title: Passing object in arrow-function in javascript
body: <p>I have a really hard time understanding the workflow of the following code-snippet</p>
  
  <pre><code>const Circuit = require('circuit-sdk');
  const client = new Circuit.Client({
    client_id: '&lt;client_id&gt;',
    client_secret: '&lt;client_secret&gt;',
    domain: 'circuitsandbox.net'
  });
  client.logon()
    .then(user =&gt; console.log('Logged on as bot: ' + user.emailAddress))
    .catch(console.error);
  </code></pre>
  
  <p>How is, in the second last line, the object <code>user</code> defined? Or, asked another way, how can i access <code>user.emailAddress</code>, without defining it beforehand?</p>
  
  <p>This code works, its a sample from the documetation, i just can't get it into my head</p>