title: Parameter undefined in URL
body: <p>I have just started learning Angular and I am lost for this first application.
  I'm looking to pass an ID to a child component so that it executes a request to an API to retrieve data.</p>
  
  <p>Here is my code:</p>
  
  <p><strong>Child component:</strong></p>
  
  <pre><code>@Component({
    selector: 'app-set-content',
    templateUrl: './set-content.component.html',
    styleUrls: ['./set-content.component.css']
  })
  export class SetContentComponent implements OnInit {
    @Input() setId;
    equipments;
  
    constructor(
      private http: HttpClient
    ) {
      this.equipments = this.http.get('https://myapi.com/sets/'+this.setId+'/details')
     }
  
    ngOnInit() {
    }
  
  }
  </code></pre>
  
  <p><strong>HTML Code for the Parent component:</strong></p>
  
  <pre><code>&lt;div class="card" *ngFor="let set of sets | async"&gt;
      &lt;div class="card-header"&gt;
              {{ set.name }}
      &lt;/div&gt;
  
      &lt;div class="card-block"&gt;
          &lt;div class="card-text"&gt;
                  &lt;app-set-content [setId]="set._id"&gt;
                  &lt;/app-set-content&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  
  &lt;/div&gt;
  </code></pre>
  
  <p>However, in my browser console, I see that the ID passed to the child component is "undefined".</p>
  
  <pre><code>GET https://myapi.com/sets/undefined/details 400 (Bad Request)
  </code></pre>
  
  <p>I tried to pass an arbitrary parameter like "1" but I get the same error</p>