Limiting a javascript forEach loop

Question

I have some nested Node.js forEach loops which are hitting API rate limiters and I would like to slow them down so that they are not hitting the rate limit. I have tried adding a setTimeout on the end of the loop but that has no impact at all on the speed of the requests. Ideally, I would be able to send a request every 2 seconds. How can I achieve this? Everything online seems to talk about rate-limiting an API but nothing on the sending side of things.

Answer 1

To delay in a loop, first of all, your function must be async.

You can then await a promise that resolves after 2 seconds. Such a delay can be inserted as a one-liner like this:

await new Promise(resolve => setTimeout(resolve, 2000));

...or as a handy function like:

const delay = time => new Promise(resolve => setTimeout(resolve, time));

...which can then be used in a loop like:

async function fetchEverything() {
   for (let thing of things) {
      await fetchThing(thing);
      await delay(2000);
   }
}

Answer 2

You can make a generic extension on Array which can insert a delay in between each iteration, you will want to make the function async so you can await it (and therefore know when it's finished).

Array.prototype.forEachLimited = async function(fn, timeBetweenCallsMs){
    
    const delay = async () => new Promise(resolve => setTimeout(resolve, timeBetweenCallsMs));
    
    for(var i=0;i<this.length;i++){
      fn(this[i]);
      await delay();
    }
};


(async function(){
  var myArr = [1,2,3,4,5];
  await myArr.forEachLimited(i => console.log(i), 2000);
  console.log("Finished");
})();

Some may frown at adding this to the Array.prototype, it will work just as well as a normal function taking the array as an argument, so the same thing without adding it to Array.prototype

async function forEachLimited(arr, fn, timeBetweenCallsMs) {

  const delay = async() => new Promise(resolve => setTimeout(resolve, timeBetweenCallsMs));

  for (var i = 0; i < arr.length; i++) {
    fn(arr[i]);
    await delay();
  }
};


(async function() {
  var myArr = [1, 2, 3, 4, 5];
  await forEachLimited(myArr, i => console.log(i), 2000);
  console.log("Finished");
})();