Symbol
length number
name string
prototype object
for(key) function
Search the global symbol registry for a symbol with the given key. If found, return that symbol; otherwise, create a new symbol with that key and add it to the registry, then return the new symbol.
key: A string key used to identify the symbol in the global registry.
Returns: A symbol associated with the given key in the global registry.
keyFor(sym) function
Retrieve a shared symbol’s key from the global symbol registry. If the symbol is not in the global registry, return undefined.
sym: The symbol to find the key for.
Returns: The string key if 'sym' is a global symbol, otherwise undefined.