Catch browser's "zoom" event in JavaScript

There's no way to actively detect if there's a zoom. I found a good entry here on how you can attempt to implement it.

I’ve found two ways of detecting the zoom level. One way to detect zoom level changes relies on the fact that percentage values are not zoomed. A percentage value is relative to the viewport width, and thus unaffected by page zoom. If you insert two elements, one with a position in percentages, and one with the same position in pixels, they’ll move apart when the page is zoomed. Find the ratio between the positions of both elements and you’ve got the zoom level. See test case. http://web.archive.org/web/20080723161031/http://novemberborn.net/javascript/page-zoom-ff3

You could also do it using the tools of the above post. The problem is you're more or less making educated guesses on whether or not the page has zoomed. This will work better in some browsers than other.

There's no way to tell if the page is zoomed if they load your page while zoomed.

I'm using this piece of JavaScript to react to Zoom "events".
It polls the window width. (As somewhat suggested on this page (which Ian Elliott linked to): http://novemberborn.net/javascript/page-zoom-ff3 [archive])

Tested with Chrome, Firefox 3.6 and Opera, not IE.

Regards, Magnus

var zoomListeners = [];

(function(){
  // Poll the pixel width of the window; invoke zoom listeners
  // if the width has been changed.
  var lastWidth = 0;
  function pollZoomFireEvent() {
    var widthNow = jQuery(window).width();
    if (lastWidth == widthNow) return;
    lastWidth = widthNow;
    // Length changed, user must have zoomed, invoke listeners.
    for (i = zoomListeners.length - 1; i >= 0; --i) {
      zoomListeners[i]();
    }
  }
  setInterval(pollZoomFireEvent, 100);
})();

Good news everyone some people! Newer browsers will trigger a window resize event when the zoom is changed.