Inherited typing for class property initializers

Problem

Initializing a class member with things like { }, null, undefined, or [] has unexpected behavior.

class Base {
  favorites = ["red", "blue"];
}
class Derived extends Base {
  favorites = [];
  constructor() {
    this.favorites.push('green'); // Can't push string onto never[], wat?
  }
}

interface Settings {
  size?: number;
  color?: string;
}
class Base {
  settings: Settings = { size: 42 };
}
class Derived extends Base {
  settings = { };
  constructor() {
    if (big) this.settings = { siz: 100 }; // no error, wat?
  }
}

Solution

New rule: When a class property is initialized with exactly null, undefined, { }, or [], the type of the property is taken from the same property of the inherited type (if one exists), rather than the type of the initializer.

The inherited type is B & I1 & I2 & ... where B is the base class and I1, I2, ... are the implemented interfaces of the class.

Examples

interface Positionable {
  position: string | null;
}
class MyPos implements Positionable {
  position = null;
  setPos(x: string) {
    this.position = x;
  }
  getPos() {
    return this.position.subtr(3); // error detected
  }
}

class Base {
  items = ['one'];
}
class Derived extends Base {
  items = []; // no longer an implicit any
}
var x = new Derived();
x.items.push(10); // Error as expected

Bad Ideas We Thought Were good

Contextual typing plays poorly with other behavior such as unit type positions. Consider

enum E { A, B, C }
class Base {
  thing = E.A;
}
class Derived extends Base {
  thing = E.B;
  change() {
    this.thing = E.C; // Error! wat
  }
}

This turns into a big problem because the E.B expression is contextually typed by the unit-like type E.A | E.B | E.C and so acquires the specific type E.B rather than the intended type E! Daniel found this break in Azure.

/cc conspirators @DanielRosenwasser @sandersn

[paddotk]

This issue has been open since 2016, any updates?

[Veetaha]

Is there any update on this issue? I am eager to see it in the closest releases.

Regarding this feedback:

Of course, there may be some breaking changes, but those will appear in bad code (with implicit any) and some of them can be fixed, like setting the type of the property by its initializer if it is assignable to the type from base/implemented interface. I think that this feature will bring more advantages (like less boilerplate and narrowing the sources of truth throughout the types of TypeScript apps).