Promise vs Callback in JavaScript

In JavaScript, managing asynchronous operations is a key aspect of modern web development. Two popular approaches for handling these operations are Promises and Callbacks. While both techniques are designed to deal with tasks that take time to complete (like fetching data from a server), they work differently and have distinct advantages and drawbacks. This article explores Promises and Callbacks, their benefits, and the key differences between them.

What are Promises in JS?

To manage asynchronous actions in JavaScript, promises are used. It is an assurance that something will be done. The promise is used to keep track of whether the asynchronous event has been executed or not and determines what happens after the event has occurred.

A Promise has four states: 

• fulfilled: Action related to the promise succeeded
• rejected: Action related to the promise failed
• pending: Promise is still pending i.e. not fulfilled or rejected yet
• settled: Promise has fulfilled or rejected

Syntax:

Let promise = new Promise(function(resolve, reject){
    // do something
});
A promise can be created using Promise constructor.

Parameters:

• Promise constructor takes only one argument which is a callback function (and that callback function is also referred as an anonymous function too).
• Callback function takes two arguments, resolve and reject
• Perform operations inside the callback function and if everything went well then call resolve.
• If desired operations do not go well then call reject.

Example: In this example, a Promise is created to compare two strings, ‘geeksforgeeks’ and ‘geeksforgeeks’. If the strings match, the promise resolves, logging a success message. Otherwise, it rejects, logging an error message. The then method handles success, and the catch method handles errors.

let promise = new Promise(function (resolve, reject) {
    const x = "geeksforgeeks";
    const y = "geeksforgeeks"
    if (x === y) {
        resolve();
    } else {
        reject();
    }
});

promise.
    then(function () {
        console.log('Success, You are a GEEK');
    }).
    catch(function () {
        console.log('Some error has occurred');
    });

Success, You are a GEEK

Benefits of Promises:

• Improves Code Readability
• Better handling of asynchronous operations
• Better flow of control definition in asynchronous logic
• Better Error Handling

What is Callback in JS ?

Callbacks are a great approach to dealing with something once another task has been finished. Here, “something” refers to the execution of a function. Callbacks can be utilized if we wish to run a function immediately following the return of another function.

The type of JavaScript function is objects. They may therefore be supplied as an argument to any other function when calling them, just like any other objects (Strings, Arrays, etc.).

Example : In this example, the add function takes two numbers, adds them, logs the result, and executes a callback function (disp). When calling add(5, 6, disp), it outputs the sum and a message from the callback.

// The add() function is 
// called with arguments a, b
// and callback, callback 
// will be executed just
// after ending of add() function
function add(a, b, callback) {
    console.log(`The sum of ${a} 
    and ${b} is ${a + b}`);
    callback();
}

// The disp() function is called just
// after the ending of add() function
function disp() {
    console.log(`This must be printed
     after addition`);
}

// Calling add() function
add(5, 6, disp)

The sum of 5 
    and 6 is 11
This must be printed
     after addition

Explanation:

Here are the two functions – add(a, b, callback) and disp(). Here add() is called with the disp() function i.e. passed in as the third argument to the add function along with two numbers.

As a result, the add() is invoked with 1, 2, and the disp() which is the callback. The add() prints the addition of the two numbers and as soon as that is done, the callback function is fired! Consequently, we see whatever is inside the disp() as the output below the additional output.

The benefit of Callback:

• You can run another function call after waiting for the outcome of a prior function call.
• You can call the parent function from the child function and can also pass data from child to parent.

Difference:

Conclusion

While both Promises and Callbacks serve the same purpose of handling asynchronous operations, Promises offer a more organized, readable, and manageable way to write code. Promises resolve the issues associated with nested callbacks and provide better error handling. As JavaScript has evolved, Promises (along with async/await) have become the preferred method for managing asynchronous logic in modern development.

How do Promises help avoid callback hell?

Promises flatten nested callback structures, making the code easier to read and manage by chaining .then() and .catch() methods instead of deeply nested functions.

What is callback hell and how do Promises resolve it?

Callback hell occurs when multiple asynchronous operations are nested within each other, leading to unreadable code. Promises resolve this by allowing flat, sequential chaining of asynchronous operations.

Can Promises be used alongside callbacks?

Yes, Promises and callbacks can be used together. For example, you can wrap a callback-based function in a Promise to take advantage of the benefits of both approaches.

How is error handling different between Promises and Callbacks?

In callbacks, error handling can be tricky, requiring manual checks. In Promises, errors are automatically caught and handled in the .catch() block, making error management easier and more reliable.

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Article Tags :

• JavaScript
• Technical Scripter
• Web Technologies
• JavaScript-Questions
• Technical Scripter 2022

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