Simple example of a floating point precision error for monetary amounts

Most software engineers are aware that floating point numbers are not suitable for monetary amounts, or any amount requiring reliable decimal precision.

It’s nice to have a quick and simple example of this, so here’s an operation that will break with floating point numbers in many languages including JavaScript:

1.01 - 0.42

// → 0.5900000000000001

In JavaScript and TypeScript, the big.js library is good for decimal operations.

In SQL, it’s best to store monetary and other decimal amounts as DECIMAL, e.g. DECIMAL(10,2).

Note that there’s a common misconception that you shouldn’t use the number type in JSON for decimal amounts, due to these precision errors. However, JSON itself is just a string, so despite the JavaScript Number type being a floating point implementation, it’s not accurate to describe JSON numbers as being floating point. They’re just strings.

Floating point precision issues only occur when a decimal number is operated on naively in JavaScript other languages. Sending a decimal representation over the wire as {"fooNumber" 123.45} is fine so long as the receiver handles it correctly.

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