> Much more difficult is writing the garbage collector. Especially a concurrent one.

Since JS is single-threaded, you can write a simple stop-the-world mark-sweep collector as a first iteration. (Simple reference counting probably isn't a good idea since cycles are common in JS.) Then you can move to incremental and/or generational if that's giving you annoying hitches. Only after that do you need to put concurrent on the table.

I believe even world class JS engines have only relatively recently started doing GC in another thread.