Dates, Math & Numbers

The standard library ships with powerful built-ins for the three things every app eventually needs: dates (timestamps, formatting, diffs), math (rounding, randomness, ranges) and number formatting (currency, percentages, precision). Knowing these saves you from installing a library for tasks one line long.

Dates: creating

const now  = new Date();                  // current date-time (local timezone)
const ymd  = new Date(2026, 2, 5);        // 5 March 2026 — month is 0-indexed!
const iso  = new Date('2026-03-05T10:30:00Z');
const fromMs = new Date(1772889000000);   // from epoch milliseconds

console.log(now.getFullYear());           // → 2026
console.log(now.getMonth());              // → 0-11 (January = 0 😬)
console.log(now.getDate());               // → day of month
console.log(now.getHours());              // → local hour
console.log(now.toISOString());           // → '2026-03-05T10:30:00.000Z' (UTC, standard)

The zero-based month is JavaScript's most famous date gotcha. toISOString() always gives UTC — the format APIs and servers expect.

Dates: comparing, diffing, adding

const start = new Date('2026-01-01');
const end   = new Date('2026-03-05');

// difference — dates convert to epoch milliseconds in arithmetic
const days = Math.round((end - start) / 86_400_000);
console.log(days);                          // → 63

// add 7 days
const next = new Date(start);
next.setDate(next.getDate() + 7);

// comparisons
console.log(end > start);                   // → true (value comparison works)
console.log(start.getTime());               // → raw number for exact work

Rule: do date math in numbers (getTime(), milliseconds), never by parsing display strings.

Formatting dates

const d = new Date(2026, 2, 5);

console.log(d.toLocaleDateString('en-IN', { weekday: 'long', day: 'numeric', month: 'long', year: 'numeric' }));
// → 'Wednesday, 5 March 2026' (locale-aware)

console.log(d.toLocaleTimeString('en-IN', { hour: '2-digit', minute: '2-digit' }));
// → '05:30 pm' style per locale

const fmt = new Intl.DateTimeFormat('en-IN', { dateStyle: 'medium' });
console.log(fmt.format(d));                 // → '5 Mar 2026'

Prefer Intl over hand-rolled concatenation — it handles locale ordering, month names and numerals. For storage and APIs, keep ISO strings; format only at the display edge.

Math: the essentials

console.log(Math.floor(4.9));    // → 4  (round down)
console.log(Math.ceil(4.1));     // → 5  (round up)
console.log(Math.round(4.5));    // → 5  (half up)
console.log(Math.trunc(-4.9));   // → -4 (drop decimals, keep sign)
console.log(Math.abs(-7));       // → 7
console.log(Math.min(3, 9, 1));  // → 1
console.log(Math.max(3, 9, 1));  // → 9
console.log(Math.pow(2, 10));    // → 1024 (or 2 ** 10)
console.log(Math.sqrt(81));      // → 9
console.log(Math.hypot(3, 4));   // → 5 (right-triangle distance)

Random numbers

console.log(Math.random());                     // → 0 ≤ x < 1

// integer in [min, max] inclusive
const randInt = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;
console.log(randInt(1, 6));                     // → dice roll 1-6

// pick from a list
const pick = (arr) => arr[Math.floor(Math.random() * arr.length)];

Math.random() is not cryptographic — for tokens, IDs or anything security-relevant use crypto.randomUUID() / crypto.getRandomValues().

Numbers: precision and formatting

console.log((0.1 + 0.2).toFixed(2));   // → '0.30' (string! — remember float quirks)
console.log(Number((0.1 + 0.2).toFixed(2)) === 0.3);  // → true after rounding

console.log(parseInt('42abc', 10));    // → 42 (base 10 explicit)
console.log(parseFloat('3.5m'));       // → 3.5
console.log(Number('  12  '));         // → 12 (trims); Number('') is 0 ⚠️

console.log(Number.isNaN(NaN));        // → true (strict — unlike global isNaN)
console.log(Number.isFinite(1 / 0));   // → false
console.log((1234567).toString());     // → '1234567'
console.log((255).toString(16));       // → 'ff' (any radix 2-36)

Intl.NumberFormat: currency and percent done right

const inr = new Intl.NumberFormat('en-IN', { style: 'currency', currency: 'INR', maximumFractionDigits: 2 });
console.log(inr.format(123456.789));    // → ₹1,23,456.79 (Indian grouping, automatically)

const usd = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
console.log(usd.format(1999.5));        // → $1,999.50

const pct = new Intl.NumberFormat('en-IN', { style: 'percent', maximumFractionDigits: 1 });
console.log(pct.format(0.0735));        // → 7.4%

Expected output: correct grouping, symbols and decimals for any locale — never hand-concatenate currency symbols, and never toFixed alone for display (it returns a string and ignores locale rules).

Useful number utilities

const clamp = (n, min, max) => Math.min(max, Math.max(min, n));   // pin a slider
const lerp  = (a, b, t) => a + (b - a) * t;                       // interpolate 0-1
console.log(clamp(15, 0, 10));   // → 10
console.log(lerp(0, 100, 0.25)); // → 25

// round to n decimals as a NUMBER
const roundTo = (n, d = 2) => Math.round(n * 10 ** d) / 10 ** d;
console.log(roundTo(1.005));     // → 1 (float reality — check expectations)

Temporal? (what's next, briefly)

The newer Temporal API (stage 3, landing gradually) fixes Date's design flaws with immutable, timezone-aware types (Temporal.PlainDate…). Where available, evaluate it for greenfield date-heavy apps; today Date + Intl + toISOString() remains the universal baseline this course teaches.

Common mistakes

• getMonth() zero-based — 2 means March; add 1 for display, subtract 1 when constructing.
• Manual string date math — parsing 'DD/MM/YYYY' yourself breaks in other locales; use Date/Intl.
• Comparing formatted strings — compare getTime() numbers or ISO strings (both sort correctly), never display strings.
• toFixed for money storage — store minor units (paise) or decimals; format only for display.
• Local vs UTC mix-ups — a date built from '2026-03-05' (no time) parses as UTC midnight, then displays a day earlier in negative offsets; add a time component ('2026-03-05T00:00:00') when you mean “local midnight”.
• Math.random() for secrets — predictable; use crypto.

Practice checklist

1. Write timeAgo(date): “just now”, “5m ago”, “3h ago”, “2d ago”.
2. Format a list of prices with Intl.NumberFormat in two locales and compare outputs.
3. Build a randInt-based dice roller UI that rolls on click (DOM + events + math).
4. Compute the number of days between two user-picked dates with <input type="date">.

These built-ins complete your toolkit. Final chapter: pulling everything together — JavaScript Best Practices & Where to Go Next.