What is Promise chaining in JavaScript?-JS Tutorial-php.cn

Home

Web Front-end

JS Tutorial

What is Promise chaining in JavaScript?

王林

Sep 08, 2023 pm 03:45 PM

JavaScript 中的 Promise 链是什么？

In this tutorial, we will discuss Promise chaining in JavaScript. Before turning to Promise chains, let’s first talk about what Promise is

So, we know that JavaScript is single-threaded, which means that two functions are not allowed to run at the same time, because we only have one call stack, as shown below, which has one function to execute

When one function completes execution, another function starts execution, so in order to achieve two or more functions running at the same time, we use asynchronous programming in the JavaScript example - setTimeout function, which accepts a callback function and the specified time, After that it will execute the function.

setTimeout(function cT(){
   console.log("I am here after 5 seconds")
}, 5000);

Therefore, we use Promise for asynchronous operations, which gives the result of the asynchronous operation and its completion or failure.

Just like in real life, we commit to completing any work in the same way that we make a commitment, and it exists in one of three states.

Pending - This represents the initial state, meaning it is neither completed nor rejected.
Fulfilled - This means that the operation we assigned completed successfully.
Rejected - This status means that the operation cannot be performed and it is rejected.

The asynchronous method returns a value, but it does not return the final value, but returns a promise to provide the value for the function in the future.

We use the

then() method to execute the Promise after the Promise statement. If it is rejected due to some error, it will be processed using a catch block.

How to declare Promise in JavaScript?

let promise = new Promise(function (resolve, reject) {
   return resolve("Hello I am resolved");
});

There is a function inside the promise here, called the callback function, which has a parameter as two methods

resolve() and reject(), as we already know Judging from the above explanations about both.

So resolve here ensures the successful completion of the function we give, while reject will ensure its responsibility for the unsuccessful completion of the given executable callback function.

Example 1

Let’s look at a program that uses Set to find unique characters in a string.

<!DOCTYPE html>
<html>
<head>
   <title>Promise chaining in JavaScript</title>
</head>
<body>
   <h3 id="Execute-a-Promise-we-receive"> Execute a Promise we receive</h3>
   <p id ="result"></p>
   <script>
      let promise = new Promise(function (resolve, reject) {
         return resolve("Hello I am resolved");
      });
      promise.then(function(value) {
         console.log(value);
         document.getElementById("result").innerHTML += value;
      })
   </script>
</body>
</html>

Inside above. The then() method will pass a callback function, and the value variable is responsible for printing out the results from the resolve() method.

What is a promise chain?

Promise chains are basically multiple asynchronous function calls and executing them one after the other in a synchronous manner. then() method.

Example 2

Let us understand in detail through an example.

<!DOCTYPE html>
<html>
<head>
   <title>Promise chaining in JavaScript </title>
</head>
<body>
   <h3 id="Promise-Chaining"> Promise Chaining</h3>
   <p id ="result"></p>
   <script>
      let promise = new Promise(function (resolve, reject) {
         return resolve(10);
      });
      promise.then(function(firstPromise_val) {
         alert(firstPromise_val)
         document.getElementById("result").innerHTML += "First Promise val: "+firstPromise_val+"<br>";
         return firstPromise_val+10;
      }).
      then(function(secondPromise_val){
         alert(secondPromise_val)
         document.getElementById("result").innerHTML +="Second Promise val: "+ secondPromise_val+"<br>";
         return secondPromise_val+10;
      }).
      then(function(thirdpromise_val){
         alert(thirdpromise_val)
         document.getElementById("result").innerHTML +="Third Promise val: "+thirdpromise_val+"<br>";
         return thirdpromise_val+10;
      })
   </script>
</body>
</html>

Let us understand how promise chain works.

.then() method calls it to create a new Promise and get it resolved.
.then() method again, it creates a new Promise, which then also gets resolved.

So, basically the

.then() method returns a new Promise and uses .then() to call next and so on.

Example 3

<!DOCTYPE html>
<html>
<head>
   <title>Promise chaining in javascript</title>
</head>
<body>
   <h3 id="Promise-Chaining"> Promise Chaining</h3>
   <p id ="result"></p>
   <script>
      let promise = new Promise(function (resolve, reject) {
         return resolve("My");
      }).
      then(function(firstPromise_val) {
         alert(firstPromise_val)
         return firstPromise_val+" Name";
      }).
      then(function(secondPromise_val){
         alert(secondPromise_val)
         return secondPromise_val+" is";
      }).
      then(function(thirdpromise_val){
         alert(thirdpromise_val)
         return thirdpromise_val+" Kalyan";
      }).
      then(function(thirdpromise_val){
         alert(thirdpromise_val)
         document.getElementById("result").innerHTML = thirdpromise_val
      })
   </script>
</body>
</html>

So we see an in-depth concept from the basics to the promise chain on how it works.

