How to use Promise.all() with Typescript

Question

Here is what I want to do:

Promise.all([aurelia.start(), entityManagerProvider.initialize()])
    .then((results:Array) => {
        let aureli         


        

Answer 1

If you'd like to keep type-safety, it's possible to extend the native type-definition of the Promise object (of type PromiseConstructor) with additional overload signatures for when Promise.all is called with a finite number of not-necessarily inter-assignable values:

interface PromiseConstructor
{
    all(values: [T1 | PromiseLike, T2 | PromiseLike]): Promise<[T1, T2]>;
    all(values: [T1 | PromiseLike, T2 | PromiseLike, T3 | PromiseLike]): Promise<[T1, T2, T3]>;
    ...
}

Add as many overloads as you need. This approach provides full type-safety for all elements in the value argument of the onfulfilled callback:

Promise.all([1, "string", true]).then(value =>
{
    let a: number = value[0]; // OK
    let b: number = value[1]; // Type 'string' is not assignable to type 'number'.
    ...
});