Setting field in superclass isn't propagated to subclass

Question

In an effort to reduce boilerplate in my constructors, I have an abstract class implementation that tries to set fields on the implementing class. A simplified version is below. The problem is that in the superclass constructor, the fields appear to be set, but outside of that constructor, they are undefined.

Code:

abstract class BaseClass {
  constructor(args: any) {
    Object.keys(args).forEach((key) => (this[key] = args[key]));
    console.log("constructed", this);
  }
}

class ChildClass extends BaseClass {
  a: number;
  b: string;

  print() {
    console.log(this);
  }
}

const val = new ChildClass({ a: 1, b: "str" });
val.print();

The output of this code is:

constructed ChildClass { a: 1, b: 'str' }
ChildClass { a: undefined, b: undefined }

Is there any way to make it so that BaseClass's constructor sets values that are accessible from ChildClass?