Question

I don’t know why I never asked myself that questioned the last years before, but suddenly I could not find any answer for myself or with google.

Javascript is known to have no types for variables. A really nice thing.
But somehow it must determine the type and work with it.

var a = 1;
var b = 2.0;
var c = 'c';
var d = "Hello World!";

So what we have is an Integer, Double/Float, Character, String (which may be teared down as char*)

I know that JS works with a runtime interpreter, but thinking of that the logic and “type” must be implemented in any way..

So how does a Javascript Interpreter recognize and internally handle the variables?
In my imagination, assuming I would write C++, I would think of a sort of template and container and a bit of a logic that overloads operators and try to check, what it really is. But that’s not thought to the end.

Share your knowledge with me please 🙂

Answer 1

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

Useful links:

ECMAScript Language Specification

ECMAScript BNF Grammar

JAVAScript BNF Gramar