiated {

Namespaces are not allowed in global script files when 'isolatedModules' is enabled. If this file is not intended to be a global script, set 'moduleDetection' to 'force' or add an empty 'export {}' statement.1280Namespaces are not allowed in global script files when 'isolatedModules' is enabled. If this file is not intended to be a global script, set 'moduleDetection' to 'force' or add an empty 'export {}' statement.

export const x = 1;

}

`Try

This restriction doesn’t apply to .d.ts files.

References to const enum members

In TypeScript, when you reference a const enum member, the reference is replaced by its actual value in the emitted JavaScript. Changing this TypeScript:

ts

`declare const enum Numbers {

Zero = 0,

One = 1,

}

console.log(Numbers.Zero + Numbers.One);

`Try

To this JavaScript:

ts

`"use strict";

console.log(0 + 1);

`Try

Without knowledge of the values of these members, other transpilers can’t replace the references to Numbers, which would be a runtime error if left alone (since there are no Numbers object at runtime). Because of this, when isolatedModules is set, it is an error to reference an ambient const enum member.

# Preserve Symlinks - preserveSymlinks

This is to reflect the same flag in Node.js; which does not resolve the real path of symlinks.

This flag also exhibits the opposite behavior to Webpack’s resolve.symlinks option (i.e. setting TypeScript’s preserveSymlinks to true parallels setting Webpack’s resolve.symlinks to false, and vice-versa).

With this enabled, references to modules and packages (e.g. imports and /// <reference type="..." /> directives) are all resolved relative to the location of the symbolic link file, rather than relative to the path that the symbolic link resolves to.

# Verbatim Module Syntax - verbatimModuleSyntax

By default, TypeScript does something called import elision. Basically, if you write something like

ts

`import { Car } from "./car";

export function drive(car: Car) {

// ...

}

`

TypeScript detects that you’re only using an import for types and drops the import entirely. Your output JavaScript might look something like this:

js

`export function drive(car) {

// ...

}

`

Most of the time this is good, because if Car isn’t a value that’s exported from ./car, we’ll get a runtime error.

But it does add a layer of complexity for certain edge cases. For example, notice there’s no statement like import "./car"; - the import was dropped entirely. That actually makes a difference for modules that have side-effects or not.

TypeScript’s emit strategy for JavaScript also has another few layers of complexity - import elision isn’t always just driven by how an import is used - it often consults how a value is declared as well. So it’s not always clear whether code like the following

ts

`export { Car } from "./car";

`

should be preserved or dropped. If Car is declared with something like a class, then it can be preserved in the resulting JavaScript file. But if Car is only declared as a type alias or interface, then the JavaScript file shouldn’t export Car at all.

While TypeScript might be able to make these emit decisions based on information from across files, not every compiler can.

The type modifier on imports and exports helps with these situations a bit. We can make it explicit whether an import or export is only being used for type analysis, and can be dropped entirely in JavaScript files by using the type modifier.

ts

`// This statement can be dropped entirely in JS output

import type * as car from "./car";

// The named import/export 'Car' can be dropped in JS output

import { type Car } from "./car";

export { type Car } from "./car";

`

type modifiers are not quite useful on their own - by default, module elision will still drop imports, and nothing forces you to make the distinction between type and plain imports and exports. So TypeScript has the flag --importsNotUsedAsValues to make sure you use the type modifier, --preserveValueImports to prevent some module elision behavior, and --isolatedModules to make sure that your TypeScript code works across different compilers. Unfortunately, understanding the fine details of those 3 flags is hard, and there are still some edge cases with unexpected behavior.

TypeScript 5.0 introduces a new option called --verbatimModuleSyntax to simplify the situation. The rules are much simpler - any imports or exports without a type modifier are left around. Anything that uses the type modifier is dropped entirely.

ts

`// Erased away entirely.

import type { A } from "a";

// Rewritten to 'import { b } from "bcd";'

import { b, type c, type d } from "bcd";

// Rewritten to 'import {} from "xyz";'

import { type xyz } from "xyz";

`

With this new option, what you see is what you get.

