How to Return Value From an Asynchronous Callback Function in JavaScript?

Just as humans are capable of performing multiple tasks simultaneously instead of waiting for one task to finish, JavaScript also handles multiple operations efficiently with the use of asynchronous programming. Fetching data, reading files, and updating user interfaces are some examples of asynchronous tasks, and JavaScript doesn’t pause the execution of any single task. Instead, it continues running other code while waiting for asynchronous operations to complete.

In this blog, you will learn more about asynchronous functions in JavaScript, the problems associated with them, and the methods through which you can get values from asynchronous functions in JavaScript.

Table of Contents:

• The Problem
• Methods to Handle the Asynchronous Callback Functions
  • Method 1: Using a Callback Function
  • Method 2: The Promise Method
  • Method 3: Using async/await Method
• Comparison Table
• Best Practices
• Conclusion
• How to Return a Value from an Asynchronous Callback Function in JavaScript – FAQs

The Problem

Whenever you try to get the value from the asynchronous functions in JavaScript, you are not able to get it because JavaScript cannot stop or wait for the function to get and store its value. It moves to the next line of code. Let’s understand this with the help of an example where you try to return a value from the function.

Example:

Output:

Explanation: In this example, you are creating a getMessage() function that will return a string. But if you try to print the result, it gives you undefined. This happens because JavaScript doesn’t wait for the setTimeout() to finish. It immediately moves to the next line and executes console.log(result), which prints undefined in the console.

Methods to Handle the Asynchronous Callback Functions

To correctly return the value from the asynchronous functions in JavaScript, you can use any of these methods:

Method 1: Using a Callback Function

In JavaScript, a Callback is a function that you can pass as an argument to another function. The main function runs an asynchronous task and calls the Callback function when it’s done.

Example

Output:

Explanation: Here we call the getMessage() function and pass an anonymous function (result) => { console.log(result); } as the Callback. This function takes the result as a parameter and prints it to the console.

Method 2: The Promise Method

A promise in JavaScript is an object that represents the completion or failure of an asynchronous operation. A promise can have three states:

• Pending: The asynchronous task is still in progress.
• Resolved: The async task has reached completion, and you have a result.
• Rejected: The async task has failed due to an error.

Handling multiple asynchronous operations using Callbacks leads to the problem of Callback hell (nested Callbacks that are hard to read and maintain). Promises solve this problem by allowing us to write cleaner and more readable asynchronous code. 

Example:

Output:

Explanation: In this example, you are using promises instead of using Callbacks because if you are using too many nested Callbacks in your code, then it leads to the problem of Callback hell. Now, getMessage() returns a promise instead of taking a Callback. The promise is resolved with the output “Hello, From Intellipaat”.

Method 3: Using async/await Method

The most readable and simplest way to handle asynchronous operations in JavaScript is using the async/await method.

Example:

Output:

Explanation: The async/await in JavaScript is recommended for use in modern JavaScript projects because it helps to write code more quickly and efficiently.

Comparison Table

Here is the Comparison between all three methods that are used to get a value from an asynchronous callback function in JavaScript.

Best Practices

Here are some of the best practices that you need to follow while working with asynchronous functions in JavaScript:

• If you are working with the older version of JavaScript, you might use Callbacks. But it’s recommended not to use Callbacks in new projects. 
• Promises are good for use in projects, but using .then() can sometimes make it difficult to write code for a beginner.
• Use async/await whenever possible because using async/await helps you to write code that is simple and easy to understand.

Conclusion

In JavaScript, handling asynchronous operations is an important skill for developing real-world projects. JavaScript does not wait for the asynchronous tasks to complete, and moves to the next line instantly. But JavaScript provides you with Callbacks, promises, and async/await to manage asynchronous operations. Avoid using Callbacks in new projects, use promises where needed, and always prefer using async/await for writing efficient code. By following these best practices, you can easily handle the asynchronous Callback functions in JavaScript.

How to Return a Value from an Asynchronous Callback Function in JavaScript – FAQs