title: chrome.storage.local["mykey"] is undefined
body: <p>I am making a Chrome extension where I will store the password from the user in local storage. I am storing it in <code>popup.js</code> using</p>
  
  <pre><code>chrome.storage.local["mykey"] = "xxx";
  </code></pre>
  
  <p>When I use <code>chrome.storage.local["mykey"]</code>, I'm getting <code>undefined</code>. Can you tell me how to store and retrieve user data in a Chrome extension?</p>