Children propType validation

gistfile1.js

let propTypes = {
  children(props, propName) {
    let errors = [];
    React.Children.map(props[propName], child => {
      if (child.type !== Foo) {
        errors.push(`${child.key} is not a Foo element`);
      }
    });
    if (errors.length > 0) {
      return new Error(errors.join(', '));
    }
  },
};