Resize on div element

DIV does not fire a resize event, so you won't be able to do exactly what you've coded, but you could look into monitoring DOM properties.

If you are actually working with something like resizables, and that is the only way for a div to change in size, then your resize plugin will probably be implementing a callback of its own.

I was only interested for a trigger when a width of an element was changed (I don' care about height), so I created a jquery event that does exactly that, using an invisible iframe element.

$.event.special.widthChanged = {
  remove: function() {
      $(this).children('iframe.width-changed').remove();
  },
  add: function () {
      var elm = $(this);
      var iframe = elm.children('iframe.width-changed');
      if (!iframe.length) {
          iframe = $('<iframe/>').addClass('width-changed').prependTo(this);
      }
      var oldWidth = elm.width();
      function elmResized() {
          var width = elm.width();
          if (oldWidth != width) {
              elm.trigger('widthChanged', [width, oldWidth]);
              oldWidth = width;
          }
      }

      var timer = 0;
      var ielm = iframe[0];
      (ielm.contentWindow || ielm).onresize = function() {
          clearTimeout(timer);
          timer = setTimeout(elmResized, 20);
      };
  }
}

It requires the following css :

iframe.width-changed {
    width: 100%;
    display: block;
    border: 0;
    height: 0;
    margin: 0;
}

You can see it in action here widthChanged fiddle

// this is a Jquery plugin function that fires an event when the size of an element is changed
// usage: $().sizeChanged(function(){})

(function ($) {

$.fn.sizeChanged = function (handleFunction) {
    var element = this;
    var lastWidth = element.width();
    var lastHeight = element.height();

    setInterval(function () {
        if (lastWidth === element.width()&&lastHeight === element.height())
            return;
        if (typeof (handleFunction) == 'function') {
            handleFunction({ width: lastWidth, height: lastHeight },
                           { width: element.width(), height: element.height() });
            lastWidth = element.width();
            lastHeight = element.height();
        }
    }, 100);


    return element;
};

}(jQuery));