Modern JavaScript

Best of Modern JavaScript — Promise API

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Since 2015, JavaScript has improved immensely.

It’s much more pleasant to use it now than ever.

In this article, we’ll look at some JavaScript promise methods.


The finally method lets us run code in a promise chain regardless of the outcome of the promise chain.

For instance, we can write:

  .then(function(value1) {  
  .then(function(value2) {  
  .finally(function() {  
    // ...  

We call finally so that we can run a callback that runs regardless of the outcome.

This is useful for running clean up code and something like that.

ES6 Promise Libraries

There’re many promise libraries we can use.

Many of them conform to the ES6 API.

They include:

ES6 Promise API

The ES6 promise API has various parts.

The most basic part is the Promise constructor.

It takes a callback that has the resolve and reject functions as parameters.

resolve resolves the promise with the passed in value.

It’ll be forwarded to the then callback if then is called.

reject rejects the promise with the given value.

The argument of reject is usually an Error instance.

We can use it by writing:

const p = new Promise(function(resolve, reject) {  

Static Promise Methods

There’re several static promise methods in the API.

They include the Promise.resolve and Promise.reject methods.

Promise.resolve converts an arbitrary value to a promise.

We pass in an argument to it to return a value with the resolved value.

We can call Promise.resolve with:


Promise.reject lets us create a new promise that’s rejected with a reason.

We can call Promise.reject with:


There’re several methods to compose promises.

Promise.all and Promise.race lets us run multiple promises in different ways.

Promise.all takes an iterable and is fulfilled if all elements in the array of promises are fulfilled.

If any of the promises in the iterable are rejected, then the rejection value is the first rejection value from the promises.

Promise.race also takes an iterable which settles the returned promise.

It’ll return a promise that resolves to the value that’s first resolved.

Promise Instance Methods

Promise instance has various methods.

The Prototype.prototype.then(onFulfilled, onRejected) method takes a onFulfilled and onRejected callbacks.

onFulfilled is called immediately when the promise is resolved.

onRejected is called when the promise is rejected.

then returns a new promise that’s resolved to the value returned in the promise.

If the then callback throws an exception, then the promise returned is rejected.

If onFulfilled is omitted, then the fulfilled result is forwarded to the next then .

If onRejected is omitted, the rejection is forwarded to the next then .

Promise.prototype.catch(onRejected) lets us catch the error of the first rejected promise.

It’s the same as p.then(null, onRejected).


The Promise API is a standard API that lets us write async code easily.

They can be chained and we can catch errors with them.

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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