Undefined variable

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 " " or Visual Basic's " ").

Examples
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).

CLISP
*** - EVAL: variable X has no value

C
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.)

JavaScript
A ReferenceError only happens if the same piece of executed code has a let or a const (but not var) declaration later on, or if the code is executed in strict mode. In all other cases, the variable will have the special value undefined.

ReferenceError: x is not defined Source File: file:///c:/temp/foo.js

Lua
(no error, continuing) nil

ML (Standard ML of New Jersey)
stdIn:1.9 Error: unbound variable or constructor: x

MUMPS
Set Y=X



OCaml
Unbound value x

Perl
(no error)

PHP 5
(no error)

PHP Notice: Undefined variable: x in foo.php on line 3

REXX
+++ Error 30 in line 2: Label not found

VBScript
(no error)

(3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'