JWT Authentication in an Angular-WordPress App
Within the context of recent software program engineering, decoupling—breaking an software into distinct elements—has emerged as an trade commonplace. Corporations and software program engineers alike favor decoupling as a result of it permits for a transparent separation of considerations between an software’s presentation layer (entrance finish) and its information entry layer (again finish). This method enhances an app’s effectivity by permitting for parallel improvement by a number of groups whereas additionally providing the pliability to decide on optimum applied sciences for all sides.
Given its modular nature, a decoupled system’s impartial parts might be focused for scaling, modification, or outright alternative because the system’s wants evolve. This observe extends throughout numerous digital platforms, together with areas like e-commerce, on-line banking, community-driven portals, and social media.
Whereas a decoupled system presents many benefits, it additionally carries potential drawbacks. The system’s communication happens throughout totally different modules or companies and might introduce latency, which slows system efficiency. As well as, conventional browser cookie and server-side authentication strategies designed for monolithic purposes turn into difficult.
To handle these considerations, builders can leverage protocols like GraphQL, REST, and gRPC to facilitate wonderful intercomponent communication, stop delays, and construction the implementation of authentication. This tutorial demonstrates that decoupled apps can thrive: In a WordPress-powered Angular app, we’ll obtain safe communication utilizing GraphQL and JWT, a preferred token-based authentication technique.
Environment friendly Communication in Decoupled Programs: An Angular-WordPress Instance
We are going to construct a weblog software with a headless WordPress again finish and an Angular entrance finish. WordPress, a extensively adopted, sturdy content material administration system (CMS), is good for managing and serving weblog content material. The selection of Angular is strategic, because it permits for dynamic content material updates with out requiring full-page reloads, which yields accelerated consumer interactions. Communication between the 2 layers shall be managed by GraphQL.
Initially, the app shall be configured to fetch weblog publish content material and show the publish titles to customers in an inventory. After it’s up and working, you’ll improve the unprotected weblog software by integrating a JWT-based authentication function. Via this token-based authentication, you make sure that solely logged-in customers have entry. Unauthenticated guests will see the checklist of titles however be prompted to register or register in the event that they try to learn a full publish.
On the entrance finish, the route guard checks consumer permissions and determines whether or not a route might be activated, and the HTTP module facilitates HTTP communication. On the again finish, GraphQL serves because the app’s communication medium, applied as an API interface over HTTP.
Observe: The complicated difficulty of cybersecurity is a broad subject that falls exterior of the scope of this text. This tutorial focuses on the combination of disparate back and front ends by means of an efficient cross-domain answer, leveraging GraphQL to implement authentication in an Angular-WordPress app. This tutorial doesn’t, nevertheless, assure the restriction of GraphQL entry strictly to logged-in customers, as attaining that might require configuring GraphQL to acknowledge entry tokens, a activity past our scope.
Step 1: Set Up the Utility’s Surroundings
That is the launch level for this challenge:
- Use a recent or current set up of WordPress in your system.
- Log in to WordPress as an administrator and, from the menu, select Settings/Normal. Within the membership part, choose the button beside Anybody can register to allow this feature.
- Together with WordPress, you’ll use the WPGraphQL plugin. Obtain the plugin from the WordPress plugin listing and activate it.
- To additional lengthen the WPGraphQL plugin’s performance, we may also use the WPGraphQL JWT Authentication plugin. It isn’t listed in WordPress’ listing, so add this plugin in line with its directions, ensuring to outline a secret key, as detailed within the
readme.md. The plugin is not going to work with out one. - Add a recent set up of Angular to your native system. Then create a workspace and software with routing and CSS help utilizing the command
ng n my-graphql-wp-app --routing --style css.- Caveat: This tutorial was written utilizing model 16 of Angular. For subsequent variations of Angular, you could have to adapt the steps and/or modify the file names introduced herein.
Together with your WordPress setup in place, the again finish of your easy weblog web site is prepared.
Step 2: Construct Out the App’s Entrance Finish
You’ll have to have all elements in place earlier than you may set up communication between the applying’s two ends. On this step, you’ll arrange the mandatory components: create pages, add and arrange routes, and combine the HTTP module. With these items in place, we will fetch and show content material.
