为什么Flow libdefs中的某些类型在开头就有美元符号

Question

Some libdefs in flow-typed respository have types, started with the dollar sign, like $AxiosXHR :

It looks like it declares globally available type, but I am not sure, and I haven't found any documentation about it.

Answer 1

Right now there's not a good way to write types inside declare module {} bodies that aren't exported, so the best option is to put a declaration outside the declare module {} and reference it.
To prevent clashing names, it's a convention to put something like $npm$ModuleName$ before the type/var name.

Ref: flow-typed CONTRIBUTING notes