JavaScript Floating Point Numbers

Run this and be surprised:

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console.log(0.1 + 0.2);          // → 0.30000000000000004
console.log(0.1 + 0.2 === 0.3);  // → false

This is not a JavaScript bug. Python, Java and most languages show the same result. Computers store numbers in binary, and some decimals (like 0.1) cannot be written exactly in binary, just as 1/3 cannot be written exactly in decimal (0.3333…). The stored value is extremely close, but not exact.

Comparing decimals

Never compare two calculated decimals with ===. Check if the difference is tiny.

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const a = 0.1 + 0.2;
const b = 0.3;

console.log(Math.abs(a - b) < Number.EPSILON); // → true

Rounding for display

If you only need to show the number, round it.

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const total = 0.1 + 0.2;
console.log(total.toFixed(2));  // → 0.30
console.log(Number(total.toFixed(2)) === 0.3); // → true

Money

Do not store money as decimals like 19.99. Store whole units of the smallest coin (cents) and divide only for display.

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const priceInCents = 1999;
const quantity = 3;
const totalInCents = priceInCents * quantity;

console.log(totalInCents);           // → 5997
console.log((totalInCents / 100).toFixed(2)); // → 59.97

Whole numbers stay exact up to a very large size, so the calculation is safe. For display with currency symbols, use Intl.NumberFormat:

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const money = new Intl.NumberFormat("en-US", { style: "currency", currency: "USD" });
console.log(money.format(59.97)); // → $59.97

Very large and very small values

Numbers above Number.MAX_SAFE_INTEGER lose accuracy. See BigInt.