Node async loop – how to make this code run in sequential order?

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You can ditch async altogether and go for async/await quite easily.

Promisify your request and use async/await

Just turn request into a Promise so you can await on it.

Better yet just use request-promise-native that already wraps request using native Promises.

Serial example

From then on it’s a slam dunk with async/await:

const rp = require('request-promise-native')

const users = [1, 2, 3, 4]
const results = []

for (const idUser of users) {
  const result = await rp('http://foo.com/users/' + idUser)

  results.push(result)
}

Parallel example

Now, the problem with the above solution is that it’s slow – the requests run serially. That’s not ideal most of the time.

If you don’t need the result of the previous request for the next request, just go ahead and do a Promise.all to fire parallel requests.

const users = [1, 2, 3, 4]

const pendingPromises = []
for (const idUser of users) {
  // Here we won't `await` on *each and every* request.
  // We'll just prepare it and push it into an Array
  pendingPromises.push(rp('http://foo.com/users/' + idUser))
}

// Then we `await` on a a `Promise.all` of those requests
// which will fire all the prepared promises *simultaneously*, 
// and resolve when all have been completed
const results = await Promise.all(pendingPromises)

Error handling

Error handling in async/await is provided by plain-old try..catch blocks, which I’ve omitted for brevity.

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