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).
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).