How to run fetch() in a loop?

Question

I am new to nodejs and promise based request. I want to fetch the data from a remote server in a loop, and then create a JSON object from all fetched data.

const fetch = require('node-fetch'); const users = []; const ids = await fetch('https://remote-server.com/ids.json'); console.log(ids); // [1,2,3] ids.forEach(id => { var user = await fetch(`https://remote-server.com/user/${id}.json`); users.push(user); }); console.log(users);

expected output

 [ { name: 'user 1', city: 'abc' }, { name: 'user 2', city: 'pqr' }, { name: 'user 3', city: 'xyz' } ]

Console log will always return empty. Let me fiddle a promise function for you, let me warn you though. You can run into very long waiting times doing that. — jPO
– jPO, Commented Jun 5, 2019 at 10:42
So. do you want the requests to be launched in parallel or one-by-one? — spender
– spender, Commented Jun 5, 2019 at 10:42

spender · Accepted Answer · 2019-06-05 10:49:01Z

So to launch in parallel:

const ids = await fetch('https://remote-server.com/ids.json'); const userPromises = ids.map(id => fetch(`https://remote-server.com/user/${id}.json`)); const users = await Promise.all(userPromises);

to launch in sequence:

const users = []; const ids = await fetch('https://remote-server.com/ids.json'); for(const id of ids){ const user = await fetch(`https://remote-server.com/user/${id}.json`); users.push(user); }

darklightcode · Accepted Answer · 2019-06-05 10:51:46Z

-2

You forgot to add async in the forEach:

ids.forEach(async (id) => { // your promise is in another function now, so you must specify async to use await var user = await fetch(`https://remote-server.com/user/${id}.json`); users.push(user); });

answered Jun 5, 2019 at 10:51

darklightcode

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1 Comment

spender Over a year ago

Beware. Array.forEach is not promise-aware, so this represents a "parallel" launching of all requests at once in "fire-and-forget" mode (bad for error-trapping) with no guarantee that the order of the users array will retain the same order as the ids array.

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