Load Event on window:
The load event fires at the end of the document loading process. At
this point, all of the objects in the document are in the DOM, and all
the images, scripts, links and sub-frames have finished loading.
[source: developer.mozilla.org]
<script>
window.onload = function(){ init(); };
</script>
Load Event on HTML Elements:
The load
event is fired when a resource and its dependent
resources have finished loading.
[source: developer.mozilla.org]
<!-- When the image is loaded completely -->
<img onload="image_loaded()" src="w3javascript.gif">
<!-- When the frame is loaded completely (including all resources) -->
<iframe onload="frame_loaded()" src="about.html">
<!-- When body loaded completely (including all resources, images and iframes) -->
<body onload="init()">
Many forums even some answers in this site may mislead you, but the load
event on body element is not only just equivalent to load
event on window, it is the exact same event. The following quote clarifies it.
For historical reasons, some attributes/properties on the <body>
and
<frameset>
elements actually set event handlers on their parent Window
object. (The HTML specification names these: onblur, onerror, onfocus,
onload, onscroll.)
[source: developer.mozilla.org]
The DOMContentLoaded Event:
What developers should use is DOMContentLoaded
event on document. It fires when the html has loaded and parsed completely.
document.addEventListener("DOMContentLoaded", function(event) {
alert("Document is ready");
});
The DOMContentLoaded event is fired when the initial HTML document has
been completely loaded and parsed, without waiting for stylesheets,
images, and subframes to finish loading. A very different event load
should be used only to detect a fully-loaded page. It is an incredibly
popular mistake to use load where DOMContentLoaded would be much more
appropriate, so be cautious.
[source: developer.mozilla.org]
Perhaps this is the only answer regarding this topic that has proper References
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– Ghassan Elias Mar 04 '18 at 06:52