What is Promise chaining in JavaScript?
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 executeWhen 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);
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.
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");
});
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There is a function inside the promise here, called the callback function, which has a parameter as two methods let promise = new Promise(function (resolve, reject) {
return resolve("Hello I am resolved");
});
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> 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>
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> 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>
- The first initial promise was fulfilled.
- Then the
- .then() method calls it to create a new Promise and get it resolved. Calling the
- .then() method again, it creates a new Promise, which then also gets resolved. Again, it's working, we can also add another promise handler.
.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> 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>
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So we see an in-depth concept from the basics to the promise chain on how it works.
<!DOCTYPE html>
<html>
<head>
<title>Promise chaining in javascript</title>
</head>
<body>
<h3> 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>
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