Inherited typing for class property initializers #10570
Description
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 expectedBad 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