Explain Symbol in JavaScript.

1. What is a Symbol?

A Symbol is a unique and immutable primitive value introduced in ES6. Unlike strings or numbers, no two Symbols are the same, even if they have the same description.

• Purpose: They are mostly used to create unique property keys for objects to avoid property name collisions.

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2. Creating a Symbol

You create a Symbol using the Symbol() function:

const sym1 = Symbol();
const sym2 = Symbol('mySymbol');

console.log(sym1); // Symbol()
console.log(sym2); // Symbol(mySymbol)
console.log(sym1 === sym2); // false (always unique)

• Optional description is only for debugging; it doesn’t affect uniqueness.

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3. Symbols as Object Keys

Symbols can be used as property keys:

const mySymbol = Symbol('id');

const user = {
  name: 'Alice',
  [mySymbol]: 12345  // Symbol key
};

console.log(user.name);      // Alice
console.log(user[mySymbol]); // 12345

• Notice the square brackets [mySymbol] are required when using a Symbol as a key.
• Symbol-keyed properties don’t show up in for...in loops or Object.keys().

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4. Global Symbols

You can create symbols that are shared across different parts of the code using Symbol.for():

const globalSym1 = Symbol.for('app.id');
const globalSym2 = Symbol.for('app.id');

console.log(globalSym1 === globalSym2); // true

• Symbol.for() looks up a global symbol registry and returns the existing symbol if it exists; otherwise, it creates a new one.
• Use Symbol.keyFor() to get the key from a global symbol:

console.log(Symbol.keyFor(globalSym1)); // "app.id"

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5. Why Use Symbols?

1. Unique property keys: Avoid conflicts in large codebases or libraries.
2. Hidden properties: Symbols are not enumerable in loops.
3. Well-known Symbols: JavaScript uses predefined symbols like Symbol.iterator or Symbol.toStringTag for internal behaviors.

Example with Symbol.iterator:

const iterableObj = {
  [Symbol.iterator]: function* () {
    yield 1;
    yield 2;
    yield 3;
  }
};

for (const value of iterableObj) {
  console.log(value); // 1 2 3
}

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✅ Summary:

• Symbol is a unique, immutable primitive.
• Mainly used as object keys to avoid naming collisions.
• Symbols can be local (Symbol()) or global (Symbol.for()).
• Many built-in Symbols exist for customizing behavior.