JavaScript Symbols: Unique Primitive Values and Object Keys

const sym = Symbol();
// ✅ a fresh unique symbolconst named = Symbol("myKey");     // ✅ with an optional description (a label for YOU)
console.log(typeof sym);
// "symbol" — the seventh primitive ✅console.log(named);                // Symbol(myKey) — the description showsconsole.log(named.description);    // "myKey" — read the label back

const s1 = Symbol("a");
const s2 = Symbol("a");
console.log(s1 === s2);
// false — ALWAYS unique, descriptions don't matterconsole.log(s1 == s2);         // false — even loose equality fails (Lesson 6!)
// ⚠️ Important limits:new Symbol();                  // ❌ TypeError — never with new!s1 + "";                       // ❌ TypeError — symbols don't auto-convert to stringsString(s1);                    // ✅ "Symbol(a)" — explicit conversion works

const libraryUser = {
  name: "Priya", id: "LIB-123" // 🏛️ the library's property};
  // 😱 YOUR code adds an "id" too…libraryUser.id = "MY-APP-456";
  console.log(libraryUser.id);
  // "MY-APP-456" — 💥 you just destroyed the library's data!

const libraryUser = {
  name: "Priya", id: "LIB-123" // 🏛️ library's string key — safe};
  const myId = Symbol("id");
  // ❄️ YOUR unique keylibraryUser[myId] = "MY-APP-456";     // bracket notation — Lesson 47!
  console.log(libraryUser.id);
  // "LIB-123" — library's data intact ✅console.log(libraryUser[myId]);       // "MY-APP-456" — yours coexists peacefully ✅

const ID = Symbol("id");
const SCORE = Symbol("score");
const player = {
  name: "Arjun", // normal string keys
[ID]: "PLYR-001", // ❄️ computed Symbol key (note the brackets!)
[SCORE]: 9500
};
// Read them with brackets (the ONLY way — dots can't reach symbols):
console.log(player[ID]);
// "PLYR-001" ✅
console.log(player[SCORE]);
// 9500 ✅
console.log(player.ID);
// undefined — dot notation looks for a string "ID" ❌

console.log(Object.keys(player));
// ["name"] — symbols hidden! 🫥
console.log(JSON.stringify(player));
// {"name":"Arjun"} — hidden from JSON too!
for (const key in player) {
  console.log(key);
  // only "name" — for…in skips symbols
}// 🔍 But they're NOT secret — here's how to find them:
console.log(Object.getOwnPropertySymbols(player));
// [Symbol(id), Symbol(score)] ✅

const a = Symbol.for ("app.id");
// create-or-fetch from the GLOBAL registry
const b = Symbol.for ("app.id");
// same description → SAME symbol!
console.log(a === b);
// true ✅ — registry symbols are shared
// And reverse-lookup:
console.log(Symbol.keyFor(a));
// "app.id" — get the registry name back