JavaScript

A callback is a function passed as an argument to another function. It allows you to run code after something else has finished executing, making JavaScript highly flexible and asynchronous.

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🧠 What is a Callback?

In simple terms:

A callback is a function that is passed to another function and is executed later, often after an operation completes.

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✅ Basic Syntax

function greeting(name) {
  console.log('Hello, ' + name);
}

function processUserInput(callback) {
  const name = 'Alice';
  callback(name);
}

processUserInput(greeting); // Output: Hello, Alice

• greeting is the callback function.
• It is passed to processUserInput and called inside that function.

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🕰️ Why Use Callbacks?

JavaScript is asynchronous. Callbacks let us handle tasks like:

• API requests
• File reading
• User interactions
• Timers

Example with setTimeout:

setTimeout(() => {
  console.log("This runs after 2 seconds");
}, 2000);

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🎯 Custom Example: Callbacks for Tasks

function doHomework(subject, callback) {
  console.log(`Starting my ${subject} homework.`);
  callback();
}

function finished() {
  console.log('I have finished my homework!');
}

doHomework('math', finished);
// Output:
// Starting my math homework.
// I have finished my homework!

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🔄 Synchronous vs Asynchronous Callbacks

Synchronous Callback:

Runs immediately during function execution.

function logMessage(callback) {
  console.log('Before callback');
  callback();
  console.log('After callback');
}

logMessage(() => {
  console.log('Inside callback');
});

Asynchronous Callback:

Runs later, after an async operation finishes.

function fetchData(callback) {
  setTimeout(() => {
    console.log('Data fetched');
    callback();
  }, 1000);
}

fetchData(() => {
  console.log('Callback after fetch');
});

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⚠️ Callback Hell

When callbacks are nested too deeply, it becomes hard to read and maintain:

login(user, () => {
  getProfile(user, () => {
    updateSettings(user, () => {
      console.log("All done!");
    });
  });
});

➡ This is called Callback Hell or the Pyramid of Doom.

✅ Solution: Use Promises or async/await for better flow control.

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🔚 Summary

Concept	Description
Callback	A function passed to another function
Use case	Async operations like API calls, timers
Common issue	Callback Hell (deep nesting)
Modern solution	Promises & async/await

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