Global error handling in Angular - Article

docs/en/Community-Articles/2024-02-12-Global-Error-Handling-in-Angular/POST.md

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+
+# Global Error Handling in Angular
+
+![Error Handling](error_handling.jpg)
+
+Error handling is how we deal with errors that go wrong when we are running a program. There is no code that runs perfectly forever :) Things can go wrong and your application might crash. **So, in order to run your program smoothly you must handle errors.** It is just not for keeping your application in a running state. It is also useful to show messages about the error to the client. Like what went wrong, why it is not allowed to access this page etc.
+
+### How to handle errors
+
+-   First of all, you have to catch them 😀. You can catch them via **try-catch block**. See an example;
+#### Create an basic error
+```ts
+({} as any).doSomething()
+```
+
+![Error Image](error-image-1.png)
+
+#### Handle it
+```ts
+try {
+  ({} as any).doSomething();
+} catch (error) {
+  this.toastService.showError(error.message);
+}
+```
+
+![toast-gif](show-toast-2.gif)
+
+-   See, we catch the error and handle it.
+-   In this case, we know where the error will be thrown. Most of the time we won't know where the error will appear. Should we cover the entire application with try-catch blocks? Of course not 😀
+-   **We are going to handle errors globally**. Angular provides a great way to do it. Let's do it step by step;
+
+### 1.Create a **service** and implement the **`ErrorHandler`** interface.
+
+```ts
+import { ErrorHandler, Injectable, inject } from '@angular/core';
+import { ToastService } from './toast.service';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class CustomErrorHandlerService implements ErrorHandler {
+  toastService = inject(ToastService);
+
+  //This method comes from interface
+  handleError(error: any): void {
+    this.toastService.showError(error.message);
+  }
+}
+
+```
+
+### 2.Provide the service by using the **`ErrorHandler`** class from **`@angular/core`**.
+
+```ts
+import { ErrorHandler } from '@angular/core';
+
+providers: [
+  { provide: ErrorHandler, useExisting: CustomErrorHandlerService }
+]
+
+```
+
+![toast-gif](show-toast-2.gif)
+
+-  It behaves exactly the same. Nice, now we catch the entire errors in one simple service.
+-  Is it that simple? I wish it is but it's not 😀. This handling mechanism only works synchronously. When we start making http requests, our **`CustomErrorHandlerService`** won't catch the errors.
+
+## How to handle HTTP Requests
+Make an HTTP request and check if it's working.
+
+![http-request](http-request-4.gif)
+
+As you can see it doesn’t work. So how can we catch the http errors? with **`catchError()`** operator in rxjs or **`observer object`**. I will go with **`catchError()`** operator.
+
+```ts
+getTodo(id: number) {
+  this.http
+      .get(`https://jsonplaceholder.typicode.com/todos/${id}`)
+      .pipe(catchError((err) => {
+  	 this.toastService.showError(err.message);
+	 return EMPTY;
+        })
+       )
+      .subscribe(todo => this.todo = todo);
+}
+
+```
+
+![http-request](http-request-5.gif)
+
+- So are we going to add this **`catchError()`** operator to the entire http requests? **NO, we will use HTTP Interceptors!**
+- Let's do it step by step.
+
+### 1.Remove catchError pipe
+
+```ts
+getTodo(id: number) {
+  this.http.get('https://jsonplaceholder.typicode.com/todos/${id}').subscribe(todo => this.todo = todo);
+}
+```
+
+### 2.Create an HTTP Interceptor
+
+```ts
+import { Injectable, inject } from '@angular/core';
+import { HttpHandler, HttpInterceptor, HttpRequest } from '@angular/common/http';
+import { EMPTY, catchError } from 'rxjs';
+import { ToastService } from './toast.service';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class ToastInterceptor implements HttpInterceptor {
+  toastService = inject(ToastService);
+
+  intercept(req: HttpRequest<any>, next: HttpHandler) {
+    return next.handle(req).pipe(catchError((error) => {
+      this.toastService.showError(error.message);
+      return EMPTY;
+    }));
+  }
+}
+```
+
+### 3.Provide the interceptor
+
+```ts
+import { HTTP_INTERCEPTORS} from '@angular/common/http';
+
+providers: [
+  { provide: HTTP_INTERCEPTORS, useExisting: ToastInterceptor, multi: true }
+]
+```
+Now everything has set up. Let's make an HTTP request and try again.
+
+![http-request](http-request-5.gif)
+
+- So, now we are handling http errors globally. Whenever an error occurs, it will be catched from interceptor and will be showed via toast message.  
+- But this method has a one little disadvantage. What if we dont want to show toast message for a spesific case? [related issue about this problem](https://github.com/angular/angular/issues/18155).   
