Support Const Type Constraint · Issue #41114 · microsoft/TypeScript (original) (raw)

Search Terms

Const Type Constraint Literal

Suggestion

Add new syntax const T or a global generic type Constant<T = any> = T extends const T ? T : never;

type ConstString = const string is same as type ConstString = Constant<string>.

• If T extends string | number | boolean | symbol | bigint | null | undefined, requires T to be a constant.
• If T is object or Array, requires every member of T to be a constant.
• If T is enum, nothing to do, because enum is same as const enum

The difference with union literal is that type 'foo' | 'bar' can only be 'foo' or 'bar', but type const string can be any constant string.

Use Cases

• Sometimes we want to constrain a type or a parameter to be literal.
• It can help the TS compiler to recognize constant types better

Examples

let foo = 'foo'; // string const bar= 'bar'; // 'bar'

const notConstant: const string = foo; // error const trulyConstant: const string = bar; // success

type Baz = { baz: string } ; const baz1: const Baz = { baz: 'baz' }; // success const baz2: Baz = { baz: 'baz' }; // not a constant const baz3: Baz = { baz: 'baz' } as const; // current available, same as baz1

type ConstArray = const Array; // same as type AnotherCosntArray = Array;

function tuple(...args: T) { return args; } const t1 = tuple(foo); // error! const t2 = tuple(bar); // success! typeof t2 should be ['bar'] const t3 = tuple('a', 1, true, ['1'], baz1) // ['a', 1, true, ['1'], { baz: 'baz' }]; const t4 = tuple(baz2) // error!

let variable: const string = 'variable'; // this is possible variable = 'another variable'; // success variable = document.title; // error

let reactiveObject: const { foo: string } = { foo: 'foo' }; // success; reactiveObject.foo = 'bar' // success

In fact, const only affects the assignment behavior, and a const type is considered to be the same as a non const type when read it.
A const type doesn't mean immutable, just means every primitive value should be literal, so maybe should call it literal type.

Alternatives

• Only add extends const syntax, and add global generic type Constant<T = any> = T extends const ? T : never;
• Use finally or static or literal(not exist in current keywords) keyword instead of const
• Use type Literal instead of type Constant.

Related issues

#30680 Suggestion: Const contexts for generic type inference

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.