We can see many HTML page using src="//example.com/myjavascript.js" to include a .js and let the browser use http:// / https:// depending on the current scheme of the page URL.
Is this a valid URI ?
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Yes, it is definitely valid. It is a “scheme relative” or “protocol relative” URI. In the spec since the beginning. Quite helpful to deal with http/https issues.
Read a much better description and caveats:
Is it valid to replace http:// with // in a <script src="http://…">?
A few points to keep in mind:
file://, and not load your JS file from the server like it would with a full URL starting withhttp://orhttps://.Edit for modern webdev practices:
While the URL is still valid, it is now recommended to use
httpsfor third party resources (and, serve those resources from secure pages). The performance or compatibility issues from years ago are largely resolved with updated protocols and browsers.