That does have some implications when it comes to module interop though. Under this flag, ECMAScript imports and exports won’t be rewritten to require calls when your settings or file extension implied a different module system. Instead, you’ll get an error. If you need to emit code that uses require and module.exports, you’ll have to use TypeScript’s module syntax that predates ES2015:

Input TypeScript Output JavaScript
tsimport foo = require("foo"); jsconst foo = require("foo");
tsfunction foo() {}function bar() {}function baz() {}export = { foo, bar, baz,}; jsfunction foo() {}function bar() {}function baz() {}module.exports = { foo, bar, baz,};

While this is a limitation, it does help make some issues more obvious. For example, it’s very common to forget to set the type field in package.json under --module node16. As a result, developers would start writing CommonJS modules instead of ES modules without realizing it, giving surprising lookup rules and JavaScript output. This new flag ensures that you’re intentional about the file type you’re using because the syntax is intentionally different.

Because --verbatimModuleSyntax provides a more consistent story than --importsNotUsedAsValues and --preserveValueImports, those two existing flags are being deprecated in its favor.

For more details, read up on the original pull request and its proposal issue.

#Backwards Compatibility

# Charset - charset

In prior versions of TypeScript, this controlled what encoding was used when reading text files from disk. Today, TypeScript assumes UTF-8 encoding, but will correctly detect UTF-16 (BE and LE) or UTF-8 BOMs.

# Imports Not Used As Values - importsNotUsedAsValues

Deprecated in favor of verbatimModuleSyntax.

This flag controls how import works, there are 3 different options:

This flag works because you can use import type to explicitly create an import statement which should never be emitted into JavaScript.

# Keyof Strings Only - keyofStringsOnly

This flag changes the keyof type operator to return string instead of string | number when applied to a type with a string index signature.

This flag is used to help people keep this behavior from before TypeScript 2.9’s release.

# No Implicit Use Strict - noImplicitUseStrict

You shouldn’t need this. By default, when emitting a module file to a non-ES6 target, TypeScript emits a "use strict"; prologue at the top of the file. This setting disables the prologue.

# No Strict Generic Checks - noStrictGenericChecks

TypeScript will unify type parameters when comparing two generic functions.

ts

`type A = <T, U>(x: T, y: U) => [T, U];

type B = (x: S, y: S) => [S, S];

function f(a: A, b: B) {

b = a; // Ok

a = b; // Error

Type 'B' is not assignable to type 'A'. Types of parameters 'y' and 'y' are incompatible. Type 'U' is not assignable to type 'T'. 'T' could be instantiated with an arbitrary type which could be unrelated to 'U'.2322Type 'B' is not assignable to type 'A'. Types of parameters 'y' and 'y' are incompatible. Type 'U' is not assignable to type 'T'. 'T' could be instantiated with an arbitrary type which could be unrelated to 'U'.

}

`Try

This flag can be used to remove that check.

# Out - out

Use outFile instead.

The out option computes the final file location in a way that is not predictable or consistent. This option is retained for backward compatibility only and is deprecated.

# Preserve Value Imports - preserveValueImports

Deprecated in favor of verbatimModuleSyntax.

There are some cases where TypeScript can’t detect that you’re using an import. For example, take the following code:

ts

`import { Animal } from "./animal.js";

eval("console.log(new Animal().isDangerous())");

`

or code using ‘Compiles to HTML’ languages like Svelte or Vue. preserveValueImports will prevent TypeScript from removing the import, even if it appears unused.

When combined with isolatedModules: imported types must be marked as type-only because compilers that process single files at a time have no way of knowing whether imports are values that appear unused, or a type that must be removed in order to avoid a runtime crash.

# Suppress Excess Property Errors - suppressExcessPropertyErrors

This disables reporting of excess property errors, such as the one shown in the following example:

ts

`type Point = { x: number; y: number };

const p: Point = { x: 1, y: 3, m: 10 };

Object literal may only specify known properties, and 'm' does not exist in type 'Point'.2353Object literal may only specify known properties, and 'm' does not exist in type 'Point'.`Try

This flag was added to help people migrate to the stricter checking of new object literals in TypeScript 1.6.

