更加简便的注入方法，不用事先声明及使用this来赋值

src/posts/posts.controller.ts

@@ -5,12 +5,8 @@ import { DemoService } from './providers/demo/demo.service'
 
 @Controller('posts')
 export class PostsController {
-    private readonly demoService;
-
-    constructor(demoService: DemoService) {
-        this.demoService = demoService
-    }
 
+    constructor(private readonly demoService: DemoService) {}
 
     @Get()
     index() {