It adds overhead on initial write, saves massive overhead every day after that. There is JS code I deal with once a month where the manipulation of types is so

It adds overhead on initial write, saves massive overhead every day after that. There is JS code I deal with once a month where the manipulation of types is so complex I probably burn ~30 minutes every time I have to touch it. If that was was transformed into TS (which is going to happen eventually) that'd be 30 minutes saved per month, on this one particular flow of data. I've done refactorings that were only possible because TS existed. A lot of JS unit tests consist of "ensure these fields exist on this object after it has been called by these functions." Typescript removes the need for those tests. And it removes the need to update those tests every time the code changes (just change the declarations appropriately!). And it removes the need to run those tests on every commit. That said, the overall code/compile/run time savings is possibly not in TS's favor due to how slow the compiler is. :/