How do i make javascript loop wait for each iteration of loop to finish before starting the next?

Question

I have a loop that gets iterated through 1000 times where each iteration makes a request and then prints the result of that request.

Similar to below.

let start = console.log(Date.now())
for (i = 0; i < 1000; i++) {
    request.then(data => {
        console.log(Date.now() - start)
    })
}

This however results in the first request taking much longer to be completed then if there was just 1 iteration of the loop.

with i < 1000:

5860
...

with i < 1:

220,
...

For the aim of this script however, I want to wait for each result to be received before starting the next iteration of the loop.

Answer 1

If you want to stick with the ES5 way of handling Promises, you could use the following approach:

const start = console.log(Date.now())

// Create a instantly resolved promise
for (let i = 0, p = Promise.resolve(); i < 1000; i++) {
  // append new promise to the chain
  p = p.then(() => request())
       .then(() => console.log(Date.now() - start));
}

If you can use ES6 methods, you could write this using the async/await pattern like so:

const asyncLoop = async () => {
  const start = console.log(Date.now())

  for (let i = 0; i < 1000; i++) {
    const data = await request();
    console.log(Date.now() - start);
  }
}

asyncLoop();

The chance that you really want to make the requests one after the other is actually pretty small though, so in case you want to make the request simultaneously, but do something after all of them resolved, you can use Promise.all(...)

const start = console.log(Date.now())
const requests = [request1, request2, ...];

Promise.all(requests)
       .then(() => console.log(Date.now() - start));