title: What does 'symbol-keyed' mean in `JSON.stringify`
body: <p>There is a Object generated by Node.js, it looks like this when I use <code>console.log</code>:</p>
  <pre><code>{ dataValues: { a: 1, b: 2 }, fn1: function(){}, fn2: function(){} }
  </code></pre>
  <p>when I use <code>JSON.stringify</code>, it return this string:</p>
  <pre><code>{&quot;a&quot;:1,&quot;b&quot;:1}
  </code></pre>
  <p>I checked the mozilla developer center and found <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify" rel="nofollow noreferrer">this</a>:</p>
  <blockquote>
  <p>All <strong>symbol-keyed</strong> properties will be completely ignored, even when using the replacer function.</p>
  </blockquote>
  <p>I think the 'dataValues' must be the 'symbol-keyed' property, but what does 'symbol-keyed' mean?</p>
  <p>btw, I use the <a href="http://sequelizejs.com/" rel="nofollow noreferrer">sequelizejs</a> ORM lib to generate this object.</p>