Implementation of Open/Closed Principle

src/admin.type.ts

@@ -0,0 +1,9 @@
+import Admin from './admin';
+import User from './user';
+import UserFactory from './user.factory';
+
+export default class AdminType implements UserFactory {
+  create(pseudo: string): User {
+    return new Admin(pseudo);
+  }
+}

src/application.ts

@@ -1,11 +1,11 @@
-import Admin from './admin';
+import UserFactoryProvider from './user.factory.provider';
 import Article from './article';
 import ArticleJsonFactory from './article.json.factory';
 import ArticleRepository from './article.repository';
-import Contributor from './contributor';
 import Database from './database';
 import { UserType } from './types';
 import User from './user';
+import UserFactory from './user.factory';
 
 export default class Application {
   run(): number {
@@ -16,14 +16,9 @@ export default class Application {
       const userPseudo = 'pierre';
       const userType: UserType = 'admin';
 
-      const user: User = (() => {
-        switch (userType as UserType) {
-          case 'admin':
-            return new Admin(userPseudo);
-          case 'contributor':
-            return new Contributor(userPseudo);
-        }
-      })();
+      const userFactory: UserFactory =
+        UserFactoryProvider.provideFromUserType(userType);
+      const user: User = userFactory.create(userPseudo);
 
       const article = new Article(user, 'Mon premier article');
 

src/contributor.type.ts

@@ -0,0 +1,9 @@
+import Contributor from './contributor';
+import User from './user';
+import UserFactory from './user.factory';
+
+export default class ContributorType implements UserFactory {
+  create(pseudo: string): User {
+    return new Contributor(pseudo);
+  }
+}

src/user.factory.provider.ts

@@ -0,0 +1,15 @@
+import AdminType from './admin.type';
+import ContributorType from './contributor.type';
+import { UserType } from './types';
+import UserFactory from './user.factory';
+
+export default abstract class UserFactoryProvider {
+  static provideFromUserType(userType: UserType): UserFactory {
+    if (userType === 'admin') {
+      return new AdminType();
+    } else if (userType === 'contributor') {
+      return new ContributorType();
+    }
+    throw new Error(`"${userType} is not a valid user type"`);
+  }
+}

src/user.factory.ts

@@ -0,0 +1,5 @@
+import User from './user';
+
+export default interface UserFactory {
+  create(pseudo: string): User;
+}