title: variable === undefined vs. typeof variable === "undefined"
body: <p>The <a href="https://contribute.jquery.org/style-guide/js/#type-checks" rel="noreferrer">jQuery Core Style Guidelines</a> suggest two different ways to check whether a variable is defined.</p>
  <ul>
  <li>Global Variables: <code>typeof variable === &quot;undefined&quot;</code></li>
  <li>Local Variables: <code>variable === undefined</code></li>
  <li>Properties: <code>object.prop === undefined</code></li>
  </ul>
  <p>Why does jQuery use one approach for global variables and another for locals and properties?</p>