What is the most effective method for extracting a specific type from a union type?

I'm working with this code snippet:

interface MyInterface {
  name: string;
}

type MyType = string | MyInterface;

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {

  constructor() {
    console.log(this.getValueAsString('Hello World'));
    console.log(this.getValueAsString({name: 'Hello World'}));
  }

  // I need to return 'Hello World' regardless of the type
  getValueAsString(myValue: MyType): string {
    // current implementation is causing compiler errors
    // return myValue.name ? myValue.name : myValue;
  }
}

The issue lies in resolving the type mismatch error for the function getValueAsString.

What would be the optimal solution for handling getValueAsString?

(stackblitz: https://stackblitz.com/edit/angular-jrty3q)

Answer №1

To distinguish between types, you can utilize a type guard:

getValueAsString(myValue: MyType): string {
  return typeof myValue === "string" ? myValue : myValue.name;
}

In this scenario, by using the type guard typeof myValue === "string", the TypeScript compiler recognizes that myValue will be either a string or a MyInterface. This guard informs the compiler that in the true branch of the conditional (after the ?), myValue is a string, and in the false branch (after the :), it is a MyInterface.

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