In JavaScript, when I want to run a script once when the page has loaded, should I use window.onload or just write the script?
For example, if I want to have a pop-up, should I write (directly inside the <script> tag):
alert("hello!");
Or:
window.onload = function() {
alert("hello!");
}
Both appear to run just after the page is loaded. What is the the difference?
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The other answers all seem out of date
First off, putting scripts at the top and using window.onload is an anti-pattern. It's left over from IE days at best or mis-understandings of JavaScript and the browser at worst.
You can just move your scripts the the bottom of your html
<html>
<head>
<title>My Page</title>
</head>
<body>
content
</body>
<script src="some-external.js"></script>
<script>
some in page code
</script>
</html>
The only reason people used window.onload is because they mistakenly believed scripts needed to go in the head section. Because things are executed in order if your script was in the head section then the body and your content didn't yet exist by definition of execute in order.
The hacky workaround was to use window.onload to wait for the rest of the page to load. Moving your script to the bottom also solved that issue and now there's no need to use window.onload since your body and content will have already been loaded.
The more modern solution is to use the defer tag on your scripts but to use that your scripts need to all be external.
<head>
<script src="some-external.js" defer></script>
<script src="some-other-external.js" defer></script>
</head>
This has the advantage that the browser will start downloading the scripts immediately and it will execute them in the order specified but it will wait to execute them until after the page has loaded, no need for window.onload or the better but still unneeded window.addEventListener('load', ...
