Providing source code to your extensions is the simplest way to give something back to the open source community. Since you are using at least one open source application (Chrome) you should think about open-sourcing your projects as paying back the debt.
You should also understand that GitHub/Bitbucket profile is a developer portfolio. Many companies look at these before offering a job. Nowadays, each and every good developer has an extensive list of open source projects. This allows anyone to get an idea of how good is the code they write, how well they can collaborate with other developers, how much attention do they pay to the details etc.
Last but not least, if your extension is really good, if you constantly maintain it and develop new features and your users enjoy using it, you may be sure that it will defend itself from any kind of copies. If you open source your code you'll surely find some collaborators that will provide translations, new features and will help you to fix the bugs.
Anyway, an answer to your question is this: if you are afraid that someone will use your JavaScript code you can obfuscate it. Of course, obfuscated code can be prettified by someone who is desperate to read it but without the comments and meaningful variable names it will be a pain to do so. Other solution is to write a NPAPI plugin or Native Client app because these allow you to compile parts of the code.