The WPGraphQL plugin activated throughout setup will allow WordPress to reveal information by means of the app’s GraphQL API. By default, the GraphQL endpoint is situated at YOUR-SITE-URL/graphql the place YOUR-SITE-URL is changed with the URL related to the WordPress set up. For instance, if the positioning URL is instance.com, the app’s GraphQL API endpoint is instance.com/graphql.
Create the App’s Pages
This easy app will encompass simply two pages initially: posts (itemizing all publish titles) and publish (displaying a complete publish).
Generate the app’s content material pages utilizing Angular’s CLI technique. Utilizing your most well-liked terminal app, entry the Angular root listing and kind:
ng generate element posts && ng generate element publish
However these new pages received’t be seen with out a rendering container and routes.
Add Routes
A route permits customers to entry a web page instantly through a corresponding URL or navigation hyperlink. Though your recent Angular set up contains routing, the function is just not supported by default.
So as to add routes to the app, change the contents of the src/app/app-routing.module.ts file with:
import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { PostComponent } from './publish/publish.element';
import { PostsComponent } from './posts/posts.element';
const routes: Routes = [
{ path: 'post/:id', component: PostComponent },
{ path: 'posts', component: PostsComponent },
];
@NgModule( {
imports: [ RouterModule.forRoot( routes ) ],
exports: [ RouterModule ]
} )
export class AppRoutingModule { }
With the previous code, we’ve added two routes to the app: one path to the posts web page, the opposite to the publish web page.
Add the Router Outlet Part
To utilize routing help, we want the router-outlet that permits Angular to render the app’s content material pages because the consumer navigates to totally different routes.
Use your most well-liked code editor and change the contents of Angular’s src/app/app.element.html file with:
<router-outlet></router-outlet>
Now the route setup is full. However earlier than we will fetch content material, we’ve got to arrange the HTTP module middleware.
Combine the HTTP Module
To fetch content material for visiting customers, a web page must ship an HTTP request to the again finish. Change the contents of the src/app/app.module.ts file with:
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { HttpClientModule } from '@angular/frequent/http';
import { AppRoutingModule } from './app-routing.module';
import { PostComponent } from './publish/publish.element';
import { PostsComponent } from './posts/posts.element';
import { AppComponent } from './app.element';
@NgModule( {
declarations: [
AppComponent,
PostComponent,
PostsComponent,
],
imports: [
BrowserModule,
HttpClientModule,
AppRoutingModule
],
suppliers: [],
bootstrap: [ AppComponent ]
} )
export class AppModule { }
With this code, we’ve got built-in Angular’s native HTTP module, which permits us to ship HTTP requests to fetch content material.
Set As much as Fetch and Show Content material
Let’s now begin fetching and displaying content material on the weblog’s pages.
The Posts Web page
Change the contents of the src/app/posts/posts.element.ts file with:
import { Part } from '@angular/core';
import { HttpClient } from '@angular/frequent/http';
@Part( {
selector: 'app-posts',
templateUrl: './posts.element.html',
styleUrls: ['./posts.component.css']
} )
export class PostsComponent
{
posts = [];
constructor( non-public http: HttpClient ) { }
async send_graphql_request( question: string )
{
const response = await this.http.publish<any>( HERE_GOES_YOUR_GRAPHQL_API_ENDPOINT, { question: question }, { } ).toPromise()
return response;
}
ngOnInit()
{
this.send_graphql_request(
`question GetPostsQuery {
posts(the place: {orderby: {subject: DATE, order: DESC}}) {
nodes {
databaseId
featuredImage {
node {
sourceUrl
}
}
title
excerpt
}
}
}`
)
.then( response =>
{
if( typeof response.errors == 'undefined' && typeof response.information !== 'undefined' )
{
this.posts = response.information.posts.nodes;
}
else
{
console.log( 'One thing went flawed! Please strive once more.' );
}
} )
}
}
When a consumer accesses the posts web page, this code is triggered and sends an HTTP request to the again finish. The request leverages a GraphQL schema to fetch the newest posts from the WordPress database.
