typedef downleveling to use with typesVersions

[mikehaas763]

Search Terms

typedef
downleveling
typesVersions

Suggestion

@RyanCavanaugh mentioned in #22605 (comment) regarding the typesVersion feature for typedef files that he "would expect these files to be either hand-created, or generated through some downleveling process". I think it'd be a great addition if the compiler would optionally emit the typedef files for any versions specified. I really don't want to manage my typedef files by hand. 🙂

Use Cases

Examples

For example, using tsc --build --downlevel-version 2.8 with typescript 3.1 would emit something like this dir structure

index.d.ts
ts3.0/index.d.ts
ts2.9/index.d.ts
ts2.8/index.d.ts

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.

[cordiosd]

I personally would like this change to be practically transparent and instantly applied. I had not found all of these suggestions when I suggested the following: #31907

[RyanCavanaugh]

We've created downlevel-dts, which is as far as we intend to go in this space.