42. What is a Callback Function?

What is a Callback Function?

A callback function is a function that is passed as an argument to another function and is executed after the completion of that function.

Why Do We Need Callbacks?

1. Asynchronous Execution: Callbacks are used to handle asynchronous operations, such as reading files, making API calls, or handling user inputs without blocking the execution of the code.
2. Control Flow: They allow you to control the flow of execution, making sure certain code executes only after specific operations are complete.
3. Reusability: Callbacks allow you to pass custom logic to a function, making the function more flexible and reusable.

Example 1: Synchronous Callback

In this example, the callback function is executed immediately after the main function finishes execution:

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

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

greet('John', sayGoodbye);

Output:

nginx
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Hello John
Goodbye!

Example 2: Asynchronous Callback

In this example, the callback is used to handle an asynchronous operation:

javascript
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function fetchData(callback) {
    setTimeout(() => {
        console.log('Data fetched');
        callback(); // Execute the callback after the operation is complete
    }, 2000);
}

function processData() {
    console.log('Processing data...');
}

fetchData(processData);

Output:

kotlin
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Data fetched
Processing data...

✅ Explanation:

• In Example 1, sayGoodbye is a synchronous callback — it gets executed immediately after greet finishes.
• In Example 2, processData is an asynchronous callback — it waits for setTimeout to complete before executing.
• This allows you to handle tasks that take time (like API calls) without blocking the rest of your code.

🔥 Why Callbacks Are Useful:

• They enable non-blocking code execution.
• They allow you to define custom behavior that can be executed after an operation completes.
• They improve code organization and modularity.