The problem is WHEN the event is added and EXECUTED via triggering (the document onload property modification can be verified by examining the properties list).
When does this execute and modify onload relative to the onload event trigger:
document.addEventListener('load', ... );
before, during or after the load and/or render of the page's HTML?
This simple scURIple (cut & paste to URL) "works" w/o alerting as naively expected:
data:text/html;charset=utf-8, <html content editable><head> <script> document.addEventListener('load', function(){ alert(42) } ); </script> </head><body>goodbye universe - hello muiltiverse</body> </html>
Does loading imply script contents have been executed?
A little out of this world expansion ...
Consider a slight modification:
data:text/html;charset=utf-8, <html content editable><head> <script> if(confirm("expand mind?"))document.addEventListener('load', function(){ alert(42) } ); </script> </head><body>goodbye universe - hello muiltiverse</body> </html>
and whether the HTML has been loaded or not.
Rendering is certainly pending since goodbye universe - hello muiltiverse is not seen on screen but, does not the confirm( ... ) have to be already loaded to be executed? ... and so document.addEventListener('load', ... ) ... ?
In other words, can you execute code to check for self-loading when the code itself is not yet loaded?
Or, another way of looking at the situation, if the code is executable and executed then it has ALREADY been loaded as a done deal and to retroactively check when the transition occurred between not yet loaded and loaded is a priori fait accompli.
So which comes first: loading and executing the code or using the code's functionality though not loaded?
onload as a window property works because it is subordinate to the object and not self-referential as in the document case, ie. it's the window's contents, via document, that determine the loaded question err situation.
PS.: When do the following fail to alert(...)? (personally experienced gotcha's):
caveat: unless loading to the same window is really fast ... clobbering is the order of the day
so what is really needed below when using the same named window:
window.open(URIstr1,"w") . addEventListener('load', function(){ alert(42); window.open(URIstr2,"w") . addEventListener('load', function(){ alert(43); window.open(URIstr3,"w") . addEventListener('load', function(){ alert(44); /* ... */ } ) } ) } )
alternatively, proceed each successive window.open with:
alert("press Ok either after # alert shows pending load is done or inspired via divine intervention" );
data:text/html;charset=utf-8, <html content editable><head><!-- tagging fluff --><script> window.open( "data:text/plain, has no DOM or" ,"Window" ) . addEventListener('load', function(){ alert(42) } ) window.open( "data:text/plain, has no DOM but" ,"Window" ) . addEventListener('load', function(){ alert(4) } ) window.open( "data:text/html,<html><body>has DOM and", "Window" ) . addEventListener('load', function(){ alert(2) } ) window.open( "data:text/html,<html><body>has DOM and", "noWindow" ) . addEventListener('load', function(){ alert(1) } ) /* etc. including where body has onload=... in each appropriate open */ </script><!-- terminating fluff --></head></html>
which emphasize onload differences as a document or window property.
Another caveat concerns preserving XSS, Cross Site Scripting, and SOP, Same Origin Policy rules which may allow loading an HTML URI but not modifying it's content to check for same. If a scURIple is run as a bookmarklet/scriplet from the same origin/site then there maybe success.
ie. From an arbitrary page, this link will do the load but not likely do alert('done'):
<a href="javascript:window.open('view-source:http://google.ca') . addEventListener( 'load', function(){ alert('done') } )"> src. vu </a>
but if the link is bookmarked and then clicked when viewing a google.ca page, it does both.
test environment:
window.navigator.userAgent = Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.4) Gecko/2008102920 Firefox/3.0.4 (Splashtop-v1.2.17.0)
window.window.addEventListener()?windowis much more reliable.addEventListeners are being appended towindow(even the one which is meant to be hooked to document as the comment in the code and the message in the console says). There's nodocument.addEventListener('load' ...in the source code :-/