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JavaScript Numbers

How JavaScript Counts

JavaScript Numbers

JavaScript has one number type for both integers and decimals. Learn its quirks — Infinity, NaN, and floating-point precision.

5 min read Level 2/5 #numbers#floating-point#NaN
What you'll learn
  • Write integer and decimal number literals
  • Recognize `NaN` and `Infinity`
  • Understand why `0.1 + 0.2 !== 0.3`

JavaScript has one number type for both integers and decimals. There’s no separate int or float.

Number literals script.js 
console.log(42);          // integer
console.log(3.14);        // decimal
console.log(-7);          // negative
console.log(1e6);         // 1,000,000 (scientific notation)
console.log(0xff);        // hex — 255
console.log(0b1010);      // binary — 10
console.log(0o17);        // octal — 15
console.log(1_000_000);   // numeric separators (ES2021) — same as 1000000
▶ Preview: console

Special Number Values

JavaScript reserves a few special values within the number type.

Infinity

What you get when a number goes too big — or you divide by zero.

Infinity script.js 
console.log(1 / 0);            // Infinity
console.log(-1 / 0);           // -Infinity
console.log(Number.MAX_VALUE * 2); // Infinity
console.log(Infinity + 1);     // Infinity (still)
▶ Preview: console

NaN (Not a Number)

You get NaN when a math operation doesn’t make sense.

NaN script.js 
console.log("hello" * 2);    // NaN
console.log(Math.sqrt(-1));  // NaN
console.log(0 / 0);          // NaN

// NaN is the one value not equal to itself:
console.log(NaN === NaN);    // false
console.log(Number.isNaN(NaN)); // true  ← use this to test
▶ Preview: console

Floating-Point Precision

This catches every JavaScript beginner exactly once.

Floating-point math script.js 
console.log(0.1 + 0.2);          // 0.30000000000000004
console.log(0.1 + 0.2 === 0.3);  // false 😱
▶ Preview: console

This isn’t a JavaScript bug — it’s how almost every programming language stores decimal numbers (IEEE-754). Numbers like 0.1 can’t be represented exactly in binary, so tiny rounding errors creep in.

For most code, this doesn’t matter. For money or anything where exactness counts:

  • Multiply to work in cents: store $1.10 as 110.
  • Use Number.EPSILON for “close enough” comparisons.
  • For truly arbitrary precision, use BigInt (next-but-one lesson).
Comparing with EPSILON script.js 
const a = 0.1 + 0.2;
const b = 0.3;

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

Safe Integer Range

Regular number values are accurate up to Number.MAX_SAFE_INTEGER (about 9 quadrillion). Beyond that, integers start losing precision.

Past the safe integer limit script.js 
console.log(Number.MAX_SAFE_INTEGER);     // 9007199254740991
console.log(Number.MAX_SAFE_INTEGER + 1); // 9007199254740992 ✅
console.log(Number.MAX_SAFE_INTEGER + 2); // 9007199254740992 😱 — same!
▶ Preview: console

For bigger integers, use BigInt.

Up Next

Now: the methods for converting strings to numbers, formatting, and checking.

JavaScript Number Methods →