title: JavaScript `of` Keyword (for...of loops)
body: <p>I just spotted, <a href="https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/tabs#Access_tabs" rel="noreferrer">in Firefox SDK JavaScript (on MDN)</a>, the use of a keyword I have never seen before:</p>
  
  <pre><code>var tabs = require('sdk/tabs');
  for (let tab of tabs)
    console.log(tab.title);
  </code></pre>
  
  <p>Is the <code>of</code> keyword made up by Mozilla or is it standardized?</p>