Symbol

Learn unique identifiers with Symbol.

Symbol

A Symbol is a unique and immutable primitive type in JavaScript. Each call to Symbol() creates a new, unique value.

Example: const a = Symbol('id') and const b = Symbol('id'). Even though the descriptions match, a === b returns false; they are two different symbols.

Symbols are usually used to add private keys to objects; other code cannot accidentally overwrite them.

Symbol uniqueness

Two Symbols are never equal to each other, even if their descriptions match. This makes them 'clash-free keys'.

Example: const a = Symbol('x'), const b = Symbol('x'). a === b returns false. The description is just a label for debugging; it does not affect uniqueness.

If you really want to share the 'same' Symbol, you can use Symbol.for('key'); this looks up a global registry. But this is an advanced topic.

Exercise Compare two Symbols

Define two separate Symbols: const a = Symbol('id') and const b = Symbol('id'). Check equality with console.log(a === b); also print its type with console.log(typeof a).

Show example solution

const a = Symbol("id");
const b = Symbol("id");
console.log(a === b);
console.log(typeof a);

Test yourself

What is Symbol used for?
- For unique keys
- To create arrays
- To concatenate strings
- To round numbers
Show answer

For unique keys — Symbol creates unique keys.
Match the Symbol properties

Symbol()

Unique value
Symbol('id')

Symbol with description
typeof s

'symbol'
s === s

Only equal to itself

Define a Symbol

const id = ____;

Show answer

const id = Symbol('id');

Next lesson: typeof

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