microsoft / microsoft/TypeScript

Promise constructor's return type doesn't unwrap nested promises

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#52,764 1 comment 3 reactions 1 assignee View on GitHub

@rbuckton is already working on this.

Since Feb 16, 2023.

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Description

Bug Report

🔎 Search Terms

promise constructor

🕗 Version & Regression Information
  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about Promises
⏯ Playground Link

Playground link with relevant code

💻 Code
const nestedPromise = (function() {
    return new Promise<Promise<number>>(resolve1 => {
        
        resolve1(new Promise<number>(resolve2 => {
            resolve2(5);
        }));

    })
})();

// according to TS, `result` should be a Promise<number>, but at runtime, it's actually just a number
nestedPromise.then(result => console.log('first', result, typeof result));
//                 ^?


// when using Promise.resolve(), the type is correct
const nestedPromiseResolve = (function() { return Promise.resolve(Promise.resolve(Promise.resolve(5)))})();
nestedPromiseResolve.then(result => console.log('second', result, typeof result));
//                        ^?



🙁 Actual behavior

Return type of the promise constructor doesn't unwrap nested promises

🙂 Expected behavior

The return type of the promise constructor should match the runtime behavior

The current constructor type is { new <T>(executor: (resolve: (value: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void): Promise<T>; }

Shouldn't the return type actually be Promise<Awaited<T>> like it is for Promise.resolve()? Is there something I'm missing here?

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