Promise chains in JavaScript

— 5 minute read

In this article, I will tell you more about promise chaining. For this specific use case, I needed a promise chain in node.js, but the same applies to vanilla js

What is a promise?

Sounds pretty logical, right, and it is. You tell the script a promise is going to end eventually.

A basic promise looks like this:

let promise = new Promise(function (resolve, reject) {
  setTimeout(() => resolve(1), 1000);
});

promise
  .then(function (data) {
    console.log(data); // Returns 1
  })
  .catch(function (error) {
    console.log(error);
  });

You can see we define the promise as a let. In this case, it can as well be a function. The promise is called, and the then identifies the first result. We include a catch to catch any errors in the promise (reject)

Then what is promise chaining for?

You use promise chaining if you want to run code in sequences. Let's say the following example:

  1. Return default number from API
  2. Multiple that number by 2
  3. Multiple the multiplied number by 2

In a more real-world example, these would be API calls asynchronous but waiting for each other.

This looks as follows:

let promise = new Promise(function (resolve, reject) {
  setTimeout(() => resolve(1), 1000);
});

promise
  .then(function (result) {
    console.log(result); // 1
    return new Promise((resolve, reject) => {
      setTimeout(() => resolve(result * 2), 1000);
    });
  })
  .then(function (result) {
    console.log(result); // 2
    return new Promise((resolve, reject) => {
      setTimeout(() => resolve(result * 2), 1000);
    });
  })
  .then(function (result) {
    console.log(result); // 4
  })
  .catch(function (error) {
    console.log(error);
  });

As you can see, we included some timeouts in this. When you run this code, you will see a 1000ms wait, then log 1 then 1000 ms wait for => log 2 another 1000ms wait log 4.

This is our chain of promises. Each promise will wait for the previous one to complete.

See below for a demo on CodePen.

See the Pen Promise chain JavaScript by Chris Bongers (@rebelchris) on CodePen.

Promise chains are a boss, right?

Hopefully, you are as excited about promise chains as I am. Feel free to send me a message on Facebook or Twitter and let me know what topics you want to read more about!