Promise.all() with dynamically sized array of requests using await

Question

I'm new to JavaScript and Promises. I need to send an array of requests using Promise.all and await. Unfortunately, I do not know the size of the array, so it needs to be dynamic. The array would be requests. Ex:

let arrayOfApiCreateRecords = [];
arrayOfApiCreateRecords.push(apiCreateRecords(req, { clientHeaders: headers, record }));
let responses = await Promise.all( arrayOfApiCreateRecords );

I tried to write my code like this, but I seem to be stuck. Is it possible to rewrite the code using Promise.all and await with a dynamic array of requests? Please advise. Below is what I have:

'use strict';

const { apiCreateRecords } = require('../../../records/createRecords');

const createRecords = async (req, headers) => {
  let body = [];
  let status;
  for(let i = 0; i < req.body.length; i++) {
    let r = req.body[i];
    let record = {
      recordId: r.record_Id,
      recordStatus: r.record_status,
    };
    const response = await apiCreateRecords(req, { clientHeaders: headers, record });
    status = (status != undefined || status >= 300) ? status : response.status;
    body.push(response.body);
    };
  return { status, body };
};

module.exports = {
  createRecords,
};

That's precisely what `Promise.all()` is for. The array can be any size. — Barmar, Jan 09 '21 at 06:43
Push the promises onto the array in the loop, then call `Promise.all()` when the loop is done. — Barmar, Jan 09 '21 at 06:44
@Barmar wouldn't this call all the promises? I wanted them to be executed all at once. — cccap1550, Jan 09 '21 at 06:49
Promises start executing when you call the async functions. `Promise.all()` is for waiting for all of them to be resolved. — Barmar, Jan 09 '21 at 06:50
@Barmar https://stackoverflow.com/q/37801654/14970505 In this example, The Promise.all behavior will be executed when the previous 2 promises are solved, BUT, before those 2 promises were solved, a third promise where added and this new one won't be take in account. Is this correct? — cccap1550, Jan 09 '21 at 06:52
That's correct. `Promise.all()` will only wait for the promises that are in the array when it's called. It doesn't monitor the array for asynchronous changes. — Barmar, Jan 09 '21 at 06:56

score 0 · Accepted Answer · answered Jan 09 '21 at 07:08

Okay, I'm going to use fetch API to demonstrate the usage of Promise.all()

Normal usage (for one fetch call)

let user = { username: 'john.doe', password: 'secret' };

try{
    let res = await fetch('https://example.com/user/', {
        method: 'POST',
        headers: {
            'Content-Type': 'application/json'
        },
        body: JSON.stringify(user)
    })

    console.log('User creation response: ', res);
}
catch(err){
    console.error('User creation error: ', err);
}

Now let's use Promise.all()

const users = [
    { username: 'john.doe', password: 'secret' },
    { username: 'jane.doe', password: 'i-love-my-secret' }
];

const requests = [];

// push first request into array
requests.push(
    fetch('https://example.com/user/', {
        method: 'POST',
        headers: {
            'Content-Type': 'application/json'
        },
        body: JSON.stringify(user[0])
    })
);

// push second request into array
requests.push(
    fetch('https://example.com/user/', {
        method: 'POST',
        headers: {
            'Content-Type': 'application/json'
        },
        body: JSON.stringify(user[1])
    })
);

try{
    const responses = await Promise.all(requests);

    console.log('User creation responses: ', responses);
}
catch(err){
    console.log('User creation error: ', err);
}

Promise.all() with dynamically sized array of requests using await

1 Answers1