JavaScript sets the variable type based on the value assignment. For example when JavaScript encounters the following code it knows that myVariable should be of type number:
var myVariable = 10;
Similarly, JavaScript will detect in the following example that the variable type is string:
var myVariable = "Hello World!";
JavaScript is also much more flexible than many other programming languages. With languages such as Java a variable must be declared to be a particular type when it is created and once created, the type cannot be changed. This is referred to as strong typing. JavaScript, on the other hand, allows the type of a variable to be changed at any time simply by assigning a value of a different type (better known as loose typing).
The following example is perfectly valid use of a variable in JavaScript. At creation time, the variable is clearly of type number. A later assignment of a string to this variable changes the type from number to string.
var myVariable = 10;
myVariable = "This is now a string type variable";
The variable’s data type is the JavaScript scripting engine’s interpretation of the type of data that variable is currently holding. A string variable holds a string; a number variable holds a number value, and so on. However, unlike many other languages, in JavaScript, the same variable can hold different types of data, all within the same application. This is a concept known by the terms loose typing and dynamic typing, both of which mean that a JavaScript variable can hold different data types at different times depending on context.
Complete article here: http://www.techotopia.com/index.php/JavaScript_Variable_Types
Another Article Which may help you: http://oreilly.com/javascript/excerpts/learning-javascript/javascript-datatypes-variables.html
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