What is a callback function and what are common use cases? Callback function क्या है और common use cases क्या हैं?

A callback function is a function that is passed as an argument to another function, and the receiving function will 'call back' (execute) the callback at some point during its execution.

// Simple callback example
function greet(name, callback) {
    console.log('Hello ' + name);
    callback(); // 'calling back' the callback function
}

function sayGoodbye() {
    console.log('Goodbye!');
}

greet('John', sayGoodbye);
// Output:
// Hello John
// Goodbye!

// Callback with arguments
function add(a, b, callback) {
    const result = a + b;
    callback(result); // Passing result to callback
}

add(5, 3, function(sum) {
    console.log('Sum is: ' + sum); // Sum is: 8
});

// Array methods with callbacks
const numbers = [1, 2, 3, 4, 5];

// forEach callback
numbers.forEach(function(num) {
    console.log(num * 2); // 2, 4, 6, 8, 10
});

// map callback
const doubled = numbers.map(function(num) {
    return num * 2;
}); // [2, 4, 6, 8, 10]

// filter callback
const evenNumbers = numbers.filter(function(num) {
    return num % 2 === 0; // [2, 4]
});

// Asynchronous callback - setTimeout
console.log('Start');

setTimeout(function() {
    console.log('After 2 seconds');
}, 2000);

console.log('End');
// Output:
// Start
// End
// After 2 seconds

// Reading file with callback (Node.js)
const fs = require('fs');

fs.readFile('file.txt', 'utf8', function(err, data) {
    if (err) {
        console.log('Error reading file:', err);
    } else {
        console.log('File content:', data);
    }
});

// AJAX request with callback
function fetchData(url, callback) {
    fetch(url)
        .then(response => response.json())
        .then(data => callback(data))
        .catch(error => callback(null, error));
}

fetchData('/api/users', function(data, error) {
    if (error) {
        console.log('Error:', error);
    } else {
        console.log('Users:', data);
    }
});

// Callback hell / Pyramid of doom (PROBLEM)
getUser(userId, function(user) {
    getOrders(user.id, function(orders) {
        getOrderDetails(orders[0].id, function(details) {
            getPayment(details.id, function(payment) {
                console.log('Payment:', payment);
            });
        });
    });
});

// Solution: Use Promises or async/await
// Modern approach with async/await
async function getPaymentInfo(userId) {
    try {
        const user = await getUser(userId);
        const orders = await getOrders(user.id);
        const details = await getOrderDetails(orders[0].id);
        const payment = await getPayment(details.id);
        return payment;
    } catch (error) {
        console.error('Error:', error);
    }
}

// Higher-order function with callback
function doubleAsync(number, callback) {
    setTimeout(function() {
        callback(number * 2);
    }, 1000);
}

doubleAsync(5, function(result) {
    console.log('Result:', result); // Result: 10 (after 1 second)
});

Interview tip: Mention that callbacks are fundamental to JavaScript's asynchronous programming model. Discuss callback hell (pyramid of doom) and why Promises and async/await are now preferred. Explain that callbacks are still used in array methods (map, filter, forEach) even though they're not asynchronous.