JavaScript defines two special types, which both have only one value assigned: null and undefined.
Those are very special.
null
null is a special value that indicates the absence of a value.
It’s a common concept in other languages as well, can be known as nil or None in Python for example.
undefined
undefined indicates that a variable has not been initialized and the value is absent.
It’s commonly returned by functions with no return value.
When a function accepts a parameter but that’s not set by the caller, it’s undefined.