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
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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