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Angular 4 error handling and logging

Angular 4 Error Handling and Logging!

How To Get and Log an error in Angular 4?
Two types of errors -
1.     If the backend returns an unsuccessful response like - 404, 500 and so on
2.     If something goes wrong in the client side like -network error etc.

In the both cases - We are using HttpErrorResponse and return the useful information on what went wrong in this call!

Example –
http.get('/api/users')
  .subscribe(data => {console.log(data);}, //Successful responses call the first callback.
      (err: HttpErrorResponse) => {
        if (err.error instanceof Error) {
           console.log('Error - ', err.error.message);
        }else {
            console.log('Error status - ${err.status}, and Error Detail - ${err.error}');
        }
      }
});

How To handle and retry the failed request due to Poor Network Connection in Angular 4?

In Angular 4, simply retry the request using RxJS operator called .retry (). It is automatically presubscribed to an Observable, thus reissuing the request when an error occurred!

Import RxJS for using retry () method-
import 'rxjs/add/operator/retry';

HTTP Observables for retry the failed request 
http.get('/api/users')
  .retry(2) //Retry this request up to 2 times.
  .subscribe(data => {console.log(data);}, //Successful responses call the first callback.
      (err: HttpErrorResponse) => {
        if (err.error instanceof Error) {
           console.log('Error - ', err.error.message);
        }else {
            console.log('Error status - ${err.status}, and Error Detail - ${err.error}');
        }
      }
});

Angular 5 ErrorHandler - Custom and Global Error Handling -

The default implementation of ErrorHandler log error messages, status, and stack to the console. To intercept error handling, we write a custom exception handler that replaces this default as appropriate for your app.

import { Injectable, ErrorHandler } from '@angular/core';
import {ErrorLoggService} from './error-logg.service';

// Global error handler for logging errors
@Injectable()
export class GlobalErrorHandler extends ErrorHandler {
    constructor(private errorLogService: ErrorLoggService) {
       //Angular provides a hook for centralized exception handling.
       //constructor ErrorHandler(): ErrorHandler
        super();
    }

    handleError(error) : void {
        this.errorLogService.logError(error);
    }
}

Custom ErrorHandler
The best way to log exceptions is to provide a specific log message for each possible exception. Always ensure that sufficient information is being logged and that nothing important is being excluded.

The multiple steps involved for creating custom error logging in Angular 5 i.e.
1.      Create a constant class for global error messages.
2.      Create an error log service – ErrorLoggService.
3.      Create a global error handler for using the error log service for logging errors.

Example -
Steps 1 – Firstly, I will CREATE a constant class for global error messages and its looks like.
export class AppConstants {
public static get baseURL(): string { return 'http://localhost:4200/api'; }
public static get httpError(): string { return 'There was an HTTP error.'; }
public static get typeError(): string { return 'There was a Type error.'; }
public static get generalError(): string { return 'There was a general error.'; }
public static get somethingHappened(): string { return 'Nobody threw an Error but something happened!'; }
}

Steps 2 – In the second steps, I will create an error log service (ErrorLoggService) for error logging and it looks like.
import { Injectable} from '@angular/core';
import { HttpErrorResponse } from '@angular/common/http';
import{ AppConstants} from '../app/constants'

//#region Handle Errors Service
@Injectable()
export class ErrorLoggService {

  constructor() {  }

  //Log error method
  logError(error: any) {
    //Returns a date converted to a string using Universal Coordinated Time (UTC).
    const date = new Date().toUTCString();

    if (error instanceof HttpErrorResponse) {
      //The response body may contain clues as to what went wrong,
      console.error(date, AppConstants.httpError, error.message, 'Status code:',
                                                  (<HttpErrorResponse>error).status);
    }
    else if (error instanceof TypeError) {
      console.error(date, AppConstants.typeError, error.message, error.stack);
    }
    else if (error instanceof Error) {
      console.error(date, AppConstants.generalError, error.message, error.stack);
    }
    else if(error instanceof ErrorEvent){
      //A client-side or network error occurred. Handle it accordingly.
      console.error(date, AppConstants.generalError, error.message);
    }
    else {
      console.error(date, AppConstants.somethingHappened, error.message, error.stack);
    }
  }
}
//#endregion


Steps 3 – In the 3rd steps, I will create a global error handler for using the error log service for logging errors and its looks like.
import { Injectable, ErrorHandler } from '@angular/core';
import {ErrorLoggService} from './error-logg.service';

// Global error handler for logging errors
@Injectable()
export class GlobalErrorHandler extends ErrorHandler {
    constructor(private errorLogService: ErrorLoggService) {
       //Angular provides a hook for centralized exception handling.
       //constructor ErrorHandler(): ErrorHandler
        super();
    }

    handleError(error) : void {
        this.errorLogService.logError(error);
    }
}


Steps 4 – In the 4th step, I will Import services, global handler and error handler and it looks like.
…...
import { NgModule, ErrorHandler } from '@angular/core';
import {ErrorLoggService} from './error-logg.service';
import {GlobalErrorHandler} from './global-error.service';

@NgModule({
  declarations: [
    AppComponent,
    LoginComponent,
    UserComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    HttpClientModule,
    RouterModule.forRoot([
      { path: '', component: AppComponent, pathMatch: 'full', canActivate: [AuthGuard]},
      { path: 'user', component: UserComponent, canActivate: [AuthGuard]},
      { path: 'login', component: LoginComponent}])
  ],
  providers: [ ErrorLoggService, // register global error log service
              GlobalErrorHandler,// register global error handler
              EmployeeService, // register Employee service
              ValidationService, //register Validation Service
              AuthenticationService, //register Authentication Service
              UserService],//register User Service
  bootstrap: [AppComponent]
})
export class AppModule { }



Steps 5 – Finally, we got a custom error handler for log error in your application errors.

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ANIL SINGH

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