`readonly` mapped type modifiers for `Map` / `ReadonlyMap`

Search Terms

ReadonlyMap
ReadWrite
Writable

Suggestion

3.4.0-rc introduced the ability to use readonly mapped type modifiers with arrays.

(https://devblogs.microsoft.com/typescript/announcing-typescript-3-4-rc/)

It would be useful if the same syntax worked on

Use Cases

This is useful in any library dealing with immutable versions of types. It would allow mapped types to easily convert between Map and ReadonlyMap.

Examples

type ReadWrite<T> = { -readonly [P in keyof T]: T[P] };
const map: ReadWrite<ReadonlyMap<string, string>> = new Map();

map.put('a', 'c'); // this should work, since readonly has been removed.

type Readonly<T> = { readonly [P in keyof T]: T[P] };
const map: Readonly<Map<string, string>> = new Map();

map.put('a', 'c'); // this should not work, since readonly has been applied

Similar examples using Array instead of Map didn't work before 3.4 and did work afterwards.

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.

首先重现 issue 中的 ReadWrite 和 Readonly 映射类型示例，并将它们的行为与 TypeScript 3.4 中引入的 Array 行为进行比较。跟踪编译器对映射类型的处理，以确定所需的设计；当 Map/ReadonlyMap 示例按指定方式运行且不改变生成的 JavaScript 时，即表示完成。