Practically, number of browsers + 2 (Node and Deno, and Deno is stretching the definition of practical at this early stage).
Edit: + Electron + the JS runtime used for Windows ecosystem based on Chakra (not sure if that's still alive though?) + a few other similar ones in that vein for desktop or mobile OS'.
+ whatever is available for embedded & similar devices, (I guess there will be a few but I know little about that area).
+ various [highly] specialised business/academic/hobby/PoC/etc runtimes, but they're not realistically things that many people care about.
Comments
Practically, number of browsers + 2 (Node and Deno, and Deno is stretching the definition of practical at this early stage).
Edit: + Electron + the JS runtime used for Windows ecosystem based on Chakra (not sure if that's still alive though?) + a few other similar ones in that vein for desktop or mobile OS'.
+ whatever is available for embedded & similar devices, (I guess there will be a few but I know little about that area).
+ various [highly] specialised business/academic/hobby/PoC/etc runtimes, but they're not realistically things that many people care about.