api.service.ts

// Code sample demonstrating use of Rust-style Result for an API Service in Angular + Typescript
// Posted: 2023-07-24
// Paul Barton
// Released to the public domain

import { Injectable } from '@angular/core';
import { environment } from '../environments/environment';
import {
  HttpClient,
  HttpErrorResponse,
  HttpParams,
  HttpResponse,
} from '@angular/common/http';
import { catchError, map, Observable, of } from 'rxjs';

export enum ResultType {
  Ok,
  Err,
}

export type Ok<T> = {
  type: ResultType.Ok;
  value: T;
};

export type Err<E> = {
  type: ResultType.Err;
  error: E;
};

export type Result<T, E> = Ok<T> | Err<E>;

export enum CreateUserError {
  EmailAlreadyExists,
  Other,
}

export interface NewUser {
  email: string;
  name: string;
  age: number;
}

export interface User {
  id: number;
  email: string;
  name: string;
  age: number;
}

@Injectable()
export class ApiService {
  constructor(private http: HttpClient) {}

  createUser(newUser: NewUser): Observable<Result<User, CreateUserError>> {
    const uri = environment.backend_uri + `users/create`;
    return this.http.post<any>(uri, newUser, { observe: 'response' })
    .pipe(
      // This handles the conversion from HttpResponse to Result.
      // It needs to come before the catchError.
      map((data: HttpResponse<any>): Ok<User> => {
        return { type: ResultType.Ok, value: data.body };
      })
    )
    .pipe(
      // This should be the last thing to happen in the pipeline
      catchError(
        (
          err: HttpErrorResponse,
          caught: Observable<any>
        ): Observable<Err<CreateUserError>> => {
          if (err.status === 400 && err.error.detail === 'Invalid email') {
            return of<Err<CreateUserError>>(
              {
                type: ResultType.Err,
                error: CreateUserError.EmailAreadyExists,
              }
            )
          } else {
            return of<Err<CreateUserError>>(
              {
                type: ResultType.Err,
                error: CreateUserError.Other,
              }
            )
          }
          }
        )
      );
  }
}