add :before conditionally react code example

Example 1: conditional jsx property

render() {
  return (
    <a className="navbar-brand" {... url ? {href: url} : {}}>Logo</a>
  )
}

Example 2: react if statement

render() {
  const isLoggedIn = this.state.isLoggedIn;
  return (
    <div>
      The user is <b>{isLoggedIn ? 'currently' : 'not'}</b> logged in.    </div>
  );
}