Callback Functions - JavaScript Program

// Callback: Function passed as argument to another function // Example 1: Simple callback function greet(name, callback) { console.log("Hello, " + name); callback(); } greet("John", function() { console.log("Callback executed!"); }); // Example 2: Callback with parameters function processArray(arr, callback) { let result = []; for (let item of arr) { result.push(callback(item)); } return result; } let numbers = [1, 2, 3, 4, 5]; let doubled = processArray(numbers, function(num) { return num * 2; }); console.log("\nDoubled:", doubled); // Example 3: Using arrow functions let squared = processArray(numbers, num => num ** 2); console.log("Squared:", squared); // Example 4: setTimeout callback console.log("\nStart"); setTimeout(function() { console.log("This runs after 1 second"); }, 1000); console.log("End"); // Example 5: Array methods with callbacks let arr = [1, 2, 3, 4, 5]; // forEach arr.forEach(function(num) { console.log("Number:", num); }); // map let doubled2 = arr.map(num => num * 2); console.log("\nMapped:", doubled2); // filter let evens = arr.filter(num => num % 2 === 0); console.log("Filtered:", evens); // Example 6: Custom callback with error handling function fetchData(successCallback, errorCallback) { let success = true; // Simulate API call if (success) { successCallback({ data: "Success!" }); } else { errorCallback("Error occurred"); } } fetchData( function(data) { console.log("\nSuccess:", data); }, function(error) { console.log("Error:", error); } );

Hello, John Callback executed! Doubled: [ 2, 4, 6, 8, 10 ] Squared: [ 1, 4, 9, 16, 25 ] Start End This runs after 1 second Number: 1 Number: 2 Number: 3 Number: 4 Number: 5 Mapped: [ 2, 4, 6, 8, 10 ] Filtered: [ 2, 4 ] Success: { data: 'Success!' }

This program demonstrates callback functions in JavaScript.

Callback Function Definition

A callback is a function passed as an argument to another function, to be executed later.

Example 1: Simple Callback

Basic callback pattern:

javascript
function greet(name, callback) {
    console.log("Hello, " + name);
    callback(); // Execute callback
}

Example 2: Callback with Parameters

Pass data to callback:

javascript
function processArray(arr, callback) {
    for (let item of arr) {
        callback(item); // Pass item to callback
    }
}

Example 3: Arrow Function Callbacks

Modern syntax:

javascript
arr.map(num => num * 2);

Example 4: Asynchronous Callbacks

Used with async operations:

javascript
setTimeout(function() {
    console.log("Delayed execution");
}, 1000);

Example 5: Array Method Callbacks

Built-in array methods use callbacks:

forEach(): Iterate
map(): Transform
filter(): Select
reduce(): Accumulate

Example 6: Error Handling Callbacks

Common pattern:

javascript
function fetchData(success, error) {
    if (success) {
        success(data);
    } else {
        error(message);
    }
}

When to Use:

Event handlers
Array operations
Asynchronous operations
API calls
Custom iteration

Callback Hell Problem:

Nested callbacks become hard to read:

javascript
func1(function() {
    func2(function() {
        func3(function() {
            // Hard to read!
        });
    });
});

Solutions:

Promises
Async/await
Named functions instead of anonymous

Output