title: Operator '+' cannot be applied to types 'String' and 'String' in TypeScript
body: <p>I am new to TypeScript trying to play with it. But I face a wired problem. When I try to concatenate two <code>String</code> type using <code>+</code> operator it's give an error that is </p>
  
  <p><strong>Operator '+' cannot be applied to types 'String' and 'String'</strong> </p>
  
  <p>My Code snap is</p>
  
  <pre><code>var firstName: String = 'Foo';
  var lastName: String = 'Bar';
  var name = firstName + lastName;
  </code></pre>
  
  <p>If I use <code>string</code> instead  <code>String</code> or add extra <code>''</code>it works fine. I checked, within JavaScript we can use <code>+</code> on two <code>String</code> object then why it's show error in TypeScript ? Is it bug or feature ? I definitely missing something. Detail explanation appreciated.</p>