nest-next | Render Module to add Nextjs support for Nestjs | Frontend Framework library

This issue has been discussed before, for example here on SO.

The key point is this:

One might ask compiler to infer what makeSideEffect does since we can provide the source of the function. However this is not practically feasible because of ambient function and (possibly polymorphic) recursion. Compiler will be trapped in infinite loops if we instruct it to infer arbitrary deep functions, as halting problem per se.

So a realistic compiler must guess what a function does by a consistent strategy. Naturally we have two alternatives:

Assume every function does not have relevant side effect: e.g. assignment like a = null. We call this optimistic. Assume every function does have side effect. We call this strategy pessimistic. Spoiler: TypeScript uses optimistic strategy.

Source

In other words, TS will not make assumptions about what is in your map callback, including whether it has made a valid assignment in your case.

Edit: Your more detailed code illustrates the issue. TS will not analyze your callback code to determine information about your global variable.

Given your code, I don't think an imperative solution is probably the way to go. An immutable, functional solution like the on proposed in the other answer is probably better, i.e., return the values you want rather than assigning to a global variable.