Subsequent, to show the fetched posts, change the contents of src/app/posts/posts.element.html file with:
<div class="content material" function="primary">
<h2 class="title">Checklist Of Posts</h2>
<div id="information">
<li class="publish" *ngFor="let publish of posts">
<img *ngIf="publish['featuredImage']" src="{{publish['featuredImage']['node']['sourceUrl']}}">
<img *ngIf="!publish['featuredImage']" src="https://picsum.photographs/300/200">
<h3>{{publish['title']}}</h3>
<a routerLink="/publish/{{publish['databaseId']}}">View Put up</a>
</li>
</div>
</div>
Add the next CSS to the app/src/posts/posts.element.css file to offer the posts web page with a minimalistic look:
.content material {
width: 900px;
margin: 0 auto;
}
h2.title {
text-align: middle;
}
li.publish {
list-style: none;
text-align: middle;
flex: 0 0 28.333333%;
margin-bottom: 15px;
}
img {
max-width: 100%;
}
div#information {
show: flex;
flex-direction: row;
justify-content: middle;
hole: 5%;
flex-wrap: wrap;
}
The Put up Web page
The identical process readies the publish web page. Change the contents of the src/app/publish/publish.element.ts file with:
import { Part } from '@angular/core';
import { HttpClient } from '@angular/frequent/http';
import { ActivatedRoute } from '@angular/router';
@Part( {
selector: 'app-post',
templateUrl: './publish.element.html',
styleUrls: ['./post.component.css']
} )
export class PostComponent
{
publish = {
title : '',
content material : '',
};
constructor( non-public route: ActivatedRoute, non-public http: HttpClient ) { }
async send_graphql_request( question: string )
{
const response = await this.http.publish<any>( HERE_GOES_YOUR_GRAPHQL_API_ENDPOINT, { question: question }, {} ).toPromise()
return response;
}
ngOnInit()
{
const post_id = this.route.snapshot.paramMap.get( 'id' );
this.send_graphql_request(
`question GetPostsQuery {
publish(id: "${post_id}", idType: DATABASE_ID) {
content material
title
}
}`
)
.then( response =>
{
if( typeof response.errors == 'undefined' && typeof response.information !== 'undefined' )
{
this.publish = response.information.publish;
}
else
{
console.log( 'One thing went flawed! Please strive once more.' );
}
} )
}
}
Now, to show the content material fetched from publish, change the contents of the src/app/publish/publish.element.html file with:
<div class="content material" function="primary">
<h2 class="title">{{publish.title}}</h2>
<div [innerHTML]="publish.content material"></div>
</div>
Lastly, add the next CSS to the app/src/publish/publish.element.css file:
.content material {
width: 900px;
margin: 0 auto;
}
h2.title {
text-align: middle;
}
These CSS guidelines will give publish the identical appear and feel as its mate.
Progress Examine
You’ve arrange the important components for the app and established the core infrastructure required for communication between the app’s Angular entrance finish and its headless WordPress again finish. In your browser, take a look at the viewability of the app’s pattern content material.
Step 3: Add Authentication
Including authentication permits for the restriction of the publish web page to be viewable solely by approved customers. To implement this, add a register web page and a login web page to the app.
The Registration Web page
Create the Web page
Use the terminal app to reaccess Angular’s root listing and kind:
ng generate element register
This creates a brand new web page named register.
To help HTML type enter fields as Angular enter, import Angular’s FormsModule into the src/app/app.module.ts file. Change the present file contents with:
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { HttpClientModule } from '@angular/frequent/http';
import { AppRoutingModule } from './app-routing.module';
import { PostComponent } from './publish/publish.element';
import { PostsComponent } from './posts/posts.element';
import { AppComponent } from './app.element';
import { RegisterComponent } from './register/register.element';
import { FormsModule } from '@angular/types'; //<----- New line added.
@NgModule( {
declarations: [
AppComponent,
PostComponent,
PostsComponent,
RegisterComponent,
],
imports: [
BrowserModule,
HttpClientModule,
AppRoutingModule,
FormsModule //<----- New line added.
],
suppliers: [],
bootstrap: [ AppComponent ]
} )
export class AppModule { }
In-line feedback are added to pinpoint adjustments made to the code.
