JavaScript Asynchronicity, Promise Object: What is a Promise object in JavaScript?

If you’re a JavaScript developer, chances are that you’ve heard of Promises. They are an incredibly powerful tool for asynchronously handling asynchronous events in your code. In this blog post, we’ll explore what they are and how they work. We’ll also look at the Promise object’s API and some examples of using it in our own applications to handle asynchronous events with ease!

Promise Object: What is a Promise object in JavaScript?

A promise object is an object that represents the eventual result of an asynchronous operation. If it succeeds, then we call its resolve function with any arguments given to the constructor as parameters (usually these are just success data).

If it fails, then we call its reject function with any argument given at the time of construction provided as parameter and this will typically be some sort of error data related to why failure occurred.

So what does this mean for my code when I want to use Promises? It’s actually extremely simple! For example, let’s say you’re making a request using axios from your backend server. You could do something like this:

axios.get(‘/api’)

.then(response => response.data)

//resolve: handle success data

.catch(error => console.log(“oh noes, ” + error)) //reject: handle errors when they happen!

You’ll notice that the code is very readable and linear – in other words, it reads like you would read a book from start to finish (rather than jumping around all over the place). You can clearly see what happens if your request succeeds or fails and any parameters are passed directly as arguments to either resolve() or reject().Another really important point about Promises is that many JavaScript frameworks use them internally for managing asynchronous operations such as AJAX calls and window resize events.

See: JavaScript Promises

Promise Chain in JavaScript.

  • Promise objects are used to make things happen when they are not happening now.
  • You can chain promises together to do many things at one time.
  • Promises are handled asynchronously, so you can make many of these chains without slowing down your code.

Let’s say that we wanted to get the current time from a server and then calculate how long it would take for our app to start up if we’re offline. We could do something like this:

//create chain

let promiseChain = new Promise(function (resolve) { //load the network request }).then(() => loadAJAXData().then((response) => response .setStartTime()) ).catch((error) => console.log(“oh noes!”)) //start with create chain

promiseChain.then(resolve).catch((error) => console.log(“oh noes!”))

//get current time from server

let promiseStart = new Promise(function (resolve) { //load the network request }).then(() => loadAJAXData().then((response) => response.setCurrentTime()) ).catch((error) => console.log(“oh noes!”))

promiseChain .then(startPromise)).catch((error) =>console.log(“oh noe”)) //calculate how long it would take to start up if offline

promiseChain .then({ resolve: function () { return calculateOfflineDuration(); }, reject: errorFormat }); /* output */ ‘hello world oh no! ‘ + Error message here

Promise API in JavaScript.

promises are asynchronous and handle errors well when they happen

=>it’s best to use promises if you’re making AJAX requests because of their ability to keep your code readable and linear while handling asynchronously without slowing down the execution speed like using setTimeout() would do

How can you use this function? It’s important for anyone who wants to write clean, efficient asynchronous code that uses Promises! For example, let’s say we want to get an updated list of friends from Facebook every time someone likes one of our posts on Instagram. We could do something like this: Promise.all([loadFacebookData(), loadAJAXData()]).then(([facebookResponse]) => {

//resolve: handle success data

loadAJAXData().then((response) => response.setFacebookLikes()) //put likes in the database!

}, function (error) { console.log(“oh noes!”); }); /* output */ ‘hello world oh no!’ + Error message here Promise.all() is a really useful function that will execute all Promises passed to it as arguments and wait for them all to complete before executing any of their respective then or catch functions – this way, we know everything is ready before continuing with our code. It’s important for anyone who wants to write clean, efficient asynchronous code that uses promises because doing so can help you avoid errors when they happen by handling them right away – this is a lot easier than trying to figure out where you need to handle an error in a long chain of promises.

How can you use this function? It’s important for anyone who wants to write clean, efficient asynchronous code that uses Promises!