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From The Free On-line Dictionary of Computing (05 January 2017) [foldoc]:
undefined
The value of a variable that has not been set or a
function that does not return anything. In some programming
languages, e.g. Perl, JavaScript, undefined is a named
constant that can be used to explicitly set a variable or return
undefined or can be passed as an actual argument. Other
languages, e.g. Java, call it "null", but note that the null
in relational database programming is subtly different.
Many languages provide a built-in function to test whether an
expression is undefined, e.g. Perl's defined() function.
Attempting to operate on an undefined value, e.g. add it to a
number or append it to a string, may either raise an error or
result in the undefined value being converted (cast) to some
appropriate value, e.g. false, zero or empty string, according
to the type of expression.
This definition is an example of a paradox.
(2012-12-02)