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Why we use then() method in JavaScript ?

Last Updated : 15 Jul, 2025

The then() method in JavaScript is used with promises to handle asynchronous operations. It accepts two callback functions: one for handling a promise's resolved value and one for handling its rejection. It returns a new promise, allowing for method chaining.

Why we use then() method

  • Handle Results: then() processes promise outcomes, allowing actions based on successful resolution or errors.
  • Callback Functions: Provides callbacks for success and failure, ensuring proper handling of asynchronous tasks.
  • Method Chaining: Enables chaining multiple then() calls for sequential asynchronous operations, simplifying code readability and flow.

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: This method can either return a Promise (if further another then() is called) or nothing.

Example 1: In this example, we have not passed any arguments.

Output:

Function called!!

Example 2: In this example, we are Passing only the first callback.

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.

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.

Output:

Function called!!
12

Example 5: In this example, we are using then() as an asynchronous function.

Output:

Promise {status: "pending"}
1
Promise {status: "resolved", result: 2}

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