We don’t recommend using this flag in a modern codebase, you can suppress one-off cases where you need it using // @ts-ignore.

# Suppress Implicit Any Index Errors - suppressImplicitAnyIndexErrors

Turning suppressImplicitAnyIndexErrors on suppresses reporting the error about implicit anys when indexing into objects, as shown in the following example:

ts

`const obj = { x: 10 };

console.log(obj["foo"]);

Element implicitly has an 'any' type because expression of type '"foo"' can't be used to index type '{ x: number; }'. Property 'foo' does not exist on type '{ x: number; }'.7053Element implicitly has an 'any' type because expression of type '"foo"' can't be used to index type '{ x: number; }'. Property 'foo' does not exist on type '{ x: number; }'.`Try

Using suppressImplicitAnyIndexErrors is quite a drastic approach. It is recommended to use a @ts-ignore comment instead:

ts

`const obj = { x: 10 };

// @ts-ignore

console.log(obj["foo"]);

`Try

#Language and Environment

# Emit Decorator Metadata - emitDecoratorMetadata

Enables experimental support for emitting type metadata for decorators which works with the module reflect-metadata.

For example, here is the TypeScript

ts

`function LogMethod(

target: any,

propertyKey: string | symbol,

descriptor: PropertyDescriptor

) {

console.log(target);

console.log(propertyKey);

console.log(descriptor);

}

class Demo {

@LogMethod

public foo(bar: number) {

// do nothing

}

}

const demo = new Demo();

`Try

With emitDecoratorMetadata not set to true (default) the emitted JavaScript is:

ts

`"use strict";

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {

var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;

if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);

else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;

return c > 3 && r && Object.defineProperty(target, key, r), r;

};

function LogMethod(target, propertyKey, descriptor) {

console.log(target);

console.log(propertyKey);

console.log(descriptor);

}

class Demo {

foo(bar) {

    // do nothing

}

}

__decorate([

LogMethod

], Demo.prototype, "foo", null);

const demo = new Demo();

`Try

With emitDecoratorMetadata set to true the emitted JavaScript is:

ts

`"use strict";

var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {

var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;

if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);

else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;

return c > 3 && r && Object.defineProperty(target, key, r), r;

};

var __metadata = (this && this.__metadata) || function (k, v) {

if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);

};

function LogMethod(target, propertyKey, descriptor) {

console.log(target);

console.log(propertyKey);

console.log(descriptor);

}

class Demo {

foo(bar) {

    // do nothing

}

}

__decorate([

LogMethod,

__metadata("design:type", Function),

__metadata("design:paramtypes", [Number]),

__metadata("design:returntype", void 0)

], Demo.prototype, "foo", null);

const demo = new Demo();

`Try

# Experimental Decorators - experimentalDecorators

Enables experimental support for decorators, which is a version of decorators that predates the TC39 standardization process.

Decorators are a language feature which hasn’t yet been fully ratified into the JavaScript specification. This means that the implementation version in TypeScript may differ from the implementation in JavaScript when it it decided by TC39.

You can find out more about decorator support in TypeScript in the handbook.

# JSX - jsx

Controls how JSX constructs are emitted in JavaScript files. This only affects output of JS files that started in .tsx files.

