Readonly properties and index signatures by ahejlsberg · Pull Request #6532 · microsoft/TypeScript (original) (raw)

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This PR adds support for readonly properties and index signatures as suggested in #12. Changes include:

A new readonly modifier that can applied to property and index signature declarations.
A property or index signature declared with the readonly modifier is considered read-only.
Read-only properties may have initializers and may be assigned to in constructors within the same class declaration, but otherwise assignments to read-only properties are disallowed.
Assignments to read-only index signatures are always disallowed.

Entities are implicitly read-only in several situations:

A property declared with a get accessor and no set accessor is considered read-only.
In the type of an enum object, enum members are considered read-only properties.
In the type of a module object, exported const variables are considered read-only properties.
An entity declared in an import statement is considered read-only.
An entity accessed through an ES2015 namespace import is considered read-only (e.g. foo.x is read-only when foo is declared as import * as foo from "foo").

Some examples:

interface Point { readonly x: number; readonly y: number; }

var p1: Point = { x: 10, y: 20 }; p1.x = 5; // Error, p1.x is read-only

var p2 = { x: 1, y: 1 }; var p3: Point = p2; // Ok, read-only alias for p2 p3.x = 5; // Error, p3.x is read-only p2.x = 5; // Ok, but also changes p3.x because of aliasing

class Foo { readonly a = 1; readonly b: string; constructor() { this.b = "hello"; // Assignment permitted in constructor } }

// Read-only view of Array with all mutating methods removed interface ReadonlyArray { readonly [x: number]: T; readonly length: number; toString(): string; toLocaleString(): string; concat<U extends ReadonlyArray>(...items: U[]): T[]; concat(...items: T[]): T[]; join(separator?: string): string; slice(start?: number, end?: number): T[]; indexOf(searchElement: T, fromIndex?: number): number; lastIndexOf(searchElement: T, fromIndex?: number): number; every(callbackfn: (value: T, index: number) => boolean, thisArg?: any): boolean; some(callbackfn: (value: T, index: number) => boolean, thisArg?: any): boolean; forEach(callbackfn: (value: T, index: number) => void, thisArg?: any): void; map(callbackfn: (value: T, index: number) => U, thisArg?: any): U[]; filter(callbackfn: (value: T, index: number) => boolean, thisArg?: any): T[]; reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number) => T, initialValue?: T): T; reduce(callbackfn: (previousValue: U, currentValue: T, currentIndex: number) => U, initialValue: U): U; reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number) => T, initialValue?: T): T; reduceRight(callbackfn: (previousValue: U, currentValue: T, currentIndex: number) => U, initialValue: U): U; }

let a: Array = [0, 1, 2, 3, 4]; let b: ReadonlyArray = a; b[5] = 5; // Error, elements are read-only b.push(5); // Error, no push method (because it mutates array) b.length = 3; // Error, length is read-only a = b; // Error, mutating methods are missing

Note that the readonly modifier does not affect subtype and assignability relationships of the containing type for the reasons described here. Also note that this PR specifically doesn't attempt to formalize immutable objects. However, with the readonly modifier it becomes a lot more convenient to implement such objects.

The PR still needs tests, but I thought I'd get it up there for folks to take a look at.

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Conflicts: src/compiler/parser.ts src/compiler/types.ts

JsonFreeman