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Well since you are already a technical seo, then you'll easily be able to apply your learnings of JS into it.  Part of good onsite/technical SEO is ensuring that the websites are easily crawled, have minimum load times and are structurally sound.   We had three people in my agency that just did JavaScript SEO.  So it's definitely something worth learning.  I think if you have a passion for coding then you should continually learn new languages and you can apply your SEO background to helping a company decide which platform and processes to use when building sites. Google Developers has an entire training course on JS SEO.  Check it out.  https://developers.google.com/search/docs/guides/javascript-seo-basics