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javascripttypescriptecmascript-6promisees6-promise

Type definition of Promise.all doesn't match with the docs

Looking at the type definition of Promise.all, I see 10 definitions:

/**
 * Creates a Promise that is resolved with an array of results when all of the provided Promises
 * resolve, or rejected when any Promise is rejected.
 * @param values An array of Promises.
 * @returns A new Promise.
 */
all<T1, T2, T3>(values: [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>]): Promise<[T1, T2, T3]>;

I only included the one with an array of length 3, but there also exist all<T1>, all<T1, T2>, all the way up to all<T1, T2, ..., T9, T10>.

However, this doesn't match with the implementation of Promise.all, which can take as an input an array longer than 10:

let myPromise = num => Promise.resolve(num);
let myPromisesArray = (new Array(20))
  .fill()
  .map((_,i) => myPromise(i));
Promise.all(myPromisesArray).then(console.log)

I'm not the worst developer in the world but still, I make the assumption that the Microsoft developers who produced the type definition of ES2015 know more about JS/TS than me, which begs the question:

Why the type definition of Promise.all doesn't match with neither its documentation nor its implementation?

Solution

  • The generic declaration up to 10 types is only accounting for non-uniform "tuples", or arrays where different indices have explicit independent types.

    There is also a declaration that supports a uniformly typed array of unbounded length:

    all<T>(values: (T | PromiseLike<T>)[]): Promise<T[]>;

    The declarations using a tuple of up to length 10 are supposed to cover a reasonable amount of use-cases, and are due to a limitation of TypeScript as a language, since generic type inference for a tuple is a uniform array of an encompassing union type, which would use the signature above. These sort of generic declarations up to an arbitrary bound of non-homogeneous types are common-place in other langauges like C# as well. For example, Action<> and Func<> use a limit of 16 parameter types.