Encountering a console error while attempting to navigate to the 404 page on Angular

Question

Encountering a console error while attempting to navigate to the 404 page on Angular

I am working on implementing a route to a page in Angular for handling incorrect URL addresses.

Error Received in Console

While there is no error message in my IDE, the console displays the following error:

ERROR TypeError: Cannot read property 'name' of undefined
     at SingleAppareilComponent.ngOnInit (single-appareil.component.ts:19)
     at callHook (core.js:2526)
     at callHooks (core.js:2495)
     at executeInitAndCheckHooks (core.js:2446)
     at refreshView (core.js:9480)
     at refreshEmbeddedViews (core.js:10590)
     at refreshView (core.js:9489)
     at refreshComponent (core.js:10636)
     at refreshChildComponents (core.js:9261)
     at refreshView (core.js:9515)

Here are the files involved:

SingleAppareilComponent
AppModule
HTML template for 404 page
HTML template for SingleAppareilComponent
AppareilService

`single.appareil.component.ts`

import { Component, OnInit } from '@angular/core';
import { ActivatedRoute } from '@angular/router';
import { AppareilService } from 'services/appareil.service';

@Component({
  selector: 'app-single-appareil',
  templateUrl: './single-appareil.component.html',
  styleUrls: ['./single-appareil.component.scss']
})
export class SingleAppareilComponent implements OnInit {

  name: string = 'Appareil';
  status: string = 'Statut';
  constructor(private appareilService: AppareilService,
              private route: ActivatedRoute) { }

  ngOnInit(): void {
    const id = this.route.snapshot.params['id'];
    this.name = this.appareilService.getApparreilById(+id).name;
    this.status = this.appareilService.getApparreilById(+id).status;
  }
}

`app.module.ts`

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';

import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';
import { AppareilComponent } from './appareil/appareil.component';
import { FormsModule } from '@angular/forms';
import { AppareilService } from 'services/appareil.service';
import { AuthComponent } from './auth/auth.component';
import { AppareilViewComponent } from './appareil-view/appareil-view.component';
import { RouterModule, Routes } from '@angular/router';
import { AuthService } from 'services/auth.service';
import { SingleAppareilComponent } from './single-appareil/single- 
appareil.component';
import { FourOhFourComponent } from './four-oh-four/four-oh-four.component';
import { HashLocationStrategy, LocationStrategy } from '@angular/common';

const appRoutes: Routes = [
  { path: 'appareils', component: AppareilViewComponent },
  { path: 'appareils/:id', component: SingleAppareilComponent },
  { path: 'auth', component: AuthComponent },
  { path: '', component: AppareilViewComponent },
  { path: 'not-found', component: FourOhFourComponent },
  { path:'**', redirectTo: '/notfound' }
]

@NgModule({
  declarations: [
    AppComponent,
    AppareilComponent,
    AuthComponent,
    AppareilViewComponent,
    SingleAppareilComponent,
    FourOhFourComponent
  ],
  imports: [
    BrowserModule,
    AppRoutingModule,
    FormsModule,
    RouterModule.forRoot(appRoutes)
  ],
  providers: [
    AppareilService,
    AuthService,
    {provide: LocationStrategy, useClass: HashLocationStrategy}
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

The expected template to be displayed:

`four-oh-four.component.html`

<h2>Error 404</h2>
<p>The page you are looking for does not exist.</p>

However, the current template being displayed is:

`single-appareil.component.hmtl`

<h2>{{ name }}</h2>
<p>Status: {{ status }}</p>
<a routerLink="/appareils">Return to list</a>

`appareil-service.ts`

export class AppareilService {
    appareils = [
        {
          id: 1,
          name: 'Washing Machine',
          status: 'off'
        },
        {
          id:2,
          name: 'Fridge',
          status: 'on'
        },
        {
          id:3,
          name: 'Computer',
          status: 'off'
        }
      ];


    getApparreilById(id: number) {
      const appareil = this.appareils.find(
        (appareilObject) =>{
          return appareilObject.id === id;
        }
      );
      return appareil
    }

    switchOnAll() {
        for (let appareil of this.appareils) {
            appareil.status = 'on'
        }
    }

    switchOffAll() {
        for (let appareil of this.appareils) {
            appareil.status = 'off'
        }
    }
    
    switchOnOne(index: number) {
        this.appareils[index].status = 'on';
    }

    switchOffOne(index: number) {
        this.appareils[index].status = 'off';
    }

javascript angular typescript routes

Answer 1

Answer №1

What Causes this Error to Appear in the Console?

ERROR TypeError: Cannot read property 'name' of undefined at SingleAppareilComponent.ngOnInit (single-appareil.component.ts:19)

According to jabaa's comment, there seems to be a function inside your component that returns undefined. Consequently, an attempt to access a property on this non-existent object was made at line 19:

this.name = this.appareilService.getApparreilById(+id).name

How Can You Troubleshoot This Issue?

Open your browser's developer tools, navigate to the debugger, and set a breakpoint on the problematic line.
You can also add debug logging in your source code to gain more insights.

