The presence of the input variable does not automatically determine if the Angular component will be rendered conditionally or not

I have a situation where my Angular component has an array of Input variables that are asynchronously initialized in the parent component.

@Component({ ... })
export class ChildComponent {
  @Input inputVariable: string;
}
@Component({ ... })
export class ParentComponent implements OnInit {
  inputVariables: string[] = [];

  constructor( private http: HttpClient ) { }

  ngOnInit(): void {
    this.http.get<string[]>('someUrl')
      .subscribe(res => this.inputVariables = res)
  }
}

Now I am looking to display a ChildComponent within parent.component.html for each input variable like this:

<div *ngFor="let inputVariable of inputVariables">
  <child-component [inputVariable]="inputVariable" />
</div>

Even though in ChildComponent I am certain that inputVariable is defined, TypeScript raises a complaint stating that the type of inputVariable must be string or undefined. Having to check if inputVariable is defined in every usage within ChildComponent is not ideal. Is there a way for TypeScript to recognize that inputVariable will always be defined for any rendered ChildComponent?

Answer №1

The @Input value is assigned after initialization (specifically on the ngOnInit hook). To avoid TypeScript errors, set the default value for inputVariable like this:

@Component({ ... })
export class ChildComponent {
  @Input inputVariable: string = "";
}

It recommends setting the type to string | undefined, since the default value of variables in JavaScript is undefined. By assigning an empty string as the default value, you can avoid potential type issues.

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