What's the best way for me to figure out whether type T is implementing an interface?

Is it possible to set a default value for the identifier property in TypeScript based on whether the type extends from Entity or not? Here's an example of what I'm trying to achieve:

export interface Entity {
    id: number;
    // ...
}

@Component({})
export class EpicComponent<T> {
  @Input() data: T[] | null = null;
  @Input() identifier: keyof T | null = T is of type Entity ? 'id' : null; // <--
}

In the above code snippet, when the generic type T is an Entity, the identifier is automatically set to be the id. But for other types, a different property might be needed as the identifier. Is there a way to accomplish this in TypeScript, or is there a different approach that could work better? Any help or suggestions would be greatly appreciated.

Answer №1

When it comes to checking inheritance using the interface in JavaScript, it's not directly possible since interfaces don't get compiled to JS. However, you can utilize type guards, like the isEntity function demonstrated below.

Would implementing something similar to this be helpful for your situation?

export interface Entity {
  id: number;
  // ...
}

function isEntity(o: any): o is Entity {
  return (o as Entity)?.id != null;
}

@Component({})
export class EpicComponent<T> {
@Input() data: T[] | null = null;
@Input() identifier: keyof T | null = null;

ngOnInit() {
  if(isEntity(this.data?.[0])) {
    this.identifier = 'id' as keyof T;
  }
}
}

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