Javascript - pure functions

Javascript - pure functions

The why and what

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What is a pure function

A pure function is a function with the following rulset:

  • the function return values are identical for identical parameters
  • the function has zero side effects

If you think back to algebra, you've probably already saw good examples for pure functions.

f(x) = 2x

In this case f function takes an argument called x and it multiplies by two.

To "use" this we simply call our function with a provided value for x:


In algebra and in programming: this function always return identical values for identical parameters and has zero side effects. So checks all the boxes.

Why should you start write pure functions

  • extreme independence: easy to move around, refactor and reorganize if needed. Meaning your software can be more flexible to changes and it will easier to adapt changes in the future
    • used heavily in functional programming
    • easy to test - remember it will always produce the same results with the same parameters
    • generally easier to maintain the codebase


Let's first write our f function in javascript:

const f = (x) => 2*x;

And call it as you'd call a regular function:

f(2); // 4

We have some built-in pure function as well:

Math.max(1,3,5,8,13); // 13

Math.floor( 45.95); //  45
Math.floor( 45.05); //  45
Math.floor(  4   ); //   4
Math.floor(-45.05); // -46
Math.floor(-45.95); // -46

Let's see an impure function and refactor into a pure function:

const addATodo = (todos, todo) => {
  return todos;

const todosInitial = ['hello'];

const todos = addATodo(todosInitial, 'foo');

You might first assume, that it's okay. But js arguments are references, which means that this has a side effect: changing the outer world. Not okay. Let's fix it.

const addATodo = (todos, todo) => {
  const clonedTodos = [...todos];
  return clonedTodos;

const todosInitial = ['hello'];

const todos = addATodo(todosInitial, 'foo');

Now our function doesn't interfere with the outer world, copies the original data and pushes the new todo into our copied array.

Closing thoughts

Hopefully I demonstrated good the benefits of using pure functions. Of course you mix and max pure and impure functions. Let me know if I missed anything important about pure functions 😊