I was reading a source code about next-firebase-auth.

I am confused about the following code syntax

export const useAuthUser: () => AuthUserContext

export const verifyIdToken: (token: string) => Promise<AuthUser>

The right side look like an arrow function. The left side is declaring varibles. Intuitively, I am thinking is the same as the following:

export const useAuthUser = () => AuthUserContext

However, it doesn't. I did some test on Chrome console. Here is the code

adder : () => (a   b)

When I hit enter, it shows

() => (a b)

But If I type adder, it shows

Uncaught ReferenceError: adder is not defined

I looked other posts here. It seems like the colon (:) is related to type annotation. So does it mean the right side is a type or interface? But how is an arrow function type or interface? It does not make sense.

Thanks for your help!

CodePudding user response：

The stuff after the colon is the type. As a type, () => AuthUserContext means a function that doesn't require any arguments, and which returns an AuthUserContext. (token: string) => Promise<AuthUser> Means a function that requires a single argument which is a string, and then returns a Promise that will resolve to an AuthUser

The lines of code you showed seem incomplete. Typically, a const will immediately followed by an assignment, as in:

export const useAuthUser: () => AuthUserContext = () => {
  // some code which returns an AuthUserContext
}

I did some test on Chrome console.

The console only supports javascript, not typescript