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Feature Request: expose getPromisedTypeOfPromise #30774

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@maclockard

Description

@maclockard

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.

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APIRelates to the public API for TypeScriptNeeds InvestigationThis issue needs a team member to investigate its status.

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