Closed
Description
TypeScript Version:
3.4.3
3.4.5
3.5.0-dev.20190430
Search Terms:
Promise.all, array, tuple, as const
Code
// requires `--lib es2017` to run
const p1 = Promise.resolve(1);
const p2 = Promise.resolve('two');
const p3 = Promise.resolve(true);
(async () => {
// works fine
// let [v1, v2, v3] = await Promise.all([p1, p2, p3]);
// also fine
// const promises = [p1, p2, p3] as [typeof p1, typeof p2, typeof p3];
// not fine
const promises = [p1, p2, p3] as const;
let [v1, v2, v3] = await Promise.all(promises);
// quick type test
v1.toExponential();
v2.toUpperCase();
let b: boolean = v3;
})();Expected behavior:
Promise.all accepts a tuple of Promise objects. Code compiles.
Actual behavior:
An array of promises defined using as const can't be passed to Promise.all. With the latest ts build, I get the following compilation error:
bug.ts:15:42 - error TS2345: Argument of type 'readonly [Promise<number>, Promise<string>, Promise<boolean>]' is not assignable to parameter of type 'Iterable<number | PromiseLike<number>>'.
Types of property '[Symbol.iterator]' are incompatible.
Type '() => IterableIterator<Promise<number> | Promise<string> | Promise<boolean>>' is not assignable to type '() => Iterator<number | PromiseLike<number>>'.
Type 'IterableIterator<Promise<number> | Promise<string> | Promise<boolean>>' is not assignable to type 'Iterator<number | PromiseLike<number>>'.
Types of property 'next' are incompatible.
Type '(value?: any) => IteratorResult<Promise<number> | Promise<string> | Promise<boolean>>' is not assignable to type '(value?: any) => IteratorResult<number | PromiseLike<number>>'.
Type 'IteratorResult<Promise<number> | Promise<string> | Promise<boolean>>' is not assignable to type 'IteratorResult<number | PromiseLike<number>>'.
Type 'Promise<number> | Promise<string> | Promise<boolean>' is not assignable to type 'number | PromiseLike<number>'.
Type 'Promise<string>' is not assignable to type 'number | PromiseLike<number>'.
Type 'Promise<string>' is not assignable to type 'PromiseLike<number>'.
Types of property 'then' are incompatible.
Type '<TResult1 = string, TResult2 = never>(onfulfilled?: (value: string) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>' is not assignable to type '<TResult1 = number, TResult2 = never>(onfulfilled?: (value: number) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => PromiseLike<TResult1 | TResult2>'.
Types of parameters 'onfulfilled' and 'onfulfilled' are incompatible.
Types of parameters 'value' and 'value' are incompatible.
Type 'string' is not assignable to type 'number'.
15 let [v1, v2, v3] = await Promise.all(promises);
~~~~~~~~
Found 1 error.