Numbers

Every non-BigInt number in JavaScript is a 64-bit float - understanding the precision limits prevents hard-to-debug arithmetic bugs.

Every JavaScript number is a 64-bit IEEE 754 floating-point value. There is no separate integer type - which means integers above 2⁵³ lose precision, and decimal arithmetic has well-known rounding quirks.

The classic float trap: 0.1 + 0.2 does not equal 0.3. Compare floating-point numbers using a tolerance instead of strict equality.

console.log(0.1 + 0.2 === 0.3); // false
console.log(0.1 + 0.2); // 0.30000000000000004
 
// Safe float comparison
const a = 0.1 + 0.2;
const b = 0.3;
console.log(Math.abs(a - b) < Number.EPSILON); // true

Number.MAX_SAFE_INTEGER is the largest integer that can be represented exactly. Beyond it, incrementing no longer works reliably.

console.log(Number.MAX_SAFE_INTEGER); // 9007199254740991
 
const n = Number.MAX_SAFE_INTEGER;
console.log(n + 1 === n + 2); // true - precision lost

BigInt handles arbitrarily large integers exactly. Use the n suffix for literals. BigInt and Number cannot be mixed in arithmetic - you must convert explicitly.

const big = 9007199254740991n + 1n;
console.log(big); // 9007199254740992n
 
console.log(10n * 2n); // 20n
// console.log(10n + 1); // TypeError: Cannot mix BigInt and other types

In production

Never store monetary values as Number. Representing $19.99 as 19.99 and doing arithmetic on it will produce rounding errors that compound across transactions. Store money as integer cents (1999) or use a decimal library like decimal.js. Payment systems that let floats drift ship silent bugs that accounting finds months later.

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