Why we use then() method in JavaScript ?
The then() method is a key feature of JavaScriptâs Promise API, designed specifically to handle asynchronous tasks like API calls. In the past, callback functions were commonly used for such tasks, leading to code that was difficult to maintain.
Syntax:
demo().then( (onResolved) => { // Some task on success }, (onRejected) => { // Some task on failure } ) Note: demo is a function that returns a promise prototype.
Parameters:
- onFulfilled: This is a function that is called upon to the success of the promise. This is an optional parameter.
- onRejected: This is a function that is called upon the rejection of the promise. This is an optional parameter.
Return Value:
Example 1: In this example, we have not passed any arguments
function demo() {
console.log("Function called!!<br>")
return Promise.resolve("Success");
// or
// return Promise.reject("Failure");
}
demo().then()
Output:
Function called!!
Example 2: In this example, we are Passing only the first callback
function demo() {
console.log("Function called!!")
return Promise.resolve("Success");
// or
// return Promise.reject("Failure");
}
demo().then(
(message) => {
console.log("Then success:" + message);
}
)
Output:
Function called!!
Then success:Success
Note: If the demo function returns a reject then it will generate an error.
Example 3: In this example, we are Passing both the arguments
function demo() {
console.log("Function called!!")
return Promise.resolve("Success");
// or
// return Promise.reject("Failure");
}
demo().then(
(message) => {
console.log("Then success:" + message);
},
(message) => {
console.log("Then failure:" + message);
}
)
Output:
Function called!!
Then success:Success
Example 4: In this example, we are using Chaining Multiple then() methods. Each then() can return a promise (a resolve or a reject) and therefore multiple then() methods can be chained together.
function demo() {
console.log("Function called!!")
return Promise.resolve(1);
// or
// return Promise.reject("Failure");
}
demo().then(
(value) => {
console.log(value);
return ++value;
},
(message) => {
console.log(message);
}
).then(
(value) => {
console.log(value);
},
(message) => {
console.log(message);
}
)
Output:
Function called!!
12
Example 5: In this example, we are using then() as an asynchronous function
let demo = new Promise((resolve, reject) => {
resolve(1);
})
let call = demo.then(
(value) => {
console.log(value);
return ++value;
},
(message) => {
console.log(message);
});
console.log(call);
setTimeout(() => {
console.log(call);
});
Output:
Promise {status: "pending"}
1
Promise {status: "resolved", result: 2}
Supported Browsers:
- Google Chrome 6.0 and above
- Internet Explorer 9.0 and above
- Mozilla 4.0 and above
- Opera 11.1 and above
- Safari 5.0 and above
Contact Us