Feature Request: expose getPromisedTypeOfPromise #30774

Search Terms

• getPromisedTypeOfPromise
• detect Promise

Suggestion

expose getPromisedTypeOfPromise in the TypeChecker API.

Use Cases

Writing lint rules with typechecking involving promises. One example is one checking the result of a yield in a generator. Since yields inside generators are not typed, being able to use linting to check types are properly annotated can be powerful.

Examples

// source code
const myAsyncFunc = async (num: number) => num;

function* gen() {
   const y = yield call(myFunc);
}

call is a redux saga effect that has an API contract of calling a passed function and awaiting if it is a promise. Given this contract, its possible to write a lint rule instructing a consumer to annotate y with : number.

// lint rule
...
let returnType = appliedFunctionType.getCallSignatures()[0].getReturnType();

if(// is promise) {
  returnType = typeChecker.getPromisedTypeOfPromise(returnType);
}

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.