title: Arrow Function this prints var but does not print let variable
body: <p>I have this code</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var key = "outer value";
  let obj = {
    key:"obj value",
    func:()=&gt; this.key,
  }
  
  console.log(obj.func());</code></pre>
  </div>
  </div>
  </p>
  <p>But when the var is changed to let, the result is undefined. Please guide.</p>