Is it possible for an Object.prototype function in Typescript to return an instance of a Sub type?

Question

Is it possible for an Object.prototype function in Typescript to return an instance of a Sub type?

I'm working on some code that involves defining classes like the following:

class Place {
  next: Place;
  get to() : Place {
    return this;
  }
}
let places : Place[]= [];
..

places[0].to.next = new Place();

Since there are many similar classes, I thought it would be useful to define the 'to' property on Object.prototype.

Object.defineProperty(Object.prototye,"to",{
  get: function() {
    return this;
  }
});

However, during compilation, I encountered an error:

Property 'next' does not exist on type 'Object'

Is there a way in Typescript to return a subtype using an Object.prototype function or property?

javascript typescript get subtype defineproperty

Answer 1

Answer №1

It appears that Typescript may not be able to fully capture the behavior you desire.

One approach that comes to mind is utilizing a method instead of a property. By defining a this parameter in methods, we can infer its type and use it as the return type:

class Location extends Object{
  next: Location;
}
let locations: Location[] = [];

interface Object{
  to<T>(this: T):T; 
}
Object.prototype.to = function () {
  return this;
};

locations[0].to().next = new Location();

A straightforward solution could involve creating a base class for all such objects with a property typed as polymorphic this:

class Core {
  get to(): this { return this; }
}
class Location extends Core{
  next: Location;
}
let locations: Location[] = [];
locations[0].to.next = new Location();

Note: Introducing changes to the global Object might not be ideal, so consider the implications before proceeding.

Answer 2

It appears that Typescript may not be able to fully capture the behavior you desire.

One approach that comes to mind is utilizing a method instead of a property. By defining a this parameter in methods, we can infer its type and use it as the return type:

class Location extends Object{
  next: Location;
}
let locations: Location[] = [];

interface Object{
  to<T>(this: T):T; 
}
Object.prototype.to = function () {
  return this;
};

locations[0].to().next = new Location();

A straightforward solution could involve creating a base class for all such objects with a property typed as polymorphic this:

class Core {
  get to(): this { return this; }
}
class Location extends Core{
  next: Location;
}
let locations: Location[] = [];
locations[0].to.next = new Location();

Note: Introducing changes to the global Object might not be ideal, so consider the implications before proceeding.

Answer 3

Answer №2

I managed to find a solution.

TS now includes a return type of "this".

class Entity {
  self() : this {
    return this;
  } 
}

class Place extends Entity {
  where(toGo:string) {
    ....
  }
}

By utilizing the code snippet below, I can easily interact with the place:

new Place().self().where("Canada");

The self method is declared in the superclass but it has the ability to return a subclass type.

This allows me to use the place instance without requiring any type casting.

Answer 4

I managed to find a solution.

TS now includes a return type of "this".

class Entity {
  self() : this {
    return this;
  } 
}

class Place extends Entity {
  where(toGo:string) {
    ....
  }
}

By utilizing the code snippet below, I can easily interact with the place:

new Place().self().where("Canada");

The self method is declared in the superclass but it has the ability to return a subclass type.

This allows me to use the place instance without requiring any type casting.

Is it possible for an Object.prototype function in Typescript to return an instance of a Sub type?

Answer №1

Answer №2

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