In your SingleAppareilComponent, try logging the id parameter extracted from the current route path along with the return value of getApparreilById(+id):

    const id = this.route.snapshot.params['id'];
    console.log('Routed to SingleAppareilComponent to display appareil with id: ', id)
    let appareil = this.appareilService.getApparreilById(+id);
    console.log('AppareilService returned appareil: ', appareil):
    this.name = appareil.name; // an error can occur here if appareil is undefined

Possible Explanations for the Error:

To delve deeper into the issue, it is essential to inspect your AppareilService component, particularly the getApparreilById method.

Testing Your Service:

appareils = [{
    id: 1,
    name: 'Washing Machine',
    status: 'off'
  },
  {
    id: 2,
    name: 'Fridge',
    status: 'on'
  },
  {
    id: 3,
    name: 'Computer',
    status: 'off'
  }
];

function getApparreilById(id) {
  const appareil = this.appareils.find(
    (appareilObject) => {
      return appareilObject.id === id;
    }
  );
  return appareil
}

/* Results with various inputs: if not found, then undefined!  */
console.log("getApparreilById(1) returns: ", getApparreilById(1));
console.log("getApparreilById(2) returns: ", getApparreilById(2));
console.log("getApparreilById(3) returns: ", getApparreilById(3));
console.log("getApparreilById(4) returns: ", getApparreilById(4));
console.log("getApparreilById('') returns: ", getApparreilById(''));
console.log("getApparreilById('1') returns: ", getApparreilById('1'));

The find method will consistently return undefined when no matching appareil with the specified id is found.

Moreover, undefined is essentially an identifier indicating "Oops, nothing, null," without any properties like name to access.

Hence, the error message states:

Cannot read property 'name' of undefined

An object undefined exists, but devoid of any properties, rendering all property readings or assignments impossible.

Consider an Alternative Return Approach:

You might opt to return a default value-object which is defined yet contains empty values, as illustrated below:

getApparreilById(id: number) {
  const appareil = this.appareils.find(a > a.id === id);
  if (appareil === undefined) {
    return {id: 0, name: '', status: ''};  // defined object with empty values
  }
  return appareil;
}

Scenario Involving a Backend HTTP GET Call Assumption:

If your service tries fetching the appareil resource with a specified id from the backend like: GET /appareils/:id, anticipate receiving a response featuring either an HTTP status 200 accompanied by the appareil within the body.

In instances where no corresponding appareil is detected, such as in this unideal case, assume a response bearing an HTTP status 404 denoting absence of the specified id-based appareil on the backend.

This results in an empty body causing getApparreilById to yield an undefined appareil object, thereby voiding any name property.

Programmatically Redirecting to the 404 Page:

In cases where undefined signifies "appareil not found," it would make sense to redirect to another component like your 404 page.

Your app's route definition includes the pathway to your 404 page:

{ path: 'not-found', component: FourOhFourComponent },

You can utilize Angular's Router within your SingleAppareilComponent to execute the redirection as prescribed in your routes:

import { Router } from '@angular/router'; // import the router module here

export class SingleAppareilComponent implements OnInit {

  name: string = 'Appareil';
  status: string = 'Statut';

  // inject router in constructor as parameter 
  constructor(private appareilService: AppareilService,
              private route: ActivatedRoute,
              private _router: Router) {
  }

  ngOnInit(): void {
    const id = this.route.snapshot.params['id'];
    let appareil = this.appareilService.getApparreilById(+id);

    if (typeof appareil === 'undefined') {
      // utilize router to redirect to 404 page following routes specification
      this._router.navigateByUrl('/not-found');
    }

    // continue otherwise
    this.name = appareil.name;
    this.status = appareil.status;
  }
}

Note: The call for get occurs only once for enhanced performance. Furthermore, the undefined check may be simplified to myVar === undefined for similar outcomes in most scenarios.

Exploring Alternative Error Handling Approaches:

Alternatively, you could consider returning a Promise from your service-method, thus enabling efficient handling of different responses including both the successful 200 scenario and variations involving errors like 404.

For further insight, refer to: Angular: Http vs fetch api

Additional Resources:

How can I check for "undefined" in JavaScript?
How to redirect to another component after checking conditions in angular 8?

Answer 2

What Causes this Error to Appear in the Console?

ERROR TypeError: Cannot read property 'name' of undefined at SingleAppareilComponent.ngOnInit (single-appareil.component.ts:19)

According to jabaa's comment, there seems to be a function inside your component that returns undefined. Consequently, an attempt to access a property on this non-existent object was made at line 19:

this.name = this.appareilService.getApparreilById(+id).name

How Can You Troubleshoot This Issue?

Open your browser's developer tools, navigate to the debugger, and set a breakpoint on the problematic line.
You can also add debug logging in your source code to gain more insights.

In your SingleAppareilComponent, try logging the id parameter extracted from the current route path along with the return value of getApparreilById(+id):

    const id = this.route.snapshot.params['id'];
    console.log('Routed to SingleAppareilComponent to display appareil with id: ', id)
    let appareil = this.appareilService.getApparreilById(+id);
    console.log('AppareilService returned appareil: ', appareil):
    this.name = appareil.name; // an error can occur here if appareil is undefined

Possible Explanations for the Error:

To delve deeper into the issue, it is essential to inspect your AppareilService component, particularly the getApparreilById method.

