Catch browser's "zoom" event in JavaScript

Is it possible to detect, using JavaScript, when the user changes the zoom in a page? I simply want to catch a "zoom" event and respond to it (similar to window.onresize event).

Thanks.

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.

Lets define px_ratio as below:

px ratio = ratio of physical pixel to css px.

if any one zoom The Page, the viewport pxes (px is different from pixel ) reduces and should be fit to The screen so the ratio (physical pixel / CSS_px ) must get bigger.

but in window Resizing, screen size reduces as well as pxes. so the ratio will maintain.

zooming: trigger windows.resize event --> and change px_ratio

but

resizing: trigger windows.resize event --> doesn’t change px_ratio

//for zoom detection
px_ratio = window.devicePixelRatio || window.screen.availWidth / document.documentElement.clientWidth;

$(window).resize(function(){isZooming();});

function isZooming(){
    var newPx_ratio = window.devicePixelRatio || window.screen.availWidth / document.documentElement.clientWidth;
    if(newPx_ratio != px_ratio){
        px_ratio = newPx_ratio;
        console.log("zooming");
        return true;
    }else{
        console.log("just resizing");
        return false;
    }
}

The key point is difference between CSS PX and Physical Pixel.

https://gist.github.com/abilogos/66aba96bb0fb27ab3ed4a13245817d1e

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

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);
})();

This works for me:

		var deviceXDPI = screen.deviceXDPI;
		setInterval(function(){
			if(screen.deviceXDPI != deviceXDPI){
				deviceXDPI = screen.deviceXDPI;
				... there was a resize ...
			}
		}, 500);

It's only needed on IE8. All the other browsers naturally generate a resize event.

There is a nifty plugin built from yonran that can do the detection. Here is his previously answered question on StackOverflow. It works for most of the browsers. Application is as simple as this:

window.onresize = function onresize() {
  var r = DetectZoom.ratios();
  zoomLevel.innerHTML =
    "Zoom level: " + r.zoom +
    (r.zoom !== r.devicePxPerCssPx
        ? "; device to CSS pixel ratio: " + r.devicePxPerCssPx
        : "");
}

Demo

Although this is a 9 yr old question, the problem persists!

I have been detecting resize while excluding zoom in a project, so I edited my code to make it work to detect both resize and zoom exclusive from one another. It works most of the time, so if most is good enough for your project, then this should be helpful! It detects zooming 100% of the time in what I've tested so far. The only issue is that if the user gets crazy (ie. spastically resizing the window) or the window lags it may fire as a zoom instead of a window resize.

It works by detecting a change in window.outerWidth or window.outerHeight as window resizing while detecting a change in window.innerWidth or window.innerHeight independent from window resizing as a zoom.

//init object to store window properties
var windowSize = {
  w: window.outerWidth,
  h: window.outerHeight,
  iw: window.innerWidth,
  ih: window.innerHeight
};

window.addEventListener("resize", function() {
  //if window resizes
  if (window.outerWidth !== windowSize.w || window.outerHeight !== windowSize.h) {
    windowSize.w = window.outerWidth; // update object with current window properties
    windowSize.h = window.outerHeight;
    windowSize.iw = window.innerWidth;
    windowSize.ih = window.innerHeight;
    console.log("you're resizing"); //output
  }
  //if the window doesn't resize but the content inside does by + or - 5%
  else if (window.innerWidth + window.innerWidth * .05 < windowSize.iw ||
    window.innerWidth - window.innerWidth * .05 > windowSize.iw) {
    console.log("you're zooming")
    windowSize.iw = window.innerWidth;
  }
}, false);

Note: My solution is like KajMagnus's, but this has worked better for me.

⬤ The resize event works on modern browsers by attaching the event on window, and then reading values of thebody, or other element with for example (.getBoundingClientRect()).

> In some earlier browsers it was possible to register resize event > handlers on any HTML element. It is still possible to set onresize > attributes or use addEventListener() to set a handler on any element. > However, resize events are only fired on the window object (i.e. > returned by document.defaultView). Only handlers registered on the > window object will receive resize events.

⚠️ Do resize your tab, or zoom, to trigger this snippet:

window.addEventListener("resize", getSizes, false)
        
function getSizes(){
  let body = document.body
  body.width = window.innerWidth
  body.height = window.innerHeight
  console.log(body.width +"px x "+ body.height + "px")
}

getSizes()

---

⬤ An other modern alternative: the ResizeObserver API

Depending your layout, you can watch for resizing on a particular element.

This works well on «responsive» layouts, because the container box get resized when zooming.

function watchBoxchange(e){
 info.textContent = e[0].contentBoxSize[0].inlineSize+" x  "+e[0].contentBoxSize[0].blockSize + "px"
}

new ResizeObserver(watchBoxchange).observe(fluid)

#fluid {
  width: 200px;
  height:100px;
  overflow: auto;
  resize: both;
  border: 3px black solid;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  font-size: 8vh
}

<div id="fluid">
   <info id="info"></info> 
</div>

---

 Be careful to not overload javascript tasks from user gestures events. Use requestAnimationFrame whenever you needs redraws.

I'd like to suggest an improvement to previous solution with tracking changes to window width. Instead of keeping your own array of event listeners you can use existing javascript event system and trigger your own event upon width change, and bind event handlers to it.