The above is the detailed content of What is Promise chaining in JavaScript?. For more information, please follow other related articles on the PHP Chinese website!

Statement

This article is reproduced at:tutorialspoint. If there is any infringement, please contact admin@php.cn delete

Replace String Characters in JavaScriptMar 11, 2025 am 12:07 AM

Detailed explanation of JavaScript string replacement method and FAQ This article will explore two ways to replace string characters in JavaScript: internal JavaScript code and internal HTML for web pages. Replace string inside JavaScript code The most direct way is to use the replace() method: str = str.replace("find","replace"); This method replaces only the first match. To replace all matches, use a regular expression and add the global flag g: str = str.replace(/fi

8 Stunning jQuery Page Layout PluginsMar 06, 2025 am 12:48 AM

Leverage jQuery for Effortless Web Page Layouts: 8 Essential Plugins jQuery simplifies web page layout significantly. This article highlights eight powerful jQuery plugins that streamline the process, particularly useful for manual website creation

Build Your Own AJAX Web ApplicationsMar 09, 2025 am 12:11 AM

So here you are, ready to learn all about this thing called AJAX. But, what exactly is it? The term AJAX refers to a loose grouping of technologies that are used to create dynamic, interactive web content. The term AJAX, originally coined by Jesse J

10 Mobile Cheat Sheets for Mobile DevelopmentMar 05, 2025 am 12:43 AM

This post compiles helpful cheat sheets, reference guides, quick recipes, and code snippets for Android, Blackberry, and iPhone app development. No developer should be without them! Touch Gesture Reference Guide (PDF) A valuable resource for desig

Improve Your jQuery Knowledge with the Source ViewerMar 05, 2025 am 12:54 AM

jQuery is a great JavaScript framework. However, as with any library, sometimes it’s necessary to get under the hood to discover what’s going on. Perhaps it’s because you’re tracing a bug or are just curious about how jQuery achieves a particular UI

10 jQuery Fun and Games PluginsMar 08, 2025 am 12:42 AM

10 fun jQuery game plugins to make your website more attractive and enhance user stickiness! While Flash is still the best software for developing casual web games, jQuery can also create surprising effects, and while not comparable to pure action Flash games, in some cases you can also have unexpected fun in your browser. jQuery tic toe game The "Hello world" of game programming now has a jQuery version. Source code jQuery Crazy Word Composition Game This is a fill-in-the-blank game, and it can produce some weird results due to not knowing the context of the word. Source code jQuery mine sweeping game

How do I create and publish my own JavaScript libraries?Mar 18, 2025 pm 03:12 PM

Article discusses creating, publishing, and maintaining JavaScript libraries, focusing on planning, development, testing, documentation, and promotion strategies.

jQuery Parallax Tutorial - Animated Header BackgroundMar 08, 2025 am 12:39 AM

This tutorial demonstrates how to create a captivating parallax background effect using jQuery. We'll build a header banner with layered images that create a stunning visual depth. The updated plugin works with jQuery 1.6.4 and later. Download the

See all articles

Hot AI Tools

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress images for free

Clothoff.io

AI clothes remover

AI Hentai Generator

Generate AI Hentai for free.

Hot Article

R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)

2 weeks agoBy尊渡假赌尊渡假赌尊渡假赌

Hello Kitty Island Adventure: How To Get Giant Seeds

1 months agoBy尊渡假赌尊渡假赌尊渡假赌

How Long Does It Take To Beat Split Fiction?

4 weeks agoByDDD

R.E.P.O. Save File Location: Where Is It & How to Protect It?

4 weeks agoByDDD

Two Point Museum: All Exhibits And Where To Find Them

1 months agoBy尊渡假赌尊渡假赌尊渡假赌

Hot Tools

Dreamweaver CS6

Visual web development tools

WebStorm Mac version

Useful JavaScript development tools

Notepad++7.3.1

Easy-to-use and free code editor

MinGW - Minimalist GNU for Windows

This project is in the process of being migrated to osdn.net/projects/mingw, you can continue to follow us there. MinGW: A native Windows port of the GNU Compiler Collection (GCC), freely distributable import libraries and header files for building native Windows applications; includes extensions to the MSVC runtime to support C99 functionality. All MinGW software can run on 64-bit Windows platforms.