How do you get the type of the object that is cloned from a Class Instance?

Question

assume I have this example class, but in reality it has many more properties

class Foo {
  name: string
  dob: number

  constructor(name: string, dob: number) {
    this.name = name;
    this.dob = dob;
  }

  get age() {
     return new Date().getTime() - this.dob
  }
}

Now Typescript is smart and when I instantiate the class it will give me all the right properties:

var classInstance = new Foo('name', new Date().getTime())

classInstance.name // ok
classInstance.dob // ok
classInstance.age // ok

Somewhere in my code the class gets cloned using the Spread Operator, I'm not sure what TS does behind the scene but it is really smart and gives me all the right properties

var classJSON = {...classInstance};

classJSON.name // ok
classJSON.dob // ok
classJSON.age // missing

tsplayground

This is great, however I sometime need to use the type of classJSON .. The only way I can think to extract it is to do this:

var classJSON  = {...new Foo('', 0)}
type ClassJSONType = typeof classJSON; 

Is there a way to extract the type straight out of Foo without needing Javascript to instantiate?

Answer 1

Disclaimer: this post is a mod based on Matt McCutchen's answer. It's a solid solution, pure TS without any JS runtime effect.

type IfEquals<X, Y, T> =
    (<T>() => T extends X ? 1 : 2) extends
  (<T>() => T extends Y ? 1 : 2) ? T : never;

type JSONify<T> = Pick<T, {
  [P in keyof T]: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, P>
}[keyof T]>;

---

Above trick excludes all readonly fields. To further exclude methods, use the following:

type JSONify<T> = Pick<T, {
  [P in keyof T]: IfEquals<{ [Q in P]: T[P] extends Function ? never : T[P] }, { -readonly [Q in P]: T[P] }, P>
}[keyof T]>;

Playground