In Greasemonkey, if a userscript adds things to the global namespace by using e.g. @require react-15.1.0.js, what happens if the original page also included React but an older version (say react-15.0.0.js)? Are the global names kept in separate environments somehow, or is there a possibility that a userscript may break a page (and vice versa) by redefining global names?
No, they cannot clash. I'm not clear on the details of greasemonkey's execution model, but I performed a test. I created this HTML file, which uses lodash when a button is clicked:
<html>
<head>
<script src='https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.13.1/lodash.min.js'></script>
</head>
<body>
<input type='button' id="btn" value="Invoke lodash">
<script type="text/javascript">
el = document.getElementById("btn")
el.addEventListener('click',function (argument) {
console.log("3 =", _.add(1,2));
})
</script>
</body>
</html>I also created a userscript, which removes _:
// ==UserScript==
// @name Test greasmonkey overriding
// @namespace gm-overriding
// @version 1
// @grant none
// ==/UserScript==
_ = null
console.log("here is _:", _);The userscript does not affect the lodash dependency, and the button click still prints the expected output.