TypeScript async iterate on object error

Question

Can someone explain me why this code give me error?

var promise = new Promise((resolve, reject) => {
    resolve([1, 2, 3, 4, 5]);
});

async function doSomethingAsync() {
    var data = await promise;
    data.forEach(v => console.log(v));
}

doSomethingAsync();

When i try to compile this like tsc file.ts --target ES6 i have this:

error:async.ts(7,10): error TS2339: Property 'forEach' does not exist on type '{}'.

What happens if you change new Promise(...) to new Promise<number[]>(...)? Perhaps the type checker simply cannot infer a sufficiently specific type for promise? — Mattias Buelens
– Mattias Buelens, Commented Aug 7, 2016 at 17:03
They are, but AFAIK they are not actually stable and it will be changed in future (im not sure). — user3075373
– user3075373, Commented Aug 7, 2016 at 21:24

user3075373 · Accepted Answer · 2016-08-07 17:06:33Z

1

Actually i found solution , We can just simply add a generic type into our Promise like:

'use strict';

var promise = new Promise<any[]>((resolve, reject) => {
    resolve(["gdfgdfgdf", "dfggfd", 1, 2, {}]);
});

async function doSomethingAsync() {
    let data = await promise;

    for (let i of data) {
        console.log(i);
    }
}

doSomethingAsync();

Now it works perfectly without any mess in code :)

answered Aug 7, 2016 at 17:06

user3075373

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1 Comment

Mattias Buelens Over a year ago

I see, so the type checker just needed to know that your promise would be resolved with an array. Nice find!

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