Add a Route
Now, to create the register route, change the contents of the src/app/app-routing.module.ts file with:
import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { PostComponent } from './publish/publish.element';
import { PostsComponent } from './posts/posts.element';
import { RegisterComponent } from './register/register.element'; //<----- New line added.
const routes: Routes = [
{ path: 'post/:id', component: PostComponent },
{ path: 'posts', component: PostsComponent },
{ path: 'register', component: RegisterComponent }, //<----- New line added.
];
@NgModule( {
imports: [ RouterModule.forRoot( routes ) ],
exports: [RouterModule]
} )
export class AppRoutingModule { }
With the route added, it’s time to configure the app to confirm the brand new consumer’s credentials and finalize their registration. Change the contents of the src/app/register/register.element.ts file with:
import { Part } from '@angular/core';
import { Router } from '@angular/router';
import { HttpClient } from '@angular/frequent/http';
@Part( {
selector: 'app-register',
templateUrl: './register.element.html',
styleUrls: ['./register.component.css']
} )
export class RegisterComponent
{
constructor( public router: Router, non-public http: HttpClient ) {}
username = '';
electronic mail = '';
password = '';
error_message = '';
async send_graphql_request( question: string )
{
const response = await this.http.publish<any>( HERE_GOES_YOUR_GRAPHQL_API_ENDPOINT, { question: question }, { } ).toPromise()
return response;
}
register()
{
doc.getElementsByTagName( 'button' )[0].setAttribute( 'disabled', 'disabled' );
doc.getElementsByTagName( 'button' )[0].innerHTML = 'Loading';
this.send_graphql_request(
`mutation RegisterMutation {
registerUser(enter: {username: "${this.username}", electronic mail: "${this.electronic mail}", password: "${this.password}"}) {
consumer {
databaseId
}
}
}`
)
.then( response =>
{
if( typeof response.errors == 'undefined' && typeof response.information.registerUser.consumer.databaseId !== 'undefined' )
{
this.router.navigate( ['/login'] );
}
else
{
this.error_message = this.decodeHTMLEntities( response.errors[0].message );
}
doc.getElementsByTagName( 'button' )[0].innerHTML = 'Register';
doc.getElementsByTagName( 'button' )[0].removeAttribute( 'disabled' );
} )
}
decodeHTMLEntities( textual content : string )
{
const entities = [
['amp', '&'],
['apos', '''],
['#x27', '''],
['#x2F', '/'],
['#39', '''],
['#47', '/'],
['lt', '<'],
['gt', '>'],
['nbsp', ' '],
['quot', '"']
];
for ( let i = 0, max = entities.size; i < max; ++i )
textual content = textual content.change( new RegExp( '&' + entities[i][0] + ';', 'g'), entities[i][1] );
return textual content;
}
}
The register() technique on this code sends the brand new consumer’s credentials to the app’s GraphQL API for verification. If registration is profitable, the brand new consumer is created, and the API returns a JSON response with the newly created consumer ID. In any other case, an error message guides the consumer as vital.
Add Content material
So as to add a consumer registration type to the web page, change the contents of the src/app/register/register.element.html file with:
<div class="register-form">
<h2>Register</h2>
<div [innerHTML]="error_message"></div>
<type>
<enter sort="textual content" identify="username" [(ngModel)]="username" placeholder="Username" required />
<enter sort="textual content" identify="electronic mail" [(ngModel)]="electronic mail" placeholder="Electronic mail" required />
<enter sort="password" identify="password" [(ngModel)]="password" placeholder="Password" required />
<button sort="submit" class="btn" (click on)="register()">Register</button>
</type>
</div>
Let’s repeat these steps for the login web page.
The Login Web page
Create the Web page
Utilizing the terminal app, reaccess Angular’s root listing and kind:
ng generate element login
Create the login route by changing the contents of the src/app/app-routing.module.ts file with:
import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { PostComponent } from './publish/publish.element';
import { PostsComponent } from './posts/posts.element';
import { RegisterComponent } from './register/register.element';
import { LoginComponent } from './login/login.element'; //<----- New line added.
const routes: Routes = [
{ path: 'post/:id', component: PostComponent },
{ path: 'posts', component: PostsComponent },
{ path: 'register', component: RegisterComponent },
{ path: 'login', component: LoginComponent }, //<----- New line added.
