title: Check if IndexedDB database exists
body: <p>Is there a way to check if an IndexedDB database already exists? When a program tries to open a database that does not exists the database is created.
  The only way that I can think of is something like the following, where I test if an objectStore already exists, if it doesn't, the database is deleted:</p>
  
  <pre><code>var dbexists=false;
  var request = window.indexedDB.open("TestDatabase");
  request.onupgradeneeded = function(e) {
      db = e.target.result;
      if (!db.objectStoreNames.contains('todo')) {
         db.close();
         indexedDB.deleteDatabase("TestDatabase");
      } else {
         dbexists=true;
      }
  }
  </code></pre>