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File loader.js:
function main()
{
if (typeof window !== 'undefined') {
var script = window.document.createElement('script')
script.src = 'https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.core.min.js'
script.onload = function () { console.log('script loaded') }
window.onload = function () { console.log('window loaded') }
window.document.head.appendChild(script)
} else {
console.log('window not available yet')
}
}
if (typeof module !== 'undefined' && module.exports) {
exports.main = main
}
main()
File window.html:
<!DOCTYPE html>
<html>
<head>
<title>Test</title>
<script src="loader.js"></script>
</head>
<body>
<div>Test</div>
</body>
</html>
Console output when I open this HTML page:
script loaded
window loaded
My question:
In the above code, is it guaranteed that the script onload event always fires before the window onload?
Yes, the output you see is guaranteed, because the
loadevent is fired "when a resource and its dependent resources have finished loading." Thewindowwill not be fully loaded until every element with something to download is downloaded and ready, including scripts and images. So, the<script>has to load before thewindowloads.The fact that there's a script which is created dynamically won't have an effect (assuming that
mainis called before the window is fully loaded, like in your code) - once inserted into the DOM, it's now something the window depends on, and so mustloadbefore the window does.