title: TypeScript - what is the 'unique' keyword for?
body: <p>I'm just writing some code in the TypeScript playground, and found that <code>unique</code> appears to be a reserved word, but I can't find any documentation for it.</p>
  
  <p><a href="https://i.stack.imgur.com/eQq5b.png" rel="noreferrer"><img src="https://i.stack.imgur.com/eQq5b.png" alt="enter image description here"></a></p>
  
  <p>Is there any documentation on this keyword anywhere?</p>