Symbol.for and Symbol.keyFor

Learn to get a symbol from the global registry and find its key.

The global symbol registry

Symbol() produces a fresh symbol every call. But sometimes you want to share the same symbol across different parts of your code, identified by a string key.

Symbol.for('key') first looks up that key in the global registry. If found, it returns the existing symbol; otherwise it creates one, stores it, and returns it. So Symbol.for('app') called twice returns the same symbol. These symbols are shared across the whole application.

Symbol.keyFor

Symbol.keyFor(symbol) does the reverse: given a symbol from the registry, it returns its key string. So Symbol.keyFor(Symbol.for('token')) returns 'token'.

If the symbol was created with Symbol() (not through the registry), Symbol.keyFor returns undefined. This makes it easy to tell whether a symbol is shared or local.

Exercise Use a shared symbol

Get a symbol with Symbol.for('config') into a variable first. Log its key using Symbol.keyFor(first). Then call Symbol.for('config') again into a variable second and log the comparison first === second.

Show example solution

const first = Symbol.for('config');
console.log(Symbol.keyFor(first));
const second = Symbol.for('config');
console.log(first === second);

Test yourself

What happens if Symbol.for('app') is called twice?
- A new symbol is created each time
- The first call throws an error
- The same symbol is returned and they are equal
- It returns undefined
Show answer

The same symbol is returned and they are equal — Correct. Symbol.for first checks the registry; if the key exists, it returns the stored symbol.

Create a shared symbol

const shared = ____;

Show answer

const shared = Symbol.for('token');

Symbol.for and Symbol.keyFor

Symbol.for('x')

Reads or creates from the registry
Symbol.keyFor(sym)

Returns the symbol's key
Symbol('x')

Always unique
Not in registry

keyFor returns undefined

Next lesson: Well-known symbols

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