Symbols & Well-Known Symbols | JS Mastery

Symbols

// Every Symbol is unique:
const a = Symbol("id");
const b = Symbol("id");
console.log(a === b);  // false

// Symbol as object key (no collision):
const ID = Symbol("id");
const user = {
  name: "Alice",
  [ID]: 12345  // symbol key
};
user[ID];  // 12345
Object.keys(user);  // ["name"] — symbols hidden!
JSON.stringify(user);  // {"name":"Alice"} — symbols excluded

// Global symbol registry:
const sym1 = Symbol.for("shared");
const sym2 = Symbol.for("shared");
console.log(sym1 === sym2);  // true

Well-Known Symbols

// Symbol.iterator — make objects iterable:
class Range {
  constructor(start, end) {
    this.start = start;
    this.end = end;
  }
  [Symbol.iterator]() {
    let n = this.start;
    const end = this.end;
    return {
      next() {
        return n <= end
          ? { value: n++, done: false }
          : { value: undefined, done: true };
      }
    };
  }
}
console.log([...new Range(1, 5)]);  // [1,2,3,4,5]
for (const n of new Range(1, 3)) console.log(n);  // 1, 2, 3

// Symbol.toPrimitive — control type conversion:
class Money {
  constructor(amount, currency) {
    this.amount = amount;
    this.currency = currency;
  }
  [Symbol.toPrimitive](hint) {
    if (hint === "number") return this.amount;
    return this.amount + " " + this.currency;
  }
}
const price = new Money(100, "USD");
console.log(+price);        // 100
console.log("" + price);    // "100 USD"

⚡ Key Takeaways

Symbols are always unique — Symbol("x") !== Symbol("x")
Symbol keys are hidden from Object.keys() and JSON.stringify()
Symbol.for() creates/retrieves global symbols — shared across modules
Well-known symbols customize built-in JS behavior
Symbol.iterator makes custom objects work with for...of and spread

🎯 Practice Exercises

EXERCISE 1

Add Symbol.iterator to a LinkedList class so it works with for...of. Also add Symbol.toPrimitive to return the list length as a number.

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