# What is a pure function?

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:

// 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.