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+---
+title: What is a pure function?
+type: question
+tags: javascript,function
+authors: chalarangelo
+cover: blog_images/dark-leaves.jpg
+excerpt: Pure functions are a very important concept to know, especially if you're interested in functional programming.
+firstSeen: 2021-12-19T05:00:00-04:00
+---
+
+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.