interface State {
  someProperty: string;
}

class MakeMeSafer extends React.Component<{}, State> {

  state: State = {
    someProperty: ''
  }

  set = ({ currentTarget: { name, value } }: React.SyntheticEvent<HTMLInputElement>): void => {
    this.setState({ [name as keyof State]: value });
  }

  render(): JSX.Element {
    return (
      <div>
        <input type="text" name="some-name-not-in-state" onChange={this.set} />
      </div>
    );
  }

}