Testing Your Service:

appareils = [{
    id: 1,
    name: 'Washing Machine',
    status: 'off'
  },
  {
    id: 2,
    name: 'Fridge',
    status: 'on'
  },
  {
    id: 3,
    name: 'Computer',
    status: 'off'
  }
];

function getApparreilById(id) {
  const appareil = this.appareils.find(
    (appareilObject) => {
      return appareilObject.id === id;
    }
  );
  return appareil
}

/* Results with various inputs: if not found, then undefined!  */
console.log("getApparreilById(1) returns: ", getApparreilById(1));
console.log("getApparreilById(2) returns: ", getApparreilById(2));
console.log("getApparreilById(3) returns: ", getApparreilById(3));
console.log("getApparreilById(4) returns: ", getApparreilById(4));
console.log("getApparreilById('') returns: ", getApparreilById(''));
console.log("getApparreilById('1') returns: ", getApparreilById('1'));

The find method will consistently return undefined when no matching appareil with the specified id is found.

Moreover, undefined is essentially an identifier indicating "Oops, nothing, null," without any properties like name to access.

Hence, the error message states:

Cannot read property 'name' of undefined

An object undefined exists, but devoid of any properties, rendering all property readings or assignments impossible.

Consider an Alternative Return Approach:

You might opt to return a default value-object which is defined yet contains empty values, as illustrated below:

getApparreilById(id: number) {
  const appareil = this.appareils.find(a > a.id === id);
  if (appareil === undefined) {
    return {id: 0, name: '', status: ''};  // defined object with empty values
  }
  return appareil;
}

Scenario Involving a Backend HTTP GET Call Assumption:

If your service tries fetching the appareil resource with a specified id from the backend like: GET /appareils/:id, anticipate receiving a response featuring either an HTTP status 200 accompanied by the appareil within the body.

In instances where no corresponding appareil is detected, such as in this unideal case, assume a response bearing an HTTP status 404 denoting absence of the specified id-based appareil on the backend.

This results in an empty body causing getApparreilById to yield an undefined appareil object, thereby voiding any name property.

Programmatically Redirecting to the 404 Page:

In cases where undefined signifies "appareil not found," it would make sense to redirect to another component like your 404 page.

Your app's route definition includes the pathway to your 404 page:

{ path: 'not-found', component: FourOhFourComponent },

You can utilize Angular's Router within your SingleAppareilComponent to execute the redirection as prescribed in your routes:

import { Router } from '@angular/router'; // import the router module here

export class SingleAppareilComponent implements OnInit {

  name: string = 'Appareil';
  status: string = 'Statut';

  // inject router in constructor as parameter 
  constructor(private appareilService: AppareilService,
              private route: ActivatedRoute,
              private _router: Router) {
  }

  ngOnInit(): void {
    const id = this.route.snapshot.params['id'];
    let appareil = this.appareilService.getApparreilById(+id);

    if (typeof appareil === 'undefined') {
      // utilize router to redirect to 404 page following routes specification
      this._router.navigateByUrl('/not-found');
    }

    // continue otherwise
    this.name = appareil.name;
    this.status = appareil.status;
  }
}

Note: The call for get occurs only once for enhanced performance. Furthermore, the undefined check may be simplified to myVar === undefined for similar outcomes in most scenarios.

Exploring Alternative Error Handling Approaches:

Alternatively, you could consider returning a Promise from your service-method, thus enabling efficient handling of different responses including both the successful 200 scenario and variations involving errors like 404.

For further insight, refer to: Angular: Http vs fetch api

Additional Resources:

How can I check for "undefined" in JavaScript?
How to redirect to another component after checking conditions in angular 8?

Answer 3

Answer №2

The specific mention of the line causing the issue is located within the ngOnInit function:

    this.name = this.appareilService.getApparreilById(+id).name;

My suspicion is that you should actually return an observable instead of a value at this point and then subscribe to it. This way, you can ensure that your data is loaded before proceeding in the ngOnInit function.

Answer 4

The specific mention of the line causing the issue is located within the ngOnInit function:

    this.name = this.appareilService.getApparreilById(+id).name;

My suspicion is that you should actually return an observable instead of a value at this point and then subscribe to it. This way, you can ensure that your data is loaded before proceeding in the ngOnInit function.

Encountering a console error while attempting to navigate to the 404 page on Angular

Error Received in Console

`single.appareil.component.ts`

`app.module.ts`

`four-oh-four.component.html`

`single-appareil.component.hmtl`

`appareil-service.ts`

Answer №1

What Causes this Error to Appear in the Console?

How Can You Troubleshoot This Issue?

Possible Explanations for the Error:

Testing Your Service:

Consider an Alternative Return Approach:

Scenario Involving a Backend HTTP GET Call Assumption:

Programmatically Redirecting to the 404 Page:

Exploring Alternative Error Handling Approaches:

Additional Resources:

Answer №2

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