Extending React components in TypeScript

import { Component } from 'react'


abstract class TestComponent<P = {}, S = {}, SS = any> extends Component<P, S, SS> {
  abstract test(): string
}


type Props = {
  first: string,
  last: string,
}

type State = {
  fullName: string,
}

class MyTest extends TestComponent<Props, State> {
  constructor(props: Props) {
    super(props)
    this.state = {
      fullName: `${props.first} ${props.last}`
    }
  }

  test() {
    const { fullName } = this.state
    return fullName
  }
}

You can set only a few properties of the state at once in Derived by using a type assertion:

this.setState({ b: 4 } as DerivedStates); // do this
this.setState({ a: 7 } as DerivedStates); // or this
this.setState({ a: 7, b: 4 });            // or this

By the way, no need to have different names for getInitialState... you could just do:

class GenericBase<S extends BaseStates> extends React.Component<void, S> {
    constructor() {
        super();        
        this.state = this.getInitialState();
    }

    protected getInitialState() {
        return { a: 3 } as BaseStates as S;
    }
}

class Derived extends GenericBase<DerivedStates> {
    getInitialState() {
        var initialStates = super.getInitialState();
        initialStates.b = 4;
        return initialStates;
    }
}