Symbol.keyFor()

const globalSym = Symbol.for("foo"); // Global symbol
console.log(Symbol.keyFor(globalSym));
// Expected output: "foo"
const localSym = Symbol(); // Local symbol
console.log(Symbol.keyFor(localSym));
// Expected output: undefined
console.log(Symbol.keyFor(Symbol.iterator));
// Expected output: undefined

Symbol.keyFor(sym)

const globalSym = Symbol.for("foo"); // create a new global symbol
Symbol.keyFor(globalSym); // "foo"
const localSym = Symbol();
Symbol.keyFor(localSym); // undefined
// well-known symbols are not symbols registered
// in the global symbol registry
Symbol.keyFor(Symbol.iterator); // undefined