null
null means "empty",
Use typeof (null) ;//Object ; to indicate that it is a special object.
The null type is only a member of itself. It does not contain attributes and methods.
undefined
undefined also means "empty",
Use typeof (undefined);//undefined; means that it is of undefined type.
The undefined type only contains one member of itself, and it does not contain attributes and methods.
Summary
null and undefined can be the same in a non-sense sense, for example, they can be converted to a boolean value, indicating false;
Neither null nor undefined can be used. and [] to get member values, so when using it, you should first determine whether the object is null or undefined.