Execute several promises in parallel, but only retrieve result from one of them

user_2436221 0
Viewed 72

I currently have a program where I want to call several REST API:s in parallel, but I'm only interested in the result from one of them.

Currently I've solved it like this:

private async loadData () {
    const all = [this.loadFirstData(), this.loadSecondData(), this.loadThirdData()];
    const combine = Promise.all(all);
    await combine;

    // One of the promises just puts it's return value in this global variable, so that I can access it after it is done.
    if (this.valueFromThirdAPI) {
        // Do something with value
    }
}

So what I do is just put the put the result from the promise I want a result from in a global variable that I can access after all of the promises have returned. This works, but I'm sure there must be a much better way of doing this.

Promises.all returns a array of the resolved values, but how do I distinguish them from each other if I'm only interested in the value from one of them? The other two doesn't need to return anything.

Thanks in advance!

promise typescript concurrency javascript
4 Answers

First, it makes sense to await Promise.all,

You can access the result using the returned array's index. Another option is using Array destructoring.

Below is an example..

const promise1 = Promise.resolve(1);
const promise2 = Promise.resolve(2);
const promise3 = Promise.resolve('this one');
const promise4 = Promise.resolve(4);

async function test() {
  //note the double `,` to ignore the first 2 promises.
  const [,,three] = await Promise.all([promise1, promise2, promise3, promise4]);
  console.log(three);
}

test();

user_68702280

Promise.all returns an array with the same order of the given promises:

private async loadData () {
    const all = [this.loadFirstData(), this.loadSecondData(), this.loadThirdData()];
    const combine = Promise.all(all);
    const values = await combine;

    // One of the promises just puts it's return value in this global variable, so that I can access it after it is done.
    if (values[2]) {
        // Do something with value
    }
}
user_12558190
user_80996310

Wait on the third and then await on all. So if the third finishes before first and second, you get to process it earlier.

private async loadData () {
    const all = [this.loadFirstData(), this.loadSecondData(), this.loadThirdData()];
    const third = await all[2];
    if (third) {
        // Do something with value
    }

    await Promise.all(all);
}
user_110579880
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