I have a large project that has now been migrated over from javascript to typescript and we're very pleased with the results. Bugs have fallen dramatically with the improved compiler support in combination with rigorously written code.

Ours is a server-side product that runs under node (ts-node, actually). By its nature, it is highly asynchronous, so the move to Typescript had the major benefit of providing us access to async/await. It has been working really well, and dramatically reduced our code size and complexity.

The one problem it has introduced is that we occasionally miss putting an "await" keyword in front of calls to methods returning promises (i.e., other async functions) when inside an async function. This "send and forget" is something that one occasionally might want to use, but for the most part, this creates a lot of chaos in our design pattern, as in most cases, the call is to something that will complete asynchronously, but most of the time we need to wait for that to complete. (That method cannot be synchronous because it depends on external services that are asynchronous.) It is not uncommon in our case to have 5 or 10 await statements in a single method.

A missing "await" keyword can be hard to find -- especially when the method being called doesn't return anything that is needed by the caller. (If it returns something, type-checking will usually catch the mismatch between the type of the Promise returned from the call.)

Ideally, we could have a "nowait" keyword that would go where one would otherwise put an "await" to indicate to the Typescript compiler that the design intent is to explicitly NOT WAIT for the called method to be completing. The tsc compiler could have a new flag that controls whether to create a warning if a method call returns a Promise that is missing either "await" or "nowait". The "nowait" keyword would have no effect whatsoever on the compiled code.