window.onload in external script gets ignored in Javascript

index.html

<html>
<head>
<script type="text/javascript" src="foo.js"></script>
<script type="text/javascript">
window.onload = function() {
    console.log("hello from html");
};
</script>
</head>
<body>
<div class="bar">bar</div>
</body>
</html>

foo.js

// this js file will be completely ignored with window.onload
//window.onload = function() {

    console.log("hello from external js");

    var bar = document.getElementsByClassName("bar");

    // this returns 0 instead of 1
    console.log(bar.length);
//};
  1. When window.onload is used in html, window.onload from external js will be ignored.
  2. When window.onload from external js is commented out, bar.length returns 0.
  3. When window.onload from html is removed, window.onload from external js works fine.

Can anyone explain why I can't use both window.onload?

If I had to use window.onload in html, how do tell if window is loaded from external js?

Answers:

Answer

1)The way you're binding, you can have just one method attached to an event. You need to add an event listener for what you want.

window.addEventListener("load", function() { alert("hello!");});

Setting directly a method to the onload event will replace any previously attached method. But if you use listeners instead, you can have many of them bound to an event.

2)If you comment out the onload in your external file, when the document.getElementsByClassName("bar") is called, your document isn't ready yet, then, it will return 0 items.

3)Use the addEventListener as I explained in the first point. If you apply this in both places, it will work like a charm.

Answer

onload is a property of window. It acts like any other variable property. When you try to use it twice you're overwriting the original value with your second write.

So your entire external script is ignored when you wrap it in window.onload, because window.onload is then overwritten to be

function() {
    console.log("hello from html");
};

If you want to do execute 2 functions, define 2 functions, a and b,

and set window.onload like this:

window.onload = function(){
  a();
  b();
}

Alternatively, you can bind 2 separate events in the way Alcides' answer suggests. My personal view is that its cleaner to do a single bind with multiple functions since its easier to know whats bound, know what order your functions will execute in, and see everything thats happening in one place, but its mostly a matter of style/preference if the order doesn't matter.

Answer

Thats Correct, you are overwriting your own onload, but you can always attach a new event listener to the window like this

function onLoadHandler(){
console.log("hello from external js");

var bar = document.getElementsByClassName("bar");

// page not loaded, so this returns 0 instead of 1
console.log(bar.length);
}
if (window.addEventListener) {
window.addEventListener('load', onLoadHandler); }
 else if (window.attachEvent) { window.attachEvent('onload', onLoadHandler ); }

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