];
@NgModule( {
imports: [ RouterModule.forRoot( routes ) ],
exports: [RouterModule]
} )
export class AppRoutingModule { }
To arrange the app to confirm the consumer’s credentials, change the contents of the src/app/login/login.element.ts file with:
import { Part } from '@angular/core';
import { Router } from '@angular/router';
import { HttpClient } from '@angular/frequent/http';
@Part( {
selector: 'app-login',
templateUrl: './login.element.html',
styleUrls: ['./login.component.css']
} )
export class LoginComponent
{
constructor( public router: Router, non-public http: HttpClient ) {}
username = '';
password = '';
error_message= '';
async send_graphql_request( question: string )
{
const response = await this.http.publish<any>( HERE_GOES_YOUR_GRAPHQL_API_ENDPOINT, { question: question }, { } ).toPromise()
return response;
}
login()
{
doc.getElementsByTagName( 'button' )[0].setAttribute( 'disabled', 'disabled' );
doc.getElementsByTagName( 'button' )[0].innerHTML = 'Loading';
this.send_graphql_request(
`mutation LoginMutation {
login(enter: {username: "${this.username}", password: "${this.password}"}) {
authToken
}
}`
)
.then( response =>
{
if( typeof response.errors == 'undefined' && typeof response.information.login.authToken !== 'undefined' )
{
localStorage.setItem( 'auth_token', JSON.stringify( response.information.login.authToken ) );
this.router.navigate( ['/posts'] );
}
else
{
this.error_message = this.decodeHTMLEntities( response.errors[0].message );
}
doc.getElementsByTagName( 'button' )[0].innerHTML = 'Login';
doc.getElementsByTagName( 'button' )[0].removeAttribute( 'disabled' );
} )
}
decodeHTMLEntities( textual content : string )
{
var entities = [
['amp', '&'],
['apos', '''],
['#x27', '''],
['#x2F', '/'],
['#39', '''],
['#47', '/'],
['lt', '<'],
['gt', '>'],
['nbsp', ' '],
['quot', '"']
];
for ( var i = 0, max = entities.size; i < max; ++i )
textual content = textual content.change( new RegExp( '&' + entities[i][0] + ';', 'g'), entities[i][1] );
return textual content;
}
}
Subsequent, change the contents of the src/app/login/login.element.html file with:
<div class="log-form">
<h2>Login to your account</h2>
<div [innerHTML]="error_message"></div>
<type>
<enter sort="textual content" identify="username" [(ngModel)]="username" placeholder="Username" required />
<enter sort="password" identify="password" [(ngModel)]="password" placeholder="Password" required />
<button sort="submit" class="btn" (click on)="login()">Login</button>
</type>
</div>
This snippet provides a login type to the web page with inputs for consumer credentials. Much like the best way the app’s registration web page is ready up, the code added right here sends an current consumer’s credentials to the app’s GraphQL API for validation. If the credentials are right, the API returns a JWT, saving it within the browser’s localStorage for later use. If the consumer’s credentials are invalid or if the JWT has expired, an error message guides them as vital.
Progress Examine
To check authentication, register as a brand new consumer and log in to the app. Then, to log off, take away the token from the browser’s localStorage. Your outcomes ought to look much like the screenshots under:
Step 4: Implement Restrictions
With the authentication function up and working, the following activity is to limit entry to the publish route, permitting logged-in customers solely.
Create and Set Up the Guard and Service
Utilizing the terminal app, reaccess Angular’s root listing and kind:
ng generate service auth && ng generate guard auth
You may be prompted with an inventory of interfaces to implement. Select CanActivate to ascertain a guard that confirms a consumer’s authentication by means of a service, additionally created on this step.
Subsequent, arrange your guard and repair to handle the authentication. Change the contents of the src/app/auth.service.ts file with:
import { Injectable } from '@angular/core';
import { Router } from '@angular/router';
@Injectable( {
providedIn: 'root'
} )
export class AuthService
{
router : any;
constructor( non-public route: Router )
{
this.router = route
}
loggedIn()
{
if( localStorage.getItem( 'auth_token' ) != null ) return true;
this.router.navigate( ['/login'] ); return false;
}
}
With this code, your setup of the service to handle authentication is full. If a JWT is current, the service sends an affirmative response to the guard. In any other case, it returns a false response to point that the consumer is just not logged in.
