Symbols

The plain-language idea

Symbol() creates a value that is guaranteed unique — no two symbols are ever equal, even if you give them the same description.

const a = Symbol("id");
const b = Symbol("id");
a === b; // false — description is just a debug label, not an identity

const user = {
  name: "Ada",
  [a]: 42,           // a symbol as a property key
};
Object.keys(user);           // ["name"] — symbol keys are skipped
JSON.stringify(user);        // '{"name":"Ada"}' — skipped here too

Symbols exist to solve one specific problem: adding a property to an object without any risk of colliding with a string key someone else already used (or will use later) — including on objects you don't fully control.

Well-known symbols power the language itself

You already met this without knowing it: Symbol.iterator is exactly how the iterator protocol identifies "the method that makes this object work with for...of." for (const x of arr) is really looking up arr[Symbol.iterator].

Symbol('id') creates a value guaranteed unique — even with the same description string.

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What people usually miss

Watch out

• Symbol() is not a constructor — new Symbol() throws a TypeError. You always create one by calling Symbol() directly, unlike almost every other wrapper type.
• Symbol.for(key) is different: it looks up (or creates) a symbol in a global registry keyed by string, so two separate calls with the same key do return the same symbol — the one exception to "symbols are always unique."
• You can still access a symbol property deliberately via Object.getOwnPropertySymbols() — symbols are hidden from casual enumeration, not truly private or secure.

Why this matters in interviews

Symbols come up less as their own topic and more as "wait, how does for...of actually know how to iterate a custom object?" — the honest answer routes straight through Symbol.iterator, connecting this lesson directly to generators.