For example

This sample code:

tsx

`export const HelloWorld = () =>

Hello world

;

`

React: "react-jsx"[1]

tsx

`import { jsx as _jsx } from "react/jsx-runtime";

export const HelloWorld = () => _jsx("h1", { children: "Hello world" });

`Try

React dev transform: "react-jsxdev"[1]

tsx

`import { jsxDEV as _jsxDEV } from "react/jsx-dev-runtime";

const _jsxFileName = "/home/runner/work/TypeScript-Website/TypeScript-Website/packages/typescriptlang-org/index.tsx";

export const HelloWorld = () => _jsxDEV("h1", { children: "Hello world" }, void 0, false, { fileName: _jsxFileName, lineNumber: 9, columnNumber: 32 }, this);

`Try

Preserve: "preserve"

tsx

`import React from 'react';

export const HelloWorld = () =>

Hello world

;

`Try

React Native: "react-native"

tsx

`import React from 'react';

export const HelloWorld = () =>

Hello world

;

`Try

Legacy React runtime: "react"

tsx

`import React from 'react';

export const HelloWorld = () => React.createElement("h1", null, "Hello world");

`Try

This option can be used on a per-file basis too using an @jsxRuntime comment.

Always use the classic runtime ("react") for this file:

tsx

`/* @jsxRuntime classic */

export const HelloWorld = () =>

Hello world

;

`

Always use the automatic runtime ("react-jsx") for this file:

tsx

`/* @jsxRuntime automatic */

export const HelloWorld = () =>

Hello world

;

`

# JSX Factory - jsxFactory

Changes the function called in .js files when compiling JSX Elements using the classic JSX runtime. The most common change is to use "h" or "preact.h" instead of the default "React.createElement" if using preact.

For example, this TSX file:

tsx

`import { h } from "preact";

const HelloWorld = () =>

Hello
;

`

With jsxFactory: "h" looks like:

tsx

`const preact_1 = require("preact");

const HelloWorld = () => (0, preact_1.h)("div", null, "Hello");

`Try

This option can be used on a per-file basis too similar to Babel’s /** @jsx h */ directive.

tsx

`/** @jsx h */

import { h } from "preact";

Cannot find module 'preact' or its corresponding type declarations.2307Cannot find module 'preact' or its corresponding type declarations.

const HelloWorld = () =>

Hello
;

`Try

The factory chosen will also affect where the JSX namespace is looked up (for type checking information) before falling back to the global one.

If the factory is defined as React.createElement (the default), the compiler will check for React.JSX before checking for a global JSX. If the factory is defined as h, it will check for h.JSX before a global JSX.

# JSX Fragment Factory - jsxFragmentFactory

Specify the JSX fragment factory function to use when targeting react JSX emit with jsxFactory compiler option is specified, e.g. Fragment.

For example with this TSConfig:

`{

"": {

"": "esnext",

"": "commonjs",

"": "react",

"": "h",

"": "Fragment"

}

}

`

This TSX file:

tsx

`import { h, Fragment } from "preact";

const HelloWorld = () => (

<>

<div>Hello</div>

</>

);

`

Would look like:

tsx

`const preact_1 = require("preact");

const HelloWorld = () => ((0, preact_1.h)(preact_1.Fragment, null,

(0, preact_1.h)("div", null, "Hello")));

`Try

This option can be used on a per-file basis too similar to Babel’s /* @jsxFrag h */ directive.

For example:

tsx

`/** @jsx h */

/** @jsxFrag Fragment */

import { h, Fragment } from "preact";

Cannot find module 'preact' or its corresponding type declarations.2307Cannot find module 'preact' or its corresponding type declarations.

const HelloWorld = () => (

<>

<div>Hello</div>

</>

);

`Try

# JSX Import Source - jsxImportSource

Declares the module specifier to be used for importing the jsx and jsxs factory functions when using jsx as "react-jsx" or "react-jsxdev" which were introduced in TypeScript 4.1.

With React 17 the library supports a new form of JSX transformation via a separate import.

