Being a freelancer I have very little time to contribute or post on GitHub. After 35 years in this business having code on GitHub honestly doesn't mean much at all to me, nor my clients. When it comes to "showing my stuff", I have a portfolio. I do use GitHub to keep up with things like my favorite add-on's for editors and such. I also have published some code snippets and very small projects there for others to look at if they are interested. In my experience, any company that is legitimately looking at someone to hire full-time is going to test their knowledge and people skills face to face. Javiar hit it spot on. If they are putting a lot of weight on your GitHub repos or contribution they are just being lazy IMHO.
With that said, I think think it is a great thing for a junior developer to build some projects on GitHub and contribute to an open source project or two. Anything to expand their knowledge is a great thing!
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