null type built-in as a literal value, and is often used to represent the absence of data.
undefined are similar in this regard, but null typically shows that a value has been assigned, even if it is empty.
A detailed description of
null is available on the Mozilla Developer Network.
Where a parameter is shown as accepting a
null value, or a method returning a
null value, it indicates that a literal
null is expected / returned (not to be confused with
false which are different values).