For example with this code:

tsx

`import React from "react";

function App() {

return

Hello World

;

}

`

Using this TSConfig:

`{

"": {

"": "esnext",

"": "commonjs",

"": "react-jsx"

}

}

`

The emitted JavaScript from TypeScript is:

tsx

`"use strict";

Object.defineProperty(exports, "__esModule", { value: true });

const jsx_runtime_1 = require("react/jsx-runtime");

function App() {

return (0, jsx_runtime_1.jsx)("h1", { children: "Hello World" });

}

`Try

For example if you wanted to use "jsxImportSource": "preact", you need a tsconfig like:

`{

"": {

"": "esnext",

"": "commonjs",

"": "react-jsx",

"": "preact",

"": ["preact"]

}

}

`

Which generates code like:

tsx

`function App() {

return (0, jsx_runtime_1.jsx)("h1", { children: "Hello World" });

}

`Try

Alternatively, you can use a per-file pragma to set this option, for example:

tsx

`/** @jsxImportSource preact */

export function App() {

return

Hello World

;

}

`

Would add preact/jsx-runtime as an import for the _jsx factory.

Note: In order for this to work like you would expect, your tsx file must include an export or import so that it is considered a module.

# Lib - lib

TypeScript includes a default set of type definitions for built-in JS APIs (like Math), as well as type definitions for things found in browser environments (like document). TypeScript also includes APIs for newer JS features matching the target you specify; for example the definition for Map is available if target is ES6 or newer.

You may want to change these for a few reasons:

In TypeScript 4.5, lib files can be overridden by npm modules, find out more in the blog.

High Level libraries

Name Contents
ES5 Core definitions for all ES5 functionality
ES2015 Additional APIs available in ES2015 (also known as ES6) - array.find, Promise, Proxy, Symbol, Map, Set, Reflect, etc.
ES6 Alias for “ES2015”
ES2016 Additional APIs available in ES2016 - array.include, etc.
ES7 Alias for “ES2016”
ES2017 Additional APIs available in ES2017 - Object.entries, Object.values, Atomics, SharedArrayBuffer, date.formatToParts, typed arrays, etc.
ES2018 Additional APIs available in ES2018 - async iterables, promise.finally, Intl.PluralRules, regexp.groups, etc.
ES2019 Additional APIs available in ES2019 - array.flat, array.flatMap, Object.fromEntries, string.trimStart, string.trimEnd, etc.
ES2020 Additional APIs available in ES2020 - string.matchAll, etc.
ES2021 Additional APIs available in ES2021 - promise.any, string.replaceAll etc.
ES2022 Additional APIs available in ES2022 - array.at, RegExp.hasIndices, etc.
ES2023 Additional APIs available in ES2023 - array.with, array.findLast, array.findLastIndex, array.toSorted, array.toReversed, etc.
ESNext Additional APIs available in ESNext - This changes as the JavaScript specification evolves
DOM DOM definitions - window, document, etc.
WebWorker APIs available in WebWorker contexts
ScriptHost APIs for the Windows Script Hosting System

Individual library components

Name
DOM.Iterable
ES2015.Core
ES2015.Collection
ES2015.Generator
ES2015.Iterable
ES2015.Promise
ES2015.Proxy
ES2015.Reflect
ES2015.Symbol
ES2015.Symbol.WellKnown
ES2016.Array.Include
ES2017.object
ES2017.Intl
ES2017.SharedMemory
ES2017.String
ES2017.TypedArrays
ES2018.Intl
ES2018.Promise
ES2018.RegExp
ES2019.Array
ES2019.Object
ES2019.String
ES2019.Symbol
ES2020.String
ES2020.Symbol.wellknown
ES2021.Promise
ES2021.String
ES2021.WeakRef
ESNext.AsyncIterable
ESNext.Array
ESNext.Intl
ESNext.Symbol

This list may be out of date, you can see the full list in the TypeScript source code.

# Lib Replacement - libReplacement

TypeScript 4.5 introduced the possibility of substituting the default lib files with custom ones. All built-in library files would first try to be resolved from packages named @typescript/lib-*. For example, you could lock your dom libraries onto a specific version of the @types/web package with the following package.json:

json

`{

"devDependencies": {

"@typescript/lib-dom": "npm:@types/web@0.0.199"

}

}

`

When installed, a package called @typescript/lib-dom should exist, and TypeScript would always look there when searching for lib.dom.d.ts.

