[jQuery] Am I using 'this' too often?

[George]

Being newbie in JavaScript I am trying to code my own helper object
that does pagination.
So here is a snippet of my code.

MyData.prototype = {
 ....blablabla...
   PageUp: function() {
        this.iFrom += this.pageSize;
        this.CheckPageIndex();
    },


    CheckPageIndex: function() {
        if( this.iFrom >= this.data.length )
            this.iFrom = Math.floor((this.data.length - 1)/
this.pageSize) * this.pageSize;


        if( this.iFrom < 0 )
            this.iFrom = 0;
    }



}


Why do I need to call CheckPageIndex using this.CheckPageIndex when
called from PageUp? It's in the same object...

Without 'this' I get an error 'CheckPageIndex is undefined'. Coming
from object oriented languages like C++ I have a trouble
understanding
it.
Or am I doing it wrong and there is a way not to specify 'this' to
many times?

Thanks
George