>If you write sync code and later you need it to be async, then you're in for a full rewrite If you just want it not to block using the page then you can just

>If you write sync code and later you need it to be async, then you're in for a full rewrite If you just want it not to block using the page then you can just chain the sync code into a bunch of async callbacks. For actually parallelizing tasks that weren't before, it can be harder, but in my experience still not a full rewrite. It is much harder for larger applications in other contexts like the migration of Firefox to Quantum, though, yes.