Promise.all with for loop in node.js

Question

I collect the information of each page from 1 to 10 as API in node.js.

Now I use this code.

async function myWork() {
  let results = []
  let tmp
  let param
  for (i=1; i<11; i++) {
    param = {'page': i}
    tmp = await callMyApi(param) // return a list
    results.push(...tmp)
  }
  return results
}

In this case, each callMyApi behaves like sync.

But I don't care about page order.

So, to speed it up, I want to use something like promise.all to process it in parallel.

How can I use promise.all in for loop in this case?

You use it instead of a for loop. Create an array of {page: i} objects, .map() them to the callMyApi() promise, then pass the resulting array to Promise.all and await the result. — user5734311
– user5734311, Commented Jul 16, 2021 at 7:58

Tushar Shahi · Accepted Answer · 2021-07-16 08:04:05Z

3

You can use Promise.all() with concat().


async function myWork() {
  let results = [];
  let promises = [];
  let param;
  for (i=1; i<11; i++) {
    let param = {'page': i}
    let tmpPromise = callMyApi(param);
    promises .push(tmpPromise);
  }
  //promises is now an array of promises, and can be used as a param in Promise.all()
  let resolved = await Promise.all(promises);
  //resolved is an array of resolved promises, each with value returned by async call
let indivResult = resolved.forEach(a => 
  results = results.concat(a));
 //for each item in resolved array, push them into the final results using foreach, you can use different looping constructs here but forEach works too
  return results;
}

answered Jul 16, 2021 at 8:04

Tushar Shahi

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1 Comment

Sohan Over a year ago

What happens when one of promise call fails ? This code will blow up. You should also surrond with catch block here

ikhvjs · Accepted Answer · 2021-07-16 08:03:54Z

1

Example below:

async function myWork() {
  let results = [];
  let param;
  for (i = 1; i < 11; i++) {
    param = { page: i };
    results.push(callMyApi(param));
  }
  const res = await Promise.all(results);
  return res.flat();
}

edited Jul 16, 2021 at 8:03

answered Jul 16, 2021 at 7:58

ikhvjs

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