How to solve JavaScript precision loss issue using Decimal.js

It’s not a bug. It’s a feature.

How you might feel when you encounter this

If you’ve ever built a shopping cart in JavaScript, you might have noticed something odd. Your total price sometimes looks slightly off.
At first the total amount is $60, then you remove an item worth $0.10, and suddenly your total becomes $59.89999999999999 instead of $59.9.

Say hello to JavaScript’s floating-point precision loss.

The problem: floating-point math in JavaScript

JavaScript uses 64-bit floating-point numbers (IEEE 754 standard) to represent all numeric values. This means certain decimal numbers, like 0.1 or 0.2, can’t be represented exactly in binary form.

That face when 0.1 + 0.2 ≠ 0.3

This tiny difference might seem harmless…until you’re recalculating totals in a shopping carts, payment systems, invoice apps, or any sort of scientific data that needs absolute precision.

The real-world example: shopping cart total price recalculation

Let’s go back to the shopping cart example.
Imagine a cart where users can add or remove items. Let’s say our cart total is calculated in JavaScript like this:

let total = 60;
const itemPrice = 0.1;

total -= itemPrice;
console.log(total); // 59.89999999999999

When you remove an item worth $0.10, the result is no longer clean. Remove multiple items and you will end up with even bigger difference.

It’s not a bug in your code, it’s just how floating-point math works under the hood.

But in a real application, these kind of errors can have real-world consequences.

To the rescue: Decimal.js

Decimal.js is a small but powerful library that helps you perform arbitrary-precision decimal arithmetic.

Or in simple terms, it stores numbers as real decimals, not binary floats.

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Go ahead and install it:

npm install decimal.js

In action: how Decimal.js works?

Decimal.js wraps numbers in a Decimal object that maintains exact decimal precision.

All arithmetic operations like addition, subtraction, multiplication, and division are done using its methods instead of native operators.

Here’s the same cart example using Decimal.js:

import Decimal from 'decimal.js';

let total = new Decimal(60);
const itemPrice = new Decimal(0.1);
total = total.minus(itemPrice);
console.log(total.toString()); // 59.9

Clean, precise, and exactly what you expect.

You can even set global precision or rounding behavior:

Decimal.set({ precision: 10, rounding: Decimal.ROUND_HALF_UP });

When to use Decimal.js

Use it whenever your app deals with money, measurements, or scientific data, anywhere precision matters.
If your calculations are purely integers or not financially sensitive, you can stick with regular numbers.

Conclusion

Never let your shopping cart total become 59.89999999999999 again.
Use Decimal.js !

Thanks for reading!

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