Readonly type breaks Set and Map

[falsandtru]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

Expected behavior:

However, Set and Map can't use with es3 and es5. We need any workaround.

export type Readonly<T> =
  T extends ReadonlySet<infer V> ? ReadonlySet<V> :
  T extends ReadonlyMap<infer K, infer V> ? ReadonlyMap<K, V> :
  { readonly [P in keyof T]: T[P]; };

Actual behavior:

type Readonly<T> = {
    readonly [P in keyof T]: T[P];
};

Playground Link:

Related Issues:

[ahejlsberg]

I'm not quite sure what you're trying to accomplish, but I think you can write it this way:

type ReadonlyMapped<T> = { readonly [P in keyof T]: T[P] };
type Readonly<T> =
  T extends Array<any> ? ReadonlyMapped<T> :
  T extends ReadonlyArray<any> | ReadonlySet<any> | ReadonlyMap<any, any> ? T :
  ReadonlyMapped<T>;

The extra indirection causes us to properly recognize that the mapped type is homomorphic.

[falsandtru]

I mean Readonly<T[]> should return ReadonlyArray<T> and should keep Set and Map types.

type a = Readonly<T[]>; // returns readonly T[] correctly
type ra = Readonly<ReadonlyArray<T>>; // returns readonly T[] correctly
type s = Readonly<Set<T>>; // returns mapped type, should return ReadonlySet<T>
type rs = Readonly<ReadonlySet<T>>; // returns mapped type, should return ReadonlySet<T>
type m = Readonly<Map<T, T>>; // returns mapped type, should return ReadonlyMap<T, U>
type rm = Readonly<ReadonlyMap<T, T>>; // returns mapped type, should return ReadonlyMap<T, U>

[falsandtru]

Sorry, nightly version works correctly with both Array and ReadonlyArray types. Now Readonly type isn't good only for Set and Map types.