title: What is window.localStorage
body: <p>The <a href="http://code.google.com/chrome/extensions/examples/extensions/email_this_page/options.js"><code>option.js</code></a> file of <a href="http://code.google.com/chrome/extensions/samples.html#028eb5364924344029bcbe1d527f132fc72b34e5">"<strong>Email this page</strong>"</a> (Chrome extension example) contains the following code:</p>
  
  <pre>
  if (<b>window.localStorage</b> == null) {
    ...
  if (<b>window.localStorage.customMailtoUrl</b> == null) {
  </pre>
  
  <p>What does this mean? What is <code>window.localStorage</code>?</p>