title: Can't do math operation in property declaration?
body: <p>JavaScript learner here: given this object:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var ivan = {
      name: 'Ivan',
      yearOfBirth: 1973,
      age: 2017 - this.yearOfBirth
  }
  
  console.log(ivan.age);</code></pre>
  </div>
  </div>
  </p>
  
  <p>Why the console.log says <code>NaN</code>? Can't I do simple math operations in property declaration? if not, why?</p>