An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code.
In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages such a usage is considered to be sufficiently serious that a diagnostic being issued and the compilation fails.
Some language definitions initially used the implicit declaration behavior and as they matured provided an option to disable it (e.g. Perl's "<code>use warnings</code>" or Visual Basic's "<code>Option Explicit</code>").
The following provides some examples of how various programming language implementations respond to undefined variables. Each code snippet is followed by an error message (if any).
*** - EVAL: variable X has no value
foo.c: In function `main': foo.c:2: error: `x' undeclared (first use in this function) foo.c:2: error: (Each undeclared identifier is reported only once foo.c:2: error: for each function it appears in.)
A ReferenceError only happens if the same piece of executed code has a or a (but not ) declaration later on, or if the code is executed in strict mode. In all other cases, the variable will have the special value .
ReferenceError: x is not defined Source File: file:///c:/temp/foo.js
(no error, continuing)
nil
stdIn:1.9 Error: unbound variable or constructor: x
Set Y=X
<UNDEF>
Unbound value x
(no error)
(no error)
PHP Notice: Undefined variable: x in foo.php on line 3
+++ Error 30 in line 2: Label not found
(no error)
(3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'