Delay between promises when using Promise.all

Question

Is there a way to delay the evaluation of an array of promises using Promise.all()?

Does it make sense to manually add a delay function to the end of each promise before adding them to the array?

Promise.all([p1,p2,p3]).then(res => console.log(res))

I would like to add a delay because my server can't handle too many requests at once.

Answer 1

Yes, you can delay promises using Promise.all to create staggered execution and it's quite easy to do:

// Promise.all() with delays for each promise
let tasks = [];
for (let i = 0; i < 10; i++) {
  const delay = 500 * i;
  tasks.push(new Promise(async function(resolve) {
    // the timer/delay
    await new Promise(res => setTimeout(res, delay));

    // the promise you want delayed
    // (for example):
    // let result = await axios.get(...);
    let result = await new Promise(r => {
      console.log("I'm the delayed promise...maybe an API call!");
      r(delay); //result is delay ms for demo purposes
    });

    //resolve outer/original promise with result
    resolve(result);
  }));
}

let results = Promise.all(tasks).then(results => {
  console.log('results: ' + results);
});

You can run it here too.

Rather than a delay between the chain, which can be done with .then() as shown in other answers, this is a delay that differs for each Promise so that when you call Promise.all() they will be staggered. This is useful when, say, you are calling an API with a rate limit that you'd breach by firing all the calls in parallel.

Peace