Add method to string class

I'd like to be able to say something like this in javascript :

   "a".distance("b")

How can I add my own distance function to the string class?

You can extend the String prototype;

String.prototype.distance = function (char) {
    var index = this.indexOf(char);

    if (index === -1) {
        alert(char + " does not appear in " + this);
    } else {
        alert(char + " is " + (this.length - index) + " characters from the end of the string!");
    }
};

... and use it like this;

"Hello".distance("H");

See a JSFiddle here.

String.prototype.distance = function( arg ) {
    // code
};

Minimal example:

No ones mentioned valueOf.

==================================================

String.prototype.
OPERATES_ON_COPY_OF_STRING = function ( 
    ARGUMENT 
){

    //:Get primitive copy of string:
    var str = this.valueOf();

    //:Append Characters To End:
    str = str + ARGUMENT;

    //:Return modified copy:
    return( str );
};

var a = "[Whatever]";
var b = a.OPERATES_ON_COPY_OF_STRING("[Hi]");
console.log( a ); //: [Whatever]
console.log( b ); //: [Whatever][Hi]

==================================================

From my research into it, there is no way to edit the string in place.

Even if you use a string object instead of a string primitive.

Below does NOT work and get's really weird results in the debugger.

==================================================

String.prototype.
EDIT_IN_PLACE_DOES_NOT_WORK = function ( 
    ARGUMENT 
){

    //:Get string object:
    var str = this;

    //:Append Characters To End:
    var LN = str.length;
    for( var i = 0; i < ARGUMENT.length; i++){
        str[LN+i] = ARGUMENT[ i ];
    };

};

var c = new String( "[Hello]" );
console.log( c );
c.EDIT_IN_PLACE_DOES_NOT_WORK("[World]");
console.log( c );

==================================================

after years (and ES6) … we have a new option how to do this:

Object.defineProperty( String.prototype, 'distance', {
	value: function ( param )
	{
		// your code …
		return 'counting distance between ' + this + ' and ' + param;
	}
} );

// ... and use it like this:
const result = "a".distance( "b" );
console.log(result);

You could do this:

String.prototype.distance = function (){ 
    //your code 
}