Promise rejection type.

Search Terms

Promise Reject Type

Suggestion

Add ability to type Promise rejections.

// Current Promise Constructor Implementation:
new <T>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void): Promise<T>;

// Proposed Change:
new <T, E = any>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: E) => void) => void): Promise<T, E>;

Use Cases

When handling promise rejections, the type any isn't very useful. I would be useful to have the rejection have an actual type without the need to cast or type guard.

Examples

Promise.reject<never, string>('hello world')
  .catch(reason => {
    console.error(reason.length); // `reason` is of type string.
  });

class MyError extends Error {
  // ...
}

Promise.reject<never, MyError>(new MyError(/* ... */))
  .catch(reason => {
    // `reason` is of type MyError.
    const info = reason.getMoreInfo();
    // ...
  });

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.

Beginne mit den Beispielen für den Promise constructor und Promise.reject im Issue und prüfe anschließend die umgebende Diskussion auf Designbeschränkungen und noch ungeklärte Einwände. Die Arbeit ist erst abgeschlossen, wenn sich das Team auf ein Design für den Ablehnungstyp und dessen Verhalten bei der Konstruktion, Ablehnung und catch-Behandlung geeinigt hat.