What Is a Callback Function in JavaScript?

A callback function is a function that is passed as an argument to another function and is executed after some operation has been completed. Callbacks are a core part of JavaScript and are commonly used for asynchronous operations like reading files, making API calls, or handling events.

Basic Example of a Callback

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

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

greet("Alice", sayGoodbye);

In the example above, sayGoodbye is passed as a callback to the greet function. It will run after the greeting is printed.

Why Use Callback Functions?

• To run code **after** another piece of code finishes
• To handle asynchronous tasks like HTTP requests
• To allow functions to be more flexible and reusable

Asynchronous Callback Example

console.log("Start");

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

console.log("End");

Here, the callback function inside setTimeout runs **after** 2 seconds. JavaScript continues execution in the meantime.

Callback in Array Methods

Many array methods use callbacks:

const numbers = [1, 2, 3];

numbers.forEach(function (num) {
  console.log(num * 2);
});

Named vs Anonymous Callbacks

// Named
function logNumber(num) {
  console.log(num);
}
[1, 2, 3].forEach(logNumber);

// Anonymous
[1, 2, 3].forEach(function (num) {
  console.log(num);
});

Callback Hell

Too many nested callbacks can lead to messy code:

doSomething(function (result) {
  doNextThing(result, function (nextResult) {
    doAnotherThing(nextResult, function (finalResult) {
      console.log(finalResult);
    });
  });
});

This is known as callback hell. Modern solutions like Promises and async/await help avoid this.