Dont change resolved value in .then()

user_3608334 0
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I have 2 function that return promises, P1 and P2. First I want to wait until P1() is fulfilled, then I want to execute P2 and when P2() is fulfilled I want to do something with the value P1() resolved to. It is easy to do using async/await like this:

const result = await P1()
await P2()
doSomething(result)

but I cant use async/await. Using promises I can do it like this:

P1().then((result) => P2().then(() => result)).then((result) => doSomething(result))

but is there any prettier way to do it (without nested thens)?

es6-promise promise javascript
2 Answers

You could use Promise.all like this, it waits until all promises have been resolved (or the first rejection).

Promise.all([p1, p2]).then(result => { 
  doSomething(result);
});

or the more terse:

Promise.all([p1, p2]).then(result => doSomething(result));
user_2693490

It seems that P1 and P2 are independent, as you are not using the value of P1 to start the call of P2. If that's the case you can use .all() method to run those 2 promises in parallel, and then do something with their results. 

Promise.all([P1, P2]).then(([ P1Result, P2Result ]) => {
  doSomething(P1Result, P2Result);
});

However if P2 does depends on P1, the nested .then() would be the way to go. I've created a tiny module to ease the calls of promises in series, where it is even possible to mix sync and async methods on the same pipeline. Take a look and see if that can help you on getting rid of writing the nested .then() methods: https://github.com/DiegoZoracKy/pipe-functions

user_36784470
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