JavaScript function call/apply with string

Question

I just noticed that, when I want to pass string as "this", the type cannot be obtained correctly inside a JavaScript function.

Here is an example:

var str = 'string value';
if (typeof (str) == 'string') {
    alert('string outside');
}

var fn = function(s) {
    if (typeof (str) == 'string') {
        alert('string param');
    }

    if (typeof (this) == 'string') {
        alert('string this');
    }
    else {
        alert(typeof(this));
    }
};

fn.call(str, str);

I see 3 messages: "string outside", "string param", and "object".

My goal is to write an "if" statement that says "this" is string. Something like if (typeof(this) == 'string'). This one does not work, please point me to the correct statement that will work inside the function.

Answer 1

Primitive values are embedded as objects when they're used as context.

From the MDN on the call function :

Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed.

If you want to know if the object is of type String, use :

var isString = Object.prototype.toString.call(str) == '[object String]';

This is the solution the MDN recommends for object type detection.