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

One simple way to imagine an implementation is that all values are kept in objects and that all variables are pointers... in C++:

struct Value
{
    int type;
    Value(int type) : type(type) { }
    virtual ~Value() { }

    virtual std::string toString() = 0;
};

struct String : Value
{
    std::string x;
    String(const std::string& x) : Value(STRING_TYPE), x(x) { }
    virtual std::string toString()
    {
        return x;
    }
};

struct Number : Value
{
    double x;
    Number(double x) : Value(NUMBER_TYPE), x(x) { }
    ...
};

struct Object : Value
{
    // NOTE: A javascript object is stored as a map from property
    //       names to Value*, not Value. The key is a string but
    //       the value can be anything
    std::map x;
    Object() : Value(OBJECT_TYPE), x(x) { }
    ...
};

For example whenever an operation has to be performed (e.g. a+b) you need to check the types of the objects pointed by the variables to decide what to do.

Note that this is an ultra-simplistic explanation... javascript today is much more sophisticated and optimized than this, but you should be able to get a rough picture.