JavaScript — Number Methods

Formatting for humans

const price = 1234.5;

price.toFixed(2);              // "1234.50"  ← STRING, always
price.toFixed(0);              // "1235"
(3.14159).toFixed(3);          // "3.142"

toFixed rounds then renders — the go-to for money display:

const total = 19.999;
console.log(`$${total.toFixed(2)}`);   // $20.00

Trap: it returns a string. total.toFixed(2) + 1 gives "20.001". Convert back with Number(...) if math must continue.

Locale-aware formatting — the upgrade

price.toLocaleString("en-US");    // "1,234.50"
price.toLocaleString("de-DE");    // "1.234,50"
price.toLocaleString("en-US", { style: "currency", currency: "USD" });
// "$1,234.50"

const big = 9876543210;
big.toLocaleString();             // "9,876,543,210"

Thousand separators and currency symbols handled per locale — no manual comma surgery.

Converting number → string

(42).toString();     // "42"
(42).toString(2);    // "101010" — binary! (radix argument)
(255).toString(16);  // "ff"     — hex
String(42) === (42).toString();   // true — same result, global fn style

String → number family

Call	Input	Result	Notes
Number(s)	"42" / "42x"	42 / NaN	strict whole-string
parseInt(s)	"42px" / "42.9"	42 / 42	leading digits only
parseFloat(s)	"3.5em"	3.5	keeps decimals
+s	"42"	42	unary plus = strict shortcut

parseInt("08");            // 8
parseInt("0x1F");          // 31 — auto-hex detection!
parseFloat("50%")          // 50 — ignores trailing junk
Number("")                 // 0  😐 quirk
Number(null)               // 0  quirk
Number(undefined)          // NaN

Useful static checks

Number.isInteger(10)      // true
Number.isInteger(10.5)    // false
Number.isFinite(1/0)      // false — Infinity rejected
Number.isNaN(NaN)         // true  — the safe NaN test
Number.MAX_SAFE_INTEGER   // 9007199254740991 — beyond this, use BigInt

Rounding beyond toFixed

Math.round(2.5)     // 3   half-up
Math.floor(-2.5)    // -3
Math.ceil(2.1)      // 3
// round to nearest 0.05 (pricing steps):
Math.round(7.23 * 20) / 20;   // 7.25

Chaining a real formatter

function formatEUR(n) {
    return n.toLocaleString("en-IE", { style: "currency", currency: "EUR" });
}
formatEUR(1234567.891);   // €1,234,567.89

Mini Practice

1. Render five prices with toFixed(2); prove typeof stays string
2. Localize one number in three locales via toLocaleString
3. Parse "12kg", "12.7km", "x12" — match each to its parser
4. Binary/hex round-trips: (255).toString(16) → back via parseInt(_,16)
5. Build formatPercent(0.876) → "87.6%" using toFixed + concat

Next: random →