How to extend a property by method

[popomore]

TypeScript Version: ~2.0.0

Code

interface FuncType {
  (key: any): Object;
}

class A {
  prop: FuncType;
}

class B extends A {
  prop() {}
}

Expected behavior:

I want to define a method with a FuncType, and implement it in extended class.

Actual behavior:

Got an error

Class 'A' defines instance member property 'prop', but extended class 'B' defines it as instance member function.

[aluanhaddad]

This is intentional because the semantics are very different. If you're looking at the type described by the base class, it suggests that prop does not require explicit binding of this, meaning that it can be referenced independently.

To implement the contract correctly, write

class B extends A {
  prop = () => {};
}

[RyanCavanaugh]

The fundamental restriction here is that prop() { } declares a prototype member, whereas prop: Functype declares an instance member. It's not possible for a prototype member to 'hide' an instance member, so any implementation declared this way would never be called.

It sounds like you might want to make A abstract based on your setup.