The --libReplacement flag allows you to disable this behavior. If you’re not using any @typescript/lib-* packages, you can now disable those package lookups with --libReplacement false. In the future, --libReplacement false may become the default, so if you currently rely on the behavior you should consider explicitly enabling it with --libReplacement true.

# Module Detection - moduleDetection

This setting controls how TypeScript determines whether a file is ascript or a module.

There are three choices:

# No Lib - noLib

Disables the automatic inclusion of any library files. If this option is set, lib is ignored.

TypeScript cannot compile anything without a set of interfaces for key primitives like: Array, Boolean, Function, IArguments, Number, Object, RegExp, and String. It is expected that if you use noLib you will be including your own type definitions for these.

# React Namespace - reactNamespace

Use jsxFactory instead. Specify the object invoked for createElement when targeting react for TSX files.

# Target - target

Modern browsers support all ES6 features, so ES6 is a good choice. You might choose to set a lower target if your code is deployed to older environments, or a higher target if your code is guaranteed to run in newer environments.

The target setting changes which JS features are downleveled and which are left intact. For example, an arrow function () => this will be turned into an equivalent function expression if target is ES5 or lower.

Changing target also changes the default value of lib. You may “mix and match” target and lib settings as desired, but you could just set target for convenience.

For developer platforms like Node there are baselines for the target, depending on the type of platform and its version. You can find a set of community organized TSConfigs at tsconfig/bases, which has configurations for common platforms and their versions.

The special ESNext value refers to the highest version your version of TypeScript supports. This setting should be used with caution, since it doesn’t mean the same thing between different TypeScript versions and can make upgrades less predictable.

# Use Define For Class Fields - useDefineForClassFields

This flag is used as part of migrating to the upcoming standard version of class fields. TypeScript introduced class fields many years before it was ratified in TC39. The latest version of the upcoming specification has a different runtime behavior to TypeScript’s implementation but the same syntax.

This flag switches to the upcoming ECMA runtime behavior.

You can read more about the transition in the 3.7 release notes.

#Compiler Diagnostics

# Diagnostics - diagnostics

Used to output diagnostic information for debugging. This command is a subset of extendedDiagnostics which are more user-facing results, and easier to interpret.

If you have been asked by a TypeScript compiler engineer to give the results using this flag in a compile, in which there is no harm in using extendedDiagnostics instead.

# Explain Files - explainFiles

Print names of files which TypeScript sees as a part of your project and the reason they are part of the compilation.

For example, with this project of just a single index.ts file

sh

`example

├── index.ts

├── package.json

└── tsconfig.json

`

Using a tsconfig.json which has explainFiles set to true:

json

`{

"compilerOptions": {

"target": "es5",

"module": "commonjs",

"explainFiles": true

}

}

`

Running TypeScript against this folder would have output like this:

`❯ tsc

node_modules/typescript/lib/lib.d.ts

Default library for target 'es5'

node_modules/typescript/lib/lib.es5.d.ts

Library referenced via 'es5' from file 'node_modules/typescript/lib/lib.d.ts'

node_modules/typescript/lib/lib.dom.d.ts

Library referenced via 'dom' from file 'node_modules/typescript/lib/lib.d.ts'

node_modules/typescript/lib/lib.webworker.importscripts.d.ts

Library referenced via 'webworker.importscripts' from

file 'node_modules/typescript/lib/lib.d.ts'

node_modules/typescript/lib/lib.scripthost.d.ts

Library referenced via 'scripthost'

from file 'node_modules/typescript/lib/lib.d.ts'

index.ts

Matched by include pattern '**/*' in 'tsconfig.json'

`

The output above show:

This option is intended for debugging how a file has become a part of your compile.

# Extended Diagnostics - extendedDiagnostics

You can use this flag to discover where TypeScript is spending its time when compiling. This is a tool used for understanding the performance characteristics of your codebase overall.

You can learn more about how to measure and understand the output in the performance section of the wiki.

# Generate CPU Profile - generateCpuProfile

This option gives you the chance to have TypeScript emit a v8 CPU profile during the compiler run. The CPU profile can provide insight into why your builds may be slow.

