# Higher Order Function in JS

In JavaScript, a higher-order function is a function that takes one or more functions as arguments and/or returns a function as its result. This means that higher-order functions can operate on functions themselves, treating them as values.

One example of a higher-order function is the `setTimeout` function, which is used to execute a function after a specified amount of time has elapsed. Here's an example:

```javascript
function sayHi() {
  console.log("Hi, Abdullah");
}

setTimeout(sayHI, 2000); // calls the sayHello function after 2 second
```

In this code, the `setTimeout` function takes the `sayHello` function as its first argument, and the number of milliseconds to wait as its second argument. This means that `setTimeout` is a higher-order function because it takes a function as an argument.

Another example of a higher-order function is the `Array.prototype.filter` function. This function takes a predicate function as an argument and returns a new array containing only the elements of the original array for which the predicate function returns `true`. Here's an example:

```javascript
var numbers = [1, 2, 3, 4, 5, 6];

function isEven(num) {
  return num % 2 === 0;
}

var evens = numbers.filter(isEven);

console.log(evens); // outputs [2, 4, 6]
```

In this code, the `filter` function takes the `isEven` function as its argument. The `isEven` function is a predicate function that returns `true` if its argument is even. The `filter` function applies the `isEven` function to each element in the `numbers` array and returns a new array containing only the elements for which the `isEven` function returned `true`. This means that `filter` is also a higher-order function because it takes a function as an argument.