What does double type declarations in TypeScript mean?

Question

I was doing the Angular tutorial here

The following piece of code have double type declarations, but I don't understand what it means.

handleError<T>(operation = 'operation', result?: T) {
   return (error: any): Observable<T> => {
      console.error(error);

      this.log(`${operation} failed: ${error.message}`);

      return of(result as T);
   };
}

So error is declared as of type any, then there's another colon to declare a function with Observable as the parameter. What exactly is it returning?

Answer 1

You return a type which is a function . It takes an error: any and returns Observable<T> .

(error: any): Observable<T>

After this you initialize your variable with a function which has body

=> {
      console.error(error);

      this.log(`${operation} failed: ${error.message}`);

      return of(result as T);
};

So your handleError returns a function with signature taking a parameter of type any and returning an Observable<T>

(error: any): Observable<T>

Answer 2

return (error: any): Observable<T> => { ... }

返回一个函数，取一个名为error的参数，类型为any ，其返回类型为Observable<T> ，其主体位于花括号之间。