call back

🔹 What is a Callback?

A callback is simply a function passed as an argument to another function, so that it can be executed later (or “called back”) once some task is done.

👉 In short: A function inside another function.

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🔹 Simple Example

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

function sayBye() {
  console.log("Goodbye!");
}

greet("Amit", sayBye);

Output:

Hello, Amit
Goodbye!

Here:

• sayBye is passed as a callback to greet.

• After greeting, greet executes the callback.

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🔹 Anonymous Callback

Instead of defining a separate function, you can pass an anonymous function:

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

greet("Amit", function() {
  console.log("Goodbye!");
});

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🔹 Callbacks with setTimeout

Callbacks are very common in asynchronous JavaScript:

console.log("Start");

setTimeout(function() {
  console.log("Callback executed after 2 seconds");
}, 2000);

console.log("End");

Output:

Start
End
Callback executed after 2 seconds

👉 Notice how setTimeout doesn’t block the program. The callback runs later.

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🔹 Callback Example with Data

function getUser(id, callback) {
  setTimeout(() => {
    console.log("Fetched user from database");
    callback({ id: id, name: "Amit" });
  }, 1000);
}

getUser(101, function(user) {
  console.log("User received:", user);
});

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🔹 The Problem: Callback Hell 😵

When many callbacks are nested, code becomes messy:

getUser(101, function(user) {
  getPosts(user.id, function(posts) {
    getComments(posts[0].id, function(comments) {
      console.log(comments);
    });
  });
});

This is called callback hell → solved by Promises and async/await.

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✅ So, in summary:

• Callback = function passed as argument.

• Useful for async tasks (e.g., reading files, DB calls, API requests).

• But too many nested callbacks → callback hell → solved by Promises/async-await.