undefined data type to indicate that no value has been assigned / returned to a parameter or from a function.
null are similar, but null shows that a value has been assigned, even if it is empty.
A detailed description of
undefined is available on the Mozilla Developer Network.
Where a parameter is shown as accepting a
undefined value, or a method returning a
undefined value, it indicates that
undefined is expected / returned (not to be confused with
false which are different values).