Center backgroud with JavaScript (need jQuery)

centerBackground.js

centerBackground: function () {

    if( !mobile )
    {
        var $back = $('#back-img img');
        var backSize = tablet ? 750 : 1490;

        if( backSize >= Master.data.w ) 
        {
            var decalage = ( Master.data.w - backSize) / 2;
            $back.css('marginLeft', decalage);
        }
        else {
            $back.css('marginLeft', 0);
        }
    } 
}