ES6+ Essentials You’ll Use Everywhere

The Patterns You’ll See in Every Codebase

Earlier topics introduced template literals, destructuring, spread, arrows, ?. and ??. This topic rounds out the modern toolkit.

Short-Circuit Patterns

// guard execution
isLoggedIn && showDashboard();

// conditional property (spread tricks)
const payload = {
  name,
  ...(coupon && { coupon }),     // include coupon only if truthy
};

Map — When Object Keys Aren’t Strings

const cache = new Map();
cache.set("/api/users", usersData);
cache.set(someObject, "metadata");     // ANY type as key

cache.get("/api/users");
cache.has("/api/users");    // true
cache.size;
cache.delete("/api/users");

for (const [key, value] of cache) { ... }

Use Map for dynamic key-value data (caches, lookups); plain objects for fixed-shape records.

Set — Uniqueness & Fast Membership

const seen = new Set();
seen.add("ravi");
seen.add("ravi");           // ignored
seen.has("ravi");           // true — O(1)

const unique = [...new Set(emails)];   // classic dedupe

JSON

const json = JSON.stringify({ name: "Ravi", score: 82 });
// '{"name":"Ravi","score":82}'

JSON.stringify(data, null, 2);          // pretty-printed
const obj = JSON.parse(json);

// deep copy idiom (modern version):
const copy = structuredClone(obj);

Useful String Methods

"  hi  ".trim()
"abc".includes("b")            // true
"file.pdf".endsWith(".pdf")    // true
"ha".repeat(3)                 // 'hahaha'
"5".padStart(2, "0")           // '05'
"A-B-C".split("-")             // ['A','B','C']
"hello".replaceAll("l", "L")   // 'heLLo'

Dates

const now = new Date();
now.getFullYear();             // 2026
now.toISOString();             // '2026-06-10T09:30:00.000Z'
Date.now();                    // ms timestamp

// human formatting — use Intl, not string-building
new Intl.DateTimeFormat("en-IN", { dateStyle: "medium" }).format(now);
// '10 Jun 2026'
new Intl.NumberFormat("en-IN", { style: "currency", currency: "INR" }).format(199);
// '₹199.00'

Numbers & Math

Math.round(4.6)    // 5
Math.floor(4.6)    // 4
Math.max(...arr)
Math.random()      // 0 ≤ x < 1
(1234.5678).toFixed(2)   // '1234.57' (a string!)