http://kangax.github.io/compat-table/es6/ < The most popular version of the most popular javascript engine supports tail call optimization.

That said, it would still probably be worth doing the optimization in the transpiler, to avoid having the JS compiler have to work out if your program runs into any JS pitfalls.

Wouldn't any scheme runtime/compiler need to handle that, though? In other words, wouldn't a transform to a loop (on the js side) be one possible approach?

Only if you know at compilation time what function(s) you will be calling in a tail recursive manner. If the function you call comes from a parameter or variable (for example, in continuation passing style code) then there is no straightforward way to compile it into a loop.