Question 49978 in Javascript Question posted in
A very good W3school tutorial can be found here.

How to fetch N requests sequentially – Javascript

Let’s say I request a url, which returns the max number of pages, ie 5. I then need to request page 2, 3, 4 and 5.

However, as to not get blocked, these can’t be done all at the same time but rather sequentially so Promise.all() is not gonna work (if I understand it correctly).

If I knew how many pages I have to get beforehand, I could just chain .thens, but I only get the info after the first request.

Also the data from all the requests have to be gathered, processed and passed to another function.

I suspect it will have something to do with promises? But how to chain those properly is beyond me.

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2

Answers


  1. 0

    You can either do it manually a tool like p-limit

    Example:

    import pLimit from 'p-limit';

const limit = pLimit(1);

async function getData(x: number) {
  // do whatever you neeed aasynchronously here
  return x;
}

async function otherFunction() {
  // some input parameters for the async getData function:
  const input = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

  // set concurrency limit to 5:
  const limit = pLimit(5);

  // run Promite.all on the closures processed by your `limit`
  return Promise.all(input.map(e => limit(() => getData(e))));
}
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  2. 0

    You can fetch requests sequentially by defining Promise.resolve(), and chaining it every iteration.

    let urls = [];
Array(21).fill('').forEach((_,i)=>{
  urls.push('https://dummyjson.com/carts/'+(i+1));
});

let promise = Promise.resolve();
console.time('time execution');
urls.forEach((url,i) => {
  promise = promise.then(() => 
    fetch(url)
    .then(r => r.json())
    .then(r => console.log(r))
    .catch(e => console.error(e))
    .finally(() => {
      if(i === urls.length - 1) console.timeEnd('time execution');
    }) 
  );
});

    Another way: Use async-await

    let urls = [];
Array(21).fill('').forEach((_,i)=>{
  urls.push('https://dummyjson.com/carts/'+(i+1));
});

(async () => {
  console.time('time execution')
  for(let [i,url] of urls.entries()) {
    console.log(await new Promise((resolve, reject) => {
      fetch(url)
      .then(r => r.json())
      .then(r => resolve(r))
      .catch(e => reject(e))
      .finally(() => {
        if(i === urls.length - 1) console.timeEnd('time execution');
      });
    }));
  }
})();
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