Value types

Every variable, argument and result holds exactly one of five kinds. A variable has no fixed type. It holds whatever was last assigned to it.

Kind	Holds	Comes from	Truthy when	Shown as (StringFormat)
Bool	true / false	literals, comparisons, !, &&, ||, many builtins	it is true	true / false
Integer	64-bit signed whole number	integer and hex literals, constants, integer arithmetic	it isn't 0	42, -7
Real	64-bit floating point	real literals, any arithmetic involving a Real	it isn't 0.0	always six decimals: 0.500000
Text	Unicode string	string literals, + on two Texts, Text builtins	it isn't empty	as is
Window	a window handle (HWND)	WindowGetActive, ContextGetWindow, WindowFindByTitle, …	it isn't null	0x00000000000A0B2C

"Truthy" is how if, for conditions, !, && and || read a value that isn't a Bool. It makes these everyday checks work without comparing to anything:

w = WindowFindByTitle("Calculator");
if (!w) UtilityPrint("Calculator isn't open");       // null handle is falsy

text = StringTrim(ClipboardGetText());
if (text) UtilityPrint("Clipboard has: " + text);    // empty Text is falsy

There's no null, undefined or "nothing" value. A builtin that finds nothing returns an empty Text, 0, -1, false or a null Window, depending on its return kind. Check for the one it uses.