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**A higher-order function** is a function that **takes another function as an argument** or **returns a function as its result**.

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### Detailed explanation

In JavaScript, functions are **first-class citizens**,
which means they can be:

- passed as values,
- stored in variables,
- returned from other functions.

Functions that use other functions this way are called **higher-order**.

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### Code examples

#### 1. Takes a function as an argument

```javascript
function repeat(n, action) {
  for (let i = 0; i < n; i++) {
    action(i);
  }
}

repeat(3, console.log);
// 0
// 1
// 2
```

`repeat` is a higher-order function because it calls the passed-in `action` function.

---

#### 2. Returns a new function

```javascript
function multiplier(factor) {
  return function (num) {
    return num * factor;
  };
}

const double = multiplier(2);
console.log(double(5)); // 10
```

`multiplier` is a higher-order function because it returns another function.

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#### 3. Built-in examples in JS

Many array methods are higher-order functions:

```javascript
[1, 2, 3].map(x => x * 2);     // takes a function
[1, 2, 3].filter(x => x > 1);  // takes a function
[1, 2, 3].reduce((a, b) => a + b, 0); // takes a function
```

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### Why they are needed

They increase **flexibility and code reuse**.
They let you implement **abstractions**, for example handling collections, events, and so on.
They are the core tool of **functional programming**.

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### Summary

| Property | Description |
|---|---|
| What it does | Takes or returns functions |
| Examples | `map`, `filter`, `reduce`, `setTimeout`, `addEventListener` |
| Benefits | Logic reuse, more compact code |
| Key idea | Functions are data, just like numbers or strings |

---

**In one phrase:**

> A higher-order function is a function that **treats other functions as data**,
> taking them as arguments or returning them as a result.

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