Retrieve the position (X,Y) of an HTML element relative to the browser window

function getOffset( el ) {

    var _x = 0;

    var _y = 0;

    while( el && !isNaN( el.offsetLeft ) && !isNaN( el.offsetTop ) ) {

        _x += el.offsetLeft - el.scrollLeft;

        _y += el.offsetTop - el.scrollTop;

        el = el.offsetParent;

    }

    return { top: _y, left: _x };

}

var x = getOffset( document.getElementById('yourElId') ).left; 

Object.defineProperty( Element.prototype, 'documentOffsetTop', {

    get: function () { 

        return this.offsetTop + ( this.offsetParent ? this.offsetParent.documentOffsetTop : 0 );

    }

} );



Object.defineProperty( Element.prototype, 'documentOffsetLeft', {

    get: function () { 

        return this.offsetLeft + ( this.offsetParent ? this.offsetParent.documentOffsetLeft : 0 );

    }

} );

var x = document.getElementById( 'myDiv' ).documentOffsetLeft;