key

String, required. The key for the symbol (and also used for the description of the symbol).

An existing symbol with the given key if found; otherwise, a new symbol is created and returned.

In contrast to Symbol(), the Symbol.for() function creates a symbol available in a global symbol registry list. Symbol.for() does also not necessarily create a new symbol on every call, but checks first if a symbol with the given key is already present in the registry. In that case, that symbol is returned. If no symbol with the given key is found, Symbol.for() will create a new global symbol.

Symbol.for("foo"); // create a new global symbol
Symbol.for("foo"); // retrieve the already created symbol

// Same global symbol, but not locally
Symbol.for("bar") === Symbol.for("bar"); // true
Symbol("bar") === Symbol("bar"); // false

// The key is also used as the description
const sym = Symbol.for("mario");
sym.toString(); // "Symbol(mario)"

To avoid name clashes with your global symbol keys and other (library code) global symbols, it might be a good idea to prefix your symbols:

Symbol.for("mdn.foo");
Symbol.for("mdn.bar");

• Symbol.keyFor()