It will never be, and indeed nobody has bothered to make a bitcoin minibg js. Why? Because each desktop CPU would only yield 20-30 megahashes per second, less

It will never be, and indeed nobody has bothered to make a bitcoin minibg js. Why? Because each desktop CPU would only yield 20-30 megahashes per second, less if you don't want to slow the user's computer to a crawl. A server would need to manage millions of clients just to match a single ASIC miner which has efficiency rated in terahashes. And that amount of hash power would only return about 30 dollars in bitcoin on a good day. This is why all javascript miners target Monero and the latter was often criticised for having a CPU-friendly PoW because this is just asking for botnets to form.