New rule: require explicitly typed `defaultProps`

It’s easy write a component with unsafe defaultProps:

interface MyComponentProps {
  onClick?: React.MouseEventHandler<HTMLElement>;
}

class MyComponent extends React.Component<MyComponentProps> {
  static defaultProps = {
    onClick: 42
  };

  render() {
    return <div onClick={this.props.onClick} />;
  }
}

So it’s useful to enforce declaring an explicit type for static defaultProps (which should basically always be Partial<P> where P is the type of the component’s props), which would catch such silly mistakes:

interface MyComponentProps {
  onClick?: React.MouseEventHandler<HTMLElement>;
}

class MyComponent extends React.Component<MyComponentProps> {
  static defaultProps: Partial<MyComponentProps> = {
  //     ^^^^^^^^^^^^ Type { onClick: number } is not convertible to Partial<MyComponentProps> or some error message along these lines
    onClick: 42
  };
}

[suchanlee]

I think it would be useful and would be happy to accept a PR!

[andrewbranch]

Actually, in TypeScript 3.0, you don’t want to explicitly type defaultProps. So, maybe instead of requiring an explicit type, a rule that checks that the inferred type of defaultProps is assignable to the class’s props interface would be valuable—which would definitely require the type checker.