Weird Math in JavaScript

Overcoming common calculation errors with simple techniques and libraries

Simple math isn’t always as simple as it seems.

Photo by iSawRed on Unsplash

Remember the solar calculator? The one you can buy for less than 1 euro? That seemingly outdated piece of technology is actually better at math than Node.js.

A well-known example is:

0.1 + 0.2 = 0.30000000000000004

I won’t dive into the technical details of why this happens. Instead, I’ll explain two straightforward ways to solve this issue:

1. Using integers
2. Using a library

Using Integers

Convert the numbers to integers before performing the calculation:

(0.1 * 100 + 0.2 * 100) / 100 = 0.3

Using a Library

While you can do math without a library, in some applications, precision is critical. To simplify the task, I recommend using mathjs. It's as simple as using an old solar calculator.

Get devinred 🎈’s stories in your inbox

Join Medium for free to get updates from this writer.

Remember me for faster sign in

First, install and configure mathjs:

import { create, all } from 'mathjs'

const config = {
  number: 'BigNumber',
  precision: 20
}
const mathjs = create(all, config)

Then, you can perform calculations by evaluating expressions as strings:

mathjs.evaluate('0.1 + 0.2'); // Returns 0.3

If you have variables, it’s just as easy:

let a = 0.1;
mathjs.evaluate(`${a} + 0.2`); // Returns 0.3

And that’s all there is to it.