How does Javascript know what type a variable is?

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

Answer 1

JavaScript itself does have types, and internally, each assignment receives an appropriate type. In your example var foo = 2.0; the type will be float. The programmer needn't worry about that too much (at first) because JS is loosly typed (!== type-free).
That means that if I were to compare a numeric string to a float, the engine will coerce the string to a number so that the comparison can be preformed.

The main difference between loose typed and strong typed languages is not type coercion, though. It's quite common in C(++) to cast to the type you need, and in some cases values are automatically converted to the correct type (2/2.0 == 2.0/2.0 == 1.0 ==> int is converted to float, implicitly). The main difference between loosly typed and strong typed languages is that you declare a variable with a distinct type:

int i = 0;//ok
//Later:
i = 'a';//<-- cannot assign char to int

whereas JS allows you to do:

var i = 1;//int
i = 1.123;//float
i = 'c';//char, or even strings
i = new Date();//objects

But, as functions/keywords like typeof, instanceof, parseFloat, parseInt,toString... suggest: there are types, they're just a tad more flexible. And variables aren't restricted to a single type.