I've seen so many posts like this talking about programming language demand as a number of positions available - but I don't think I've ever seen one talking about programming language supply. JavaScript is certainly in high demand, but the market is also brimming with JavaScript developers.
Some other languages on the other hand, are far less common, but because they occupy a specific niche, might actually be more "in demand" (ie, less competition for an applicant, higher salary).
That's an important distinction, but I think it's getting more common to post for positions you have filled so there are resumes available if someone takes flight. Resumes don't mean someone is available either.
Sure. Would you agree that this is something a JS-centric company is more likely to do (than a java one) because JS engineers are on average younger, and their life is more volatile, and also because the company itself is younger and more volatile?
Probably because it's difficult to determine the supply of developers for a language. Job boards tend to be companies posting jobs, not candidates posting (public) resumes.
Even if you had resume data, having a language listed as a proficiency doesn't mean that person is in the supply of developers for that language. Likewise, sometimes people get jobs for languages they don't know. I hadn't used golang before my current job, so I wouldn't have been in the "supply" of go developers despite being hired to use it.
The market may be brimming with js developers, but the bulk of them cant tell the difference between == and ===, cant get grasp the this/bind scope or lack basic understanding of inheritance, polymorphism and composition (the npm registry is full of copy pasted modules with small changes and a new name).
Getting js right is hard. Thus, competition for _skilled_ devs is rather low.
Same goes for php. Writing code full of sql injections, no design patterns, and no scalability is easy. Most php devs are limited to Laravel or Sympfony and have no clue about the language’s inner workings. Doing it right is hard and if you do you will get nice returns.
But i do agree about niche languages. The number of jobs available means nothing. Whats relevant is the (applicants/ number of jobs) formula.
Some other languages on the other hand, are far less common, but because they occupy a specific niche, might actually be more "in demand" (ie, less competition for an applicant, higher salary).