Serial and Parallel Request in Javascript

English isn’t my native language, please excuse the typing error.


Talk is cheap, show you my code.

Use async and await syntactic sugar, before u read the blog, you should have so experience on Promise and window.fetch function.

So, let’s start.

First, there are some common code in the example:

// for example, just send a get request
const fetchData = async (url) => await window.fetch(url)

const urls = [

we defined the urls list and a fetchData function, which is based on window.fetch, it’s very simple.

Parallel Request

const requests = => fetchData(url))
const responses = await Promise.all(requests)

The Promise.all(iterable) method returns a single Promise that resolves when all of the promises in the iterable have resolved or when the iterable arguments contains no promises. It rejects with the reason of the first promise that rejects.

So it’very dangerous. so use Seial Request is better than it most times.

Serial Request

There I wiil provide 2 implement:

const serialRquests = async(urls, handler) => {
  const result = [];
  for (const url of urls) {
    const ret = await handler(url);
  return result;
const responses = await serialRquests(urls,fetchData)

And other method is very intersting, and I found it form gist, it use the Array.reduce methods, the hardest build-in function.

When write the blog, I find a wery powerable Promise libary in node, which named bluebird. this libary support more methods, like Promise.reduce, Promise.filter and etc. In the next ECMAScript, developer can use Promise.finally, a very useful method like try...catch...finally

Thanks for your reading.

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