User:Pjacobi/Draft/Nulls and undefined

The NULL and/or undefined values

What is the value of variables never assigned before? And: is there a special NULL value. Are they considered to be the same.

NULL

	ECMAScript	Lua	Perl	PHP	Python	Ruby
Name	null	nil		NULL	None	nil
Sames as undefined?	-	+	+	+	-
valid in boolean context?	-	+	-/-	+	+
valid in other expressions?	-	+	-/-	-	-

ECMAScript

ECMAScript has distinct null and undefined, the latter is only for declared variables never assigned a value. Accessing undeclared variables raises an exception.

Lua

In Lua, the NULL value nil and the undefined state are equal. This is basically the same model as Perl, but with giving this state a name. So undefing a variable has the normal assignment syntax.

PERL

PERL has no named NULL value but undefining a variable using undef can mostly achieve the same, as using an undefined variable in boolean context is allowed (and evaluates to false).

PHP

Python

Ruby