CLISP

(setf y x)

*** - EVAL: variable X has no value

C

int main() {
  int y = x;
  return 0;
}

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.

"use strict";
let y = x;

let y = x;
let x; // causes error on line 1

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

Lua

y = x

(no error, continuing)

print(y)

nil

ML (Standard ML of New Jersey)

val y = x;

stdIn:1.9 Error: unbound variable or constructor: x

MUMPS

Set Y=X

<UNDEF>

OCaml

let y = x;;

Unbound value x

Perl

my $y = ($x // 0) + 1; # defined-or operator

(no error)

PHP 5

$y = $x;

(no error)

$y="";
$x="";
error_reporting(E_ALL);
$y = $x;

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

Python

x = y
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    x = y
NameError: name 'y' is not defined

>>> x = y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'y' is not defined

REXX

signal on novalue
y = x

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

Ruby

irb(main):001:0> y = x
NameError: undefined local variable or method `x' for main:Object
	from (irb):1

Tcl

% set y $x
can't read "x": no such variable

VBScript

Dim y
y = x

(no error)

Option Explicit

Dim y
y = x

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