How to express type for output class of the TS decorator

Question

The following code is from the TS docs

function classDecorator<T extends { new (...args: any[]): {} }>(
  constructor: T
) {
  return class extends constructor {
    newProperty = "new property";
    hello = "override";
  };
}

@classDecorator
class Greeter {
  property = "property";
  hello: string;
  constructor(m: string) {
    this.hello = m;
  }
}
const g = new Greeter("world");
console.log(g.newProperty) // =>>>> ERROR for type check, but correct in runtime

How do I express the type for the for the decorated class?

To be more exact, I want intellisense to see the g.newProperty

Answer 1

There is another concept called mixin , a function that returns modified class TypeScript is able to suggest correct props for the returned type of the class

function Mixin<T extends { new (...args: any[]): object }>(BaseClass: T) {
  return class extends BaseClass {
    // The following constructor signature is a must for a mixin
    // TypeScript error tells it explicitly
    constructor(...args: any[]) {
      super(...args)
      // Additional stuff
    }
    // Additional functionality here
  }
}

class A {}

const ModifiedA = Mixin(A);

const ModifiedAObject = new ModifiedA(/* params */);