String prototype modifying itself

Question

As far as i know it\'s not possible to modify an object from itself this way:

String.prototype.append = function(val){
    this = this + val;
}

Answer 1

I've been researching the same... First of all, of course you can't just do this += x, 'this' is an object, you can't use the + operator on objects.

There are 'behind the scene' methods that get called - for example

String.prototype.example = function(){ alert( this ); }

is actually calling

String.prototype.example = function(){ alert( this.valueOf() ); }

So what you need to find is a relevant value that does the the opposite - something like this.setValue(). Except that there isn't one. The same holds for Number too.

Even the built in methods are bound by that

var str = 'aaa';
str.replace( /a/, 'b' );
console.log( str ); // still 'aaa' - replace acts as static function 
str = str.replace( /a/, 'b' );
console.log( str ); // 'bbb' - assign result of method back to the object

On some other objects you can; for example on a Date:

Date.prototype.example = function(){
 this.setMonth( this.getMonth()+6 );
};
var a=new Date();
alert(a.getMonth());
a.example();
alert(a.getMonth());

It's annoying, but there you go