To limit the publish route primarily based on info acquired from the service, change the contents of the src/app/auth.guard.ts file with:
import { CanActivateFn } from '@angular/router';
import { AuthService } from './auth.service';
import { inject } from '@angular/core';
export const authGuard: CanActivateFn = ( route, state ) =>
{
// Use dependency injection to get an occasion of the AuthService.
const authService = inject( AuthService );
// Return whether or not the consumer is logged in utilizing the AuthService.
return authService.loggedIn();
};
Now the publish web page is restricted, permitting solely logged-in customers.
Prohibit the Put up Web page’s Route
To increase the publish web page’s restriction, let’s implement a route-specific restriction. Change the contents of the src/app/app-routing.module.ts file with:
import { NgModule } from '@angular/core';
import { RouterModule, Routes, CanActivate } from '@angular/router';
import { PostComponent } from './publish/publish.element';
import { PostsComponent } from './posts/posts.element';
import { LoginComponent } from './login/login.element';
import { RegisterComponent } from './register/register.element';
import { authGuard } from './auth.guard'; //<----- New line added.
const routes: Routes = [
{ path: 'post/:id', component: PostComponent, canActivate: [ authGuard ] }, //<----- New code added.
{ path: 'posts', element: PostsComponent },
{ path: 'register', element: RegisterComponent },
{ path: 'login', element: LoginComponent },
];
@NgModule( {
imports: [ RouterModule.forRoot( routes ) ],
exports: [ RouterModule ]
} )
export class AppRoutingModule { }
With the modified code, the publish web page’s route now makes use of Angular’s canActivate technique to serve the web page solely to authenticated customers.
Confirm the JWT
You at the moment are able to validate the JWT saved within the visiting consumer’s browser. Particularly, you’ll examine in actual time that the JWT is unexpired and legitimate. Change the contents of the src/app/publish/publish.element.ts file with:
import { Part } from '@angular/core';
import { HttpClient } from '@angular/frequent/http';
import { ActivatedRoute } from '@angular/router';
@Part( {
selector: 'app-post',
templateUrl: './publish.element.html',
styleUrls: ['./post.component.css']
} )
export class PostComponent
{
publish = {
title : '',
content material : '',
};
constructor( non-public route: ActivatedRoute, non-public http: HttpClient ) { }
async send_graphql_request( question: string )
{
let headers = {};
// New code begins right here.
const token = localStorage.getItem( 'auth_token' );
if( token !== null )
{
const parsedToken = JSON.parse( token );
if( parsedToken )
{
headers = { 'Authorization': 'Bearer ' + parsedToken };
}
}
// New code ends right here.
const response = await this.http.publish<any>( HERE_GOES_YOUR_GRAPHQL_API_ENDPOINT, { question: question }, { headers } ).toPromise()
return response;
}
ngOnInit()
{
const post_id = this.route.snapshot.paramMap.get( 'id' );
this.send_graphql_request(
`question GetPostsQuery {
publish(id: "${post_id}", idType: DATABASE_ID) {
content material
title
}
}`
)
.then( response =>
{
if( typeof response.errors == 'undefined' && typeof response.information !== 'undefined' )
{
this.publish = response.information.publish;
}
else
{
console.log( 'One thing went flawed! Please strive once more.' );
}
} )
}
}
This code injects the saved JWT as a bearer authorization header into every HTTP request made by the consumer visiting the publish web page. To emphasise adjustments from the code’s earlier iteration, new code is ready off by feedback.
Ultimate Output: Attaining Dynamic and Safe UX
To verify that restrictions are working correctly, guarantee you aren’t logged in and entry the posts web page. Subsequent, try to entry the publish web page. You ought to be redirected to the login web page. Log in to view fetched content material on the publish web page. If the app works as anticipated, you’ve successfully accomplished this tutorial and developed a decoupled, protected SPA.
On this digital age, offering a dynamic and safe consumer expertise is an expectation, not an enhancement. The ideas and approaches explored on this tutorial might be utilized to your subsequent decoupled challenge to attain scalability whereas providing builders flexibility in designing and delivering efficient web sites.
The editorial crew of the Toptal Engineering Weblog extends its gratitude to Branko Radulovic for reviewing the code samples and different technical content material introduced on this article.