+- **[ABP Framework](https://abp.io/) has great solution for this problem. Let's understand the solution and apply it straightforwardly.**  
+  - Create a singleton service called **`HttpErrorReporterService`.** This service is going to store **HttpError** in a subject, and share the httpError as an observable for subscribers.
+  - Create a new service called **`RestService`** which is a layer on top of **`HttpClient`**, this new service is able to get a skipHandleError parameter. If skipHandleError value is **false** then it will be reported to the **`HttpErrorReporterService`** otherwise error will be throwed.
+  - Create a service called **`ErrorHandler`**, This service is going to subscribe to observable in **`HttpErrorReporterService`** and handle the errors (in our case we will show toast message).
+- When i first see this solution, i loved it. For more information and detail you can check the source code from the links below;   
+  - [**Http Error Reporter Service**](https://github.com/abpframework/abp/blob/360a3395aa0e44fb77574ea7eac745e8ba94b82e/npm/ng-packs/packages/core/src/lib/services/http-error-reporter.service.ts#L6)
+  - [**Rest Service**](https://github.com/abpframework/abp/blob/360a3395aa0e44fb77574ea7eac745e8ba94b82e/npm/ng-packs/packages/core/src/lib/services/rest.service.ts#L4)
+  - [**Error Handler**](https://github.com/abpframework/abp/blob/360a3395aa0e44fb77574ea7eac745e8ba94b82e/npm/ng-packs/packages/theme-shared/src/lib/handlers/error.handler.ts#L19)
+
+You can copy the source codes from ABP or use ABP directly 😀
+Lets simulate the solution in our application, this simulation is not suitable for your application it just for demonstration.
+
+**Rest Service**
+
+```ts
+import { HttpClient, HttpRequest } from '@angular/common/http';
+import { Injectable, inject } from '@angular/core';
+import { HttpErrorReporterService } from './http-error-reporter.service';
+import { catchError, throwError } from 'rxjs';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class RestService {
+  http = inject(HttpClient);
+  httpErrorReporterService = inject(HttpErrorReporterService);
+
+  request(req: HttpRequest<any> | { method: string, url: string; }, skipHandleError = false) {
+    const { method, url } = req;
+    return this.http.request(method, url).pipe(catchError((err) => {
+      if (!skipHandleError) {
+        this.httpErrorReporterService.reportError(err);
+      }
+      return throwError(() => err);
+    }));
+  }
+}
+
+```
+
+**HttpErrorReporterService**
+
+```ts
+import { HttpErrorResponse } from '@angular/common/http';
+import { Injectable } from '@angular/core';
+import { Subject } from 'rxjs';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class HttpErrorReporterService {
+  private _error$ = new Subject<HttpErrorResponse>();
+
+  get error$() {
+    return this._error$.asObservable();
+  }
+
+  reportError(error: HttpErrorResponse) {
+    this._error$.next(error);
+  }
+}
+
+```
+
+**ErrorHandler**
+
+```ts
+import { Injectable, inject } from '@angular/core';
+import { HttpErrorReporterService } from './http-error-reporter.service';
+import { ToastService } from './toast.service';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class ErrorHandlerService {
+  httpErrorReporterService = inject(HttpErrorReporterService);
+  toastMessageService = inject(ToastService);
+
+  constructor(){
+    this.httpErrorReporterService.error$.subscribe((error) => {
+      this.toastMessageService.showError(error.message);
+    });
+  }
+}
+
+```
+
+Now lets make an http request to check is it working
+
+```ts
+restService = inject(RestService);
+
+getTodo() {
+  this.restService.request(
+    { method: 'GET', url: 'https://jsonplaceholder.typicode.com/todos/1111' },
+  ).subscribe(todo => {
+    this.todo = todo;
+  });
+}
+```
+
+![http-request](http-request-7.gif)
+
+Now let's pass **true to the `skipHandleError` parameter**, let's see if the **errorHandler** going to skip this error or not.
+
+```ts
+restService = inject(RestService);
+
+getTodo() {
+  this.restService.request(
+    { method: 'GET', url: '<https://jsonplaceholder.typicode.com/todos/1111>' },
+    skipHandleError: true,
+  ).subscribe(todo => {
+    this.todo = todo;
+  });
+}
+
+```
+
+![http-request](http-request-8.gif)
+
+### Conclusion
+- To handle synchronous errors globally, you can use [Error Handler](https://angular.io/api/core/ErrorHandler).
+- To handle http errors globally, you can use [HTTP - interceptor](https://angular.io/guide/http-interceptor-use-cases). But in this method, you won't be able to skip specific cases. I recommend you use the ABP Framework's solution.
+
+Thanks for reading, if you have any advice please share it with me in the comments.