fetchUser.jsx

import React from 'react';

class FetchUser extends React.Component {
  state = {
    user: undefined,
  }

  componentDidMount() {
    if (this.props.userid) {
      this.fetchUser(this.props.userid);
    }
  }

  componentWillReceiveProps(nextProps) {
    if (nextProps.userid && this.props.userid !== nextProps.userid) {
      this.fetchUser(nextProps.userid);
    }
  }
        
  fetchUser = userid => APIFetchUser(userid).then(user => this.setState({ user }));

  render() {
    this.props.children(this.state.user);
  }
}

export default FetchUser;

route.jsx

import React from 'react';

const UserRoute = ({ params: { userid } }) => (
  <FetchUser userid={userid}>
    {(user) => (user ? <User user={user} /> : <SomethingElse />)}
  </FetchUser>
);

export default UserRoute;