Why does window.onresize gets called when a page is loading?

Question 1

As far as I can tell, window.onresize does not get called on page load by default on desktop browsers

I wrote a simple html page as follows (many H1s to make the page have some content):

<!DOCTYPE html>
<html>
<head>        
    <script>
        var i = 0;
        window.onresize = function() {
            i++;
        }
        window.setTimeout(function() {
            alert("resize called " + i + " times");
        }, 2000);
    </script>
</head>
<body>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
    <h1>test</h1>
</body>
</html>

The alert shows 0 in the following browsers:

Chrome 32
Firefox 26
Opera 12
IE11
IE8
Safari 5.1.7

Mobile Browser Viewports Theory

I can see your problems seem to be on mobile devices. onresize may fire on page load due to the "visual viewport" resizing after the mobile browser has loaded the content and figured out how to scale the page to your screen size.

See here for an explanation of mobile viewports:

http://www.quirksmode.org/mobile/viewports2.html

And see here for a table of how several mobile browsers handle the onresize event for the visual viewport:

http://www.quirksmode.org/dom/events/resize_mobile.html

If this is the case then I think you might have a very tough time combating it.

Ignoring the first call to `onresize`

To avoid the first run of your event handler for onresize you could simply set a flag like so:

var initial = true;
$(window).on('resize',function(){
    if(!initial)
    {
        // do your stuff here
    }else
    { 
        // don't do your stuff here
    } 
    initial = false;
});

However, as you say in the comments this will not work if onresize is working as expected (and not running on page load). It assumes that the first run will be on page load.

Question 2

I had the same issue on Chrome 36 for Android. A resize event was triggered at some point on first page load when I was not expecting this to happen (it causes some minor visual glitch at page rendering). After investigating I tried to remove the part of my JavaScript that was dynamically appending the meta name="viewport" tag information after the document.ready event has fired (I used jQuery) and no more hazardous firing of the resize event.

One explanation could be that appending the viewport information after the page has already begun rendering will cause the resize event to trigger (which does make sense).

Of course this will only happen for browsers which do take the meta name="viewport" tag information into account i.e. mobile browsers mainly (I have not tested IE11 on 8.1 but given it is an hybrid OS it could be taking into account meta name="viewport" tag information).

Question 3

I had the issue that sometimes multiple resize events where fired after loading the page. To ensure only real resize events are considered, I used a timer to ignore the event for the first 500ms after page load. This way, it will work in all browsers, regardless whether they fire one or multiple resize events after loading.

var realResize = false;

setTimeout(function() {
  realResize = true;
}, 500);

window.onresize = function() {
  if (realResize) {
    // do something
  }
};

Why does window.onresize gets called when a page is loading?

Mobile Browser Viewports Theory

Ignoring the first call to onresize

Ignoring the first call to `onresize`