This option can only be used from the CLI via: --generateCpuProfile tsc-output.cpuprofile.

sh

`npm run tsc --generateCpuProfile tsc-output.cpuprofile

`

This file can be opened in a chromium based browser like Chrome or Edge Developer in the CPU profiler section. You can learn more about understanding the compilers performance in the TypeScript wiki section on performance.

# generateTrace - generateTrace

Generates an event trace and a list of types.

# List Emitted Files - listEmittedFiles

Print names of generated files part of the compilation to the terminal.

This flag is useful in two cases:

For example:

`example

├── index.ts

├── package.json

└── tsconfig.json

`

With:

`{

"": {

"": true,

"": true

}

}

`

Would echo paths like:

`$ npm run tsc

path/to/example/index.js

path/to/example/index.d.ts

`

Normally, TypeScript would return silently on success.

# List Files - listFiles

Print names of files part of the compilation. This is useful when you are not sure that TypeScript has included a file you expected.

For example:

`example

├── index.ts

├── package.json

└── tsconfig.json

`

With:

Would echo paths like:

`$ npm run tsc

path/to/example/node_modules/typescript/lib/lib.d.ts

path/to/example/node_modules/typescript/lib/lib.es5.d.ts

path/to/example/node_modules/typescript/lib/lib.dom.d.ts

path/to/example/node_modules/typescript/lib/lib.webworker.importscripts.d.ts

path/to/example/node_modules/typescript/lib/lib.scripthost.d.ts

path/to/example/index.ts

`

Note if using TypeScript 4.2, prefer explainFiles which offers an explanation of why a file was added too.

# noCheck - noCheck

Disable full type checking (only critical parse and emit errors will be reported).

# Trace Resolution - traceResolution

When you are trying to debug why a module isn’t being included. You can set traceResolution to true to have TypeScript print information about its resolution process for each processed file.

#Projects

# Composite - composite

The composite option enforces certain constraints which make it possible for build tools (including TypeScript itself, under --build mode) to quickly determine if a project has been built yet.

When this setting is on:

You can find documentation on TypeScript projects in the handbook.

# Disable Referenced Project Load - disableReferencedProjectLoad

In multi-project TypeScript programs, TypeScript will load all of the available projects into memory in order to provide accurate results for editor responses which require a full knowledge graph like ‘Find All References’.

If your project is large, you can use the flag disableReferencedProjectLoad to disable the automatic loading of all projects. Instead, projects are loaded dynamically as you open files through your editor.

# Disable Solution Searching - disableSolutionSearching

When working with composite TypeScript projects, this option provides a way to declare that you do not want a project to be included when using features like find all references or jump to definition in an editor.

This flag is something you can use to increase responsiveness in large composite projects.

# Disable Source Project Reference Redirect - disableSourceOfProjectReferenceRedirect

When working with composite TypeScript projects, this option provides a way to go back to the pre-3.7 behavior where d.ts files were used to as the boundaries between modules. In 3.7 the source of truth is now your TypeScript files.

# Incremental - incremental

Tells TypeScript to save information about the project graph from the last compilation to files stored on disk. This creates a series of .tsbuildinfo files in the same folder as your compilation output. They are not used by your JavaScript at runtime and can be safely deleted. You can read more about the flag in the 3.4 release notes.

To control which folders you want to the files to be built to, use the config option tsBuildInfoFile.

# TS Build Info File - tsBuildInfoFile

This setting lets you specify a file for storing incremental compilation information as a part of composite projects which enables faster building of larger TypeScript codebases. You can read more about composite projects in the handbook.

The default depends on a combination of other settings:

#Output Formatting

# No Error Truncation - noErrorTruncation

Do not truncate error messages.

