Chalarangelo/30-seconds-of-code

---
title: What is a pure function?
shortTitle: Pure function introduction
type: question
language: javascript
tags: [function]
cover: dark-leaves
excerpt: Pure functions are a very important concept to know, especially if you're interested in functional programming.
listed: true
dateModified: 2021-12-19
---

Pure functions are one of the most important concepts to learn and understand, especially if you're interested in functional programming.

A pure function is a function that satisfies the following two conditions:

- Given the **same input**, it always returns the **same output**.
- Causes **no side effects** outside the function's scope.

Let's look at some examples of pure and impure functions:

```js
// Pure
const add = (x, y) => x + y;
const concat = (arr, value) => [...arr, value];
const order = arr => [...arr].sort((a, b) => a - b);

// Impure
const addRandom = x => x + Math.random();
const pushConcat = (arr, value) => { arr.push(value); return arr; }
const reorder = arr => arr.sort((a, b) => a - b);
```

Understanding if a function is pure or impure is generally pretty easy. An easy trick to tell if a function is impure is if it makes sense to call it without using its return value or if it doesn't return any. These usually indicate that a function causes side effects.

Pure functions are more **reliable and reusable**, which in turn makes them **easier to reason about**. It's usually preferable to use a pure function over an impure one if constraints allow it.