# What is callback ?

In JavaScript, a callback is a function that is passed as an argument to another function and is intended to be executed after the completion of a specific task or at a particular event. Callbacks are commonly used in asynchronous programming, event handling, and other scenarios where you want to execute code in response to some external trigger.

let's see an example:

```javascript
function getAnswer (num, callback) {
  return callback(num)
}
function multiply (num) {
  return num*num
}

let ans = getAnswer(10, multiply)
console.log(ans);

//Output
// 100
```

In the provided code, the `getAnswer` function takes two parameters: `num` and `callback`. It expects a numerical value (`num`) and a callback function (`callback`). The `getAnswer` function then calls the provided callback function with the given numerical value (`num`) and returns the result.

The `multiply` function is a simple callback function that takes a number and returns the square of that number (`num * num`).

`multiply`: Takes a number as an argument and returns its square.

```javascript
function multiply(num) {
  return num * num;
}
```

`getAnswer`: Takes a number (`num`) and a callback function (`callback`). It calls the callback with the provided number and returns the result.

```javascript
function getAnswer(num, callback) {
  return callback(num);
}
```

The `getAnswer` function is invoked with the numerical value `10` and the `multiply` function as the callback.

```javascript
let ans = getAnswer(10, multiply);
```

The `getAnswer` function calls the provided callback function (`multiply`) with the argument `10`.

```javascript
return callback(num);
```

The `multiply` function is executed with `num` set to `10`, resulting in the square of `10` (`10 * 10`), which is `100`.

The result (`100`) returned by the callback function is assigned to the variable `ans`.

```javascript
let ans = getAnswer(10, multiply);
```

Finally, the result (`ans`) is logged to the console.

```javascript
console.log(ans); // Output: 100
```

the `getAnswer` function serves as a higher-order function that takes a numerical value and a callback function. It uses the callback function to perform a specific operation.

### Callbacks in Event Handling:

```javascript
// Example of using a callback for event handling
const btn = document.getElementById('myButton');

btn.addEventListener('click', function() {
  console.log("Button clicked!");
});
```

In this example, the anonymous function passed as a callback is executed when the 'click' event occurs on the button element.