Angular error handling using catchError(somefunctionthatreturnsanobservable)

Question

Going through the Angular docs for handling HTTP error responses in a service I came across something like this:

getContext(){
   return this.http.post(this.testUrl, {})
       .pipe(
           catchError(this.handleError)
       )
}

handleError(error: HttpErrorResponse){
    if(error.error instanceof ErrorEvent){
        console.log('An error occurred:', error.error.message);
    } else {
        console.log(
            `Backend returned code ${error.status}` +
            `body was ${error.error}`
        )
    }
    return throwError('Need to fix!');
}

I was wondering why this.handleError is not passed an argument (an HTTPErrorResponse ) and how it works without receiving one?

Answer 1

The handleError method is passed as a callback to catchError .

doSomething(callback) {
    // Call the callback with arguments
    callback('Hello world');
}

saySomething(param) {
   console.log(param);
}

doSomething(saySomething);

Try to imagine catchError is doing the same for your handleError function. Will call it with arguments, you just specify what function to be called when an error occurs.

Good luck!