Execute an Array of promises sequentially without using async/await

Question

Lets say I have an array of promises. Each element of my array is a knex.js query builder and is ready to be executed and returns a promise.

How can I run each element of this array sequentially. The array is built dynamically.

let promisesArray = [q1,q2,q3] ;

Each q is not a promise by itself but it will return a promise upon execution.

`promise.all()` ? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all — fedesc, Apr 25 '19 at 08:06
Possible duplicate of [ES6 Promises - something like async.each?](https://stackoverflow.com/questions/32028552/es6-promises-something-like-async-each) — str, Apr 25 '19 at 08:07
if `q1, q2, q3` are promises, then it's too late to run them sequentially — Jaromanda X, Apr 25 '19 at 08:08
Is it important that they *run* in order or do you just need the results in order? — , Apr 25 '19 at 08:10

Amin Fazlali · Accepted Answer · 2019-04-25T08:55:05.767

5

Here could be a possible option:

let p = Promise.resolve([]);
promisesArray.forEach(q => {
  p = p.then(responses => {
    //based on the nature of each q, to start execution
    //use either q().then() or q.then()
    return q().then(response => {
      //Any further logic can be here.
      console.log(response);
      return responses.concat([response]);
    })
  })
})

p.then(responses => {
  // here you have all of the responses.
})

edited Apr 25 '19 at 08:55

answered Apr 25 '19 at 08:11

Amin Fazlali

1,209
1
9
17

1

using reduce, this is a single line `promisesArray.reduce((p, q) => p.then(responses => q.then(response => [...responses, response])), Promise.resolve([]))` – Jaromanda X Apr 25 '19 at 08:14
not really, @Nnanyielugo - depends on what you know I guess – Jaromanda X Apr 25 '19 at 08:27
One-liners are usually not difficult to read to their authors :-) – Nnanyielugo Apr 25 '19 at 08:31

Khaled Osman · Answer 2 · 2019-04-25T08:22:56.133

You can use Array.reduce to reduce the Array into one promise that chains them one after another

let promisesArray = [q1,q2,q3] ;

function runSequentially(promiseArr) {
  return promiseArr.reduce((accum, p) => accum.then(p), Promise.resolve())
}

//Example, this prints.. 1, 2, 3 then "done".
runSequentially([Promise.resolve(1).then(console.log), Promise.resolve(2).then(console.log), Promise.resolve(3).then(console.log)]).then(() => console.log("done"))

score 0 · Answer 3 · answered Apr 25 '19 at 08:15

0

I can think of bluebird promise, which should solve your issue. Keeping the concurrency value to 1, should execute promises sequentially.

    var Promise = require("bluebird");
    Promise.map([q1,q2,q3], {concurrency: 1})

answered Apr 25 '19 at 08:15

Sreehari

1,328
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I am really curious to know, why a negative vote is given for it. Do you think that it won't work? – Sreehari Apr 25 '19 at 08:31

score 0 · Answer 4 · answered Apr 25 '19 at 08:16

Based on your claim that q1, q2, q3 are "knex.js query builder and is ready to be executed and returns a promise", get a function to execute itself with the next index when the promise is resolved. Call it with 0 first.

function awaitPromise(arr, idx) {

   arr[idx]().then(function(res) {

       console.log(res);

       if (idx < arr.length - 1)      
           awaitPromise(arr, idx + 1);
   })
}

score 0 · Answer 5 · edited Sep 13 '19 at 10:36

0

If you're using bluebird, you can use Promise.map with concurrency set to 1.

await Promise.map(arrayOfObj, async (obj) => {
    await this.someOperation();
  },
  {concurrency: 1}
);

edited Sep 13 '19 at 10:36

Deilan

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answered May 27 '19 at 13:08

swaroop ghosh

11
3

Execute an Array of promises sequentially without using async/await

5 Answers5