Variables don't have to be declared before being used : Variables « Language Basics « JavaScript Tutorial

When the JavaScript sees a declared identifier (var varName), it creates a global variable.

var sTest = "hello ";
sTest2 = sTest + "world";
alert(sTest2);

1.9.Variables
	1.9.1.	Variables
	1.9.2.	Variables can hold different types of values at different times
	1.9.3.	Naming Variables
	1.9.4.	Variables don't have to be declared before being used
	1.9.5.	Define variable and assign value
	1.9.6.	Reassigning value to Variables
	1.9.7.	Working With JavaScript Variables
	1.9.8.	Declare two variables in the same line
	1.9.9.	Reference a variable without declaration
	1.9.10.	Assign one variable to another variable
	1.9.11.	Do simple calculation
	1.9.12.	Braces indicate code blocks