What exactly are the implications of having dual type declarations in TypeScript?

I recently came across the Angular tutorial here

In the code snippet below, there are double type declarations that I am having trouble understanding.

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);
   };
}

The error is defined as type any, followed by another colon which declares a function with Observable as the parameter. What exactly does it return?

Answer №1

You provide a function type that receives an error: any and produces an output of Observable<T>.

(error: any): Observable<T>

Following this, you assign a function to your variable with the following body:

=> {
      console.error(error);

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

      return of(result as T);
};

Therefore, your handleError demonstrates a function with a parameter of type any returning an Observable<T>

(error: any): Observable<T>

Answer №2

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

This function, named execute, accepts an argument called error of type any. It returns an Observable<T> and the implementation is contained within the curly braces.

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