With false, the default.

ts

`var x: {

propertyWithAnExceedinglyLongName1: string;

propertyWithAnExceedinglyLongName2: string;

propertyWithAnExceedinglyLongName3: string;

propertyWithAnExceedinglyLongName4: string;

propertyWithAnExceedinglyLongName5: string;

propertyWithAnExceedinglyLongName6: string;

propertyWithAnExceedinglyLongName7: string;

propertyWithAnExceedinglyLongName8: string;

};

// String representation of type of 'x' should be truncated in error message

var s: string = x;

Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; propertyWithAnExceedinglyLongName6: string; propertyWithAnExceedinglyLongName7: string; propert...' is not assignable to type 'string'.
Variable 'x' is used before being assigned.2322
2454Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; propertyWithAnExceedinglyLongName6: string; propertyWithAnExceedinglyLongName7: string; propert...' is not assignable to type 'string'.
Variable 'x' is used before being assigned.`Try

With true

ts

`var x: {

propertyWithAnExceedinglyLongName1: string;

propertyWithAnExceedinglyLongName2: string;

propertyWithAnExceedinglyLongName3: string;

propertyWithAnExceedinglyLongName4: string;

propertyWithAnExceedinglyLongName5: string;

propertyWithAnExceedinglyLongName6: string;

propertyWithAnExceedinglyLongName7: string;

propertyWithAnExceedinglyLongName8: string;

};

// String representation of type of 'x' should be truncated in error message

var s: string = x;

Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; propertyWithAnExceedinglyLongName6: string; propertyWithAnExceedinglyLongName7: string; propertyWithAnExceedinglyLongName8: string; }' is not assignable to type 'string'.
Variable 'x' is used before being assigned.2322
2454Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; propertyWithAnExceedinglyLongName6: string; propertyWithAnExceedinglyLongName7: string; propertyWithAnExceedinglyLongName8: string; }' is not assignable to type 'string'.
Variable 'x' is used before being assigned.`Try

# Preserve Watch Output - preserveWatchOutput

Whether to keep outdated console output in watch mode instead of clearing the screen every time a change happened.

# Pretty - pretty

Stylize errors and messages using color and context, this is on by default — offers you a chance to have less terse, single colored messages from the compiler.

#Completeness

# Skip Default Lib Check - skipDefaultLibCheck

Use skipLibCheck instead. Skip type checking of default library declaration files.

# Skip Lib Check - skipLibCheck

Skip type checking of declaration files.

This can save time during compilation at the expense of type-system accuracy. For example, two libraries could define two copies of the same type in an inconsistent way. Rather than doing a full check of all d.ts files, TypeScript will type check the code you specifically refer to in your app’s source code.

A common case where you might think to use skipLibCheck is when there are two copies of a library’s types in your node_modules. In these cases, you should consider using a feature like yarn’s resolutionsto ensure there is only one copy of that dependency in your tree or investigate how to ensure there is only one copy by understanding the dependency resolution to fix the issue without additional tooling.

Another possibility is when you are migrating between TypeScript releases and the changes cause breakages in node_modules and the JS standard libraries which you do not want to deal with during the TypeScript update.

Note, that if these issues come from the TypeScript standard library you can replace the library using TypeScript 4.5’s lib replacement technique.

#Command Line

#Watch Options

TypeScript 3.8 shipped a new strategy for watching directories, which is crucial for efficiently picking up changes to node_modules.

On operating systems like Linux, TypeScript installs directory watchers (as opposed to file watchers) on node_modules and many of its subdirectories to detect changes in dependencies. This is because the number of available file watchers is often eclipsed by the number of files in node_modules, whereas there are way fewer directories to track.

Because every project might work better under different strategies, and this new approach might not work well for your workflows, TypeScript 3.8 introduces a new watchOptions field which allows users to tell the compiler/language service which watching strategies should be used to keep track of files and directories.

# Assume Changes Only Affect Direct Dependencies - assumeChangesOnlyAffectDirectDependencies

When this option is enabled, TypeScript will avoid rechecking/rebuilding all truly possibly-affected files, and only recheck/rebuild files that have changed as well as files that directly import them.

This can be considered a ‘fast & loose’ implementation of the watching algorithm, which can drastically reduce incremental rebuild times at the expense of having to run the full build occasionally to get all compiler error messages.