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.
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Nearby terms: uncountable « uncurry « uncurrying « undefined » undefined external reference » underflow » Undernet