I have four asynchronous processes that fetch from different sources, namely process A, B, C and D.
Process A, B and C are independent of each other, but D depends on data from C.
As far as possible, I want A and B to be fetched independently on C and D, but I want to keep the result of A, B, C and D for the procedure that follows.
So I'm trying to come up with the correct way to express this functionality.
Something like
const results = await Promise.allSettled([A, B, C]);
const resultD = await D(results[2].value);
means that I'm waiting to perform D until A, B, C are all finished, but in this case A or B might take significantly more time than C.
Likewise,
const results = await Promise.allSettled([A, B, C, D(await C)]);
Doesn't seem quite right? I don't want C performed twice, and I also want to keep the result.
What would be the correct way to go about solving this elegantly?
You can group C and D together and resolve it with A and B.
Let's understand the snippet below:
Consider the functions A, B, C & D equivalent to fetching from four different APIs. Also, notice how D needs some data, that it would get from C.
And in the main function we're resolving A, B independently and we're resolving D only after C has finished.
const sleep = (delay, data) => new Promise(res => setTimeout(() => res(data), delay));
const A = () => sleep(100).then(() => "A")
const B = () => sleep(200).then(() => "B")
const C = () => sleep(100).then(() => "C")
const D = (inp) => sleep(100).then(() => inp + "D")
function main() {
const promises = [A(), B(), C().then(res => D(res))];
Promise.all(promises).then(console.log);
}
main();You can also use Promise.allSettled but make sure you understand the difference between allSettled and all.