Does console.log treat 'this' differently?

Question

Why does the following statement:

(function(){ console.log(this); }).apply(String("hello"));

display the following output

String {0: "h", 1: "e", 2: "l", 3: "l", 4: "o", length: 5} 

and not a simple:

hello

Is this behavior built-in to the interpreter or is there a way to detect the type of reference passed?

Answer 1

The reason that you get an object and not a string as the output of your function is that by default javascript 'this' object is always forced to be an object.

If you however use javascript in a strict format with 'use strict' then this is disabled and you can get the result that you would expect.

// wrapped in a function to allow copy paste into console
(function() {
   'use strict';

   (function(){ console.log(this); }).apply(String("hello")); 
})();

A more thorough explanation about 'strict mode' and why it removes the boxing of this into an object can be found on the mozilla site here

Answer 2

In JavaScript, this can't be a primitive type; you would need to use .valueOf() to fetch the primitive value, i.e.:

(function(){ console.log(this.valueOf()); }).apply(String("hello"));

Or use 'use strict'; like DeadAlready mentioned in his answer.