$(window).bind('myZoomEvent', function() { ... });

function pollZoomFireEvent() 
{ 

    if ( ... width changed ... ) {
        $(window).trigger('myZoomEvent');
    }
}

Throttle/debounce can help with reducing the rate of calls of your handler.

Here is a clean solution:

// polyfill window.devicePixelRatio for IE
if(!window.devicePixelRatio){
  Object.defineProperty(window,'devicePixelRatio',{
    enumerable: true,
    configurable: true,
    get:function(){
      return screen.deviceXDPI/screen.logicalXDPI;
    }
  });
}
var oldValue=window.devicePixelRatio;
window.addEventListener('resize',function(e){
  var newValue=window.devicePixelRatio;
  if(newValue!==oldValue){
    // TODO polyfill CustomEvent for IE
    var event=new CustomEvent('devicepixelratiochange');
    event.oldValue=oldValue;
    event.newValue=newValue;
    oldValue=newValue;
    window.dispatchEvent(event);
  }
});

window.addEventListener('devicepixelratiochange',function(e){
  console.log('devicePixelRatio changed from '+e.oldValue+' to '+e.newValue);
});

According to MDN, "matchMedia" is the proper way to do this https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio#Monitoring_screen_resolution_or_zoom_level_changes

it's a bit finicky because each instance can only watch one MQ at a time, so if you're interested in any zoom level change you need to make a bunch of matchers.. but since the browser is in charge to emitting the events it's probably still more performant than polling, and you could throttle or debounce the callback or pin it to an animation frame or something - here's an implementation that seems pretty snappy, feel free to swap in _throttle or whatever if you're already depending on that.

Run the code snippet and zoom in and out in your browser, note the updated value in the markup - I only tested this in Firefox! lemme know if you see any issues.

const el = document.querySelector('#dppx')

if ('matchMedia' in window) {
  function observeZoom(cb, opts) {
    opts = {
      // first pass for defaults - range and granularity to capture all the zoom levels in desktop firefox
      ceiling: 3,
      floor: 0.3,
      granularity: 0.05,
      ...opts
    }
    const precision = `${opts.granularity}`.split('.')[1].length

    let val = opts.floor
    const vals = []
    while (val <= opts.ceiling) {
      vals.push(val)
      val = parseFloat((val + opts.granularity).toFixed(precision))
    }

    // construct a number of mediamatchers and assign CB to all of them
    const mqls = vals.map(v => matchMedia(`(min-resolution: ${v}dppx)`))

    // poor person's throttle
    const throttle = 3
    let last = performance.now()
    mqls.forEach(mql => mql.addListener(function() {
      console.debug(this, arguments)
      const now = performance.now()
      if (now - last > throttle) {
        cb()
        last = now
      }
    }))
  }

  observeZoom(function() {
    el.innerText = window.devicePixelRatio
  })
} else {
  el.innerText = 'unable to observe zoom level changes, matchMedia is not supported'
}

<div id='dppx'>--</div>

You can also get the text resize events, and the zoom factor by injecting a div containing at least a non-breakable space (possibly, hidden), and regularly checking its height. If the height changes, the text size has changed, (and you know how much - this also fires, incidentally, if the window gets zoomed in full-page mode, and you still will get the correct zoom factor, with the same height / height ratio).

<script>
var zoomv = function() {
  if(topRightqs.style.width=='200px){
  alert ("zoom");
  }
};
zoomv();
</script>

On iOS 10 it is possible to add an event listener to the touchmove event and to detect, if the page is zoomed with the current event.

var prevZoomFactorX;
var prevZoomFactorY;
element.addEventListener("touchmove", (ev) => {
  let zoomFactorX = document.documentElement.clientWidth / window.innerWidth;
  let zoomFactorY = document.documentElement.clientHeight / window.innerHeight;
  let pageHasZoom = !(zoomFactorX === 1 && zoomFactorY === 1);

  if(pageHasZoom) {
    // page is zoomed
    
    if(zoomFactorX !== prevZoomFactorX || zoomFactorY !== prevZoomFactorY) {
      // page is zoomed with this event
    }
  }
  prevZoomFactorX = zoomFactorX;
  prevZoomFactorY = zoomFactorY;
});

Here is a native way (major frameworks cannot zoom in Chrome, because they dont supports passive event behaviour)

//For Google Chrome
document.addEventListener("mousewheel", event => {
  console.log(`wheel`);
  if(event.ctrlKey == true)
  {
    event.preventDefault();
    if(event.deltaY > 0) {
      console.log('Down');
    }else {
      console.log('Up');
    }
  }
}, { passive: false });

// For Mozilla Firefox
document.addEventListener("DOMMouseScroll", event => {
  console.log(`wheel`);
  if(event.ctrlKey == true)
  {
    event.preventDefault();
    if(event.detail > 0) {
      console.log('Down');
    }else {
      console.log('Up');
    }
  }
}, { passive: false });

I'am replying to a 3 year old link but I guess here's a more acceptable answer,

Create .css file as,

@media screen and (max-width: 1000px) 
{
       // things you want to trigger when the screen is zoomed
}

EG:-

@media screen and (max-width: 1000px) 
{
    .classname
    {
          font-size:10px;
    }
}

The above code makes the size of the font '10px' when the screen is zoomed to approximately 125%. You can check for different zoom level by changing the value of '1000px'.