reality: I saw null in the debugger here, causing the bug

add a null check

IDE: you don't need to check for null, this can't be null

what do you do when you can't trust the tools you work with

Depends on the language... if the IDE tells you that it can't be null, then that's most probably based on type inference, which in turn means there might be something wrong with your types.

Typescript is an approximation, even if the user never makes a false statement and the compiler options are set to their strictest possible values, the engine itself regularly infers and accepts overly narrow types that exclude possible values.

Yeah, it's unlikely that the IDE would be wrong on this. It usually bases it's estimations on just the few lines in your local scope and global scope, so if it says it can't be null, then it really shouldn't be null unless something weird is going on. Is this a multithreaded application and are you accessing something that another thread can change in the meanwhile? Really weird if not, not impossible, just weird

That or docblocs are lying

Can‘t happen unless you use a retarded language.