Symbols & Well-Known Symbols

1// Basic Symbols
2const id1 = Symbol("id");
3const id2 = Symbol("id");
4console.log(id1 === id2);  // false (every Symbol is unique)
5console.log(typeof id1);   // "symbol"
6
7// As property keys (no collisions!)
8const SECRET = Symbol("secret");
9const user = {
10  name: "Alice",
11  [SECRET]: "hidden-value"
12};
13console.log(user[SECRET]);     // "hidden-value"
14console.log(Object.keys(user)); // ["name"] — Symbols are hidden!
15console.log(JSON.stringify(user)); // '{"name":"Alice"}' — excluded!
16
17// Global Symbol registry
18const s1 = Symbol.for("shared");
19const s2 = Symbol.for("shared");
20console.log(s1 === s2);  // true (shared via registry)
21console.log(Symbol.keyFor(s1)); // "shared"
22
23// Well-known Symbols
24class Money {
25  constructor(amount, currency) {
26    this.amount = amount;
27    this.currency = currency;
28  }
29  // Custom string conversion
30  [Symbol.toPrimitive](hint) {
31    if (hint === "number") return this.amount;
32    if (hint === "string") return `${this.currency}${this.amount}`;
33    return this.amount;
34  }
35  // Custom toString tag
36  get [Symbol.toStringTag]() {
37    return "Money";
38  }
39}
40const price = new Money(42, "$");
41console.log(+price);           // 42
42console.log(`${price}`);      // "$42"
43console.log(Object.prototype.toString.call(price)); // "[object Money]"
44
45// Symbol.iterator (make custom objects iterable)
46class Range {
47  constructor(start, end) {
48    this.start = start;
49    this.end = end;
50  }
51  [Symbol.iterator]() {
52    let current = this.start;
53    const end = this.end;
54    return {
55      next() {
56        return current <= end
57          ? { value: current++, done: false }
58          : { done: true };
59      }
60    };
61  }
62}
63console.log([...new Range(1, 5)]); // [1, 2, 3, 4, 5]