Number

JavaScript · Reference cheat sheet

📋 Overview

number is IEEE-754 double precision. Integers are safe up to Number.MAX_SAFE_INTEGER (2^53 - 1). Use BigInt for larger integers. Prefer Number static methods over global parseInt / isNaN when possible.

🔧 Core concepts

• Literals: 42, 3.14, 0xff, 0b1010, 1e3, separators 1_000_000.
• Constants: MAX_VALUE, MIN_VALUE, EPSILON, NaN, POSITIVE_INFINITY.
• Check: Number.isFinite, Number.isInteger, Number.isSafeInteger, Number.isNaN.
• Parse: Number(text), Number.parseInt(text, radix), Number.parseFloat.
• Format: toFixed, toPrecision, toString(radix), toLocaleString, Intl.NumberFormat.

Number.isNaN(Number("x")); // true
Number.parseInt("10px", 10); // 10

💡 Examples

const n = 1_000_000.5;
n.toFixed(2); // "1000000.50"

// Safe integer check
function addSafe(a, b) {
  const s = a + b;
  if (!Number.isSafeInteger(s)) throw new RangeError("unsafe");
  return s;
}

// Epsilon compare
function nearlyEqual(a, b, eps = Number.EPSILON) {
  return Math.abs(a - b) < eps;
}
nearlyEqual(0.1 + 0.2, 0.3);

// Parse with radix
Number.parseInt("08", 10); // 8
Number.parseInt("ff", 16); // 255

// Locale currency
new Intl.NumberFormat("en-US", {
  style: "currency",
  currency: "USD",
}).format(19.9);

// BigInt interop
const big = 10n;
Number(big); // 10 — may lose precision for huge BigInts
BigInt(42); // 42n

// Unary plus
+"42"; // 42
+""; // 0
+null; // 0
+undefined; // NaN

// Clamp percent
const pct = Math.min(100, Math.max(0, value));

⚠️ Pitfalls

• Global isNaN coerces: isNaN("foo") === true; use Number.isNaN.
• parseInt without radix can surprise on leading zeros in old engines — always pass radix.
• == coerces strings/numbers; use === after explicit parse.
• Floating point drift — don’t use binary floats for money.
• typeof NaN === "number".

🔗 Related

• math.md — Math helpers
• boolean.md — falsy 0 / NaN
• strings.md — parsing from text
• json.md — number serialization
• date.md — timestamps