Declaring additional properties on an inferred type

Issue

I have a class with a constructor that returns a complex object who's type is partly inferred. Now I want to add some properties to that returned type. I don't want to declare them on the class. Is there a way to do this?

type AdditionalProps = {
 myAdditionalProp : string
}

const myInstance: inferredType & AdditionalProps = new MyClass()
myInstance.myAdditionalProp = "I love typescript."

Solution

You can use Object.assign to add new props to newly created object. The type is inferred correctly, but can be stated explicitly as well.

class MyClass {
  id: number;

  constructor() {
    this.id = 1;
  }
}

type AdditionalProps = {
 myAdditionalProp : string
}

const myInstance: MyClass & AdditionalProps = Object.assign(new MyClass(), {myAdditionalProp: 'I love typescript.'});
// NOTE: the type can be inferred
console.log(myInstance.id);
console.log(myInstance.myAdditionalProp);

Answered By - Lesiak