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Description
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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 codeThis wouldn't change the runtime behavior of existing JavaScript codeThis could be implemented without emitting different JS based on the types of the expressionsThis 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.
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JakeTunaley commentedon Mar 28, 2019
This would actually be a breaking change if someone is using a type like
Readonly<Map<Foo, Bar>>
, because right now methods likeMap.prototype.put
exist on that type, but after this change they would not.Also, instead of just adding a special case for
Map
, why not do this in general?That would be incredibly useful, though it would benefit from a new error checking that readonly methods don't mutate class state.
bjchambers commentedon Mar 28, 2019
Hmm... you are correct that it has the potential to break existing code. However, it seems like the same argument would apply to the following change, which made similar changes to
ReadonlyArray
:#29435