title: javascript - pushing new property to objects that are in an array
body: <p>I got an array person containing many objects that look like this:</p>
  
  <pre><code>const person = [
  { first: 'firstName', last: 'lastName', year: 1439, passed: 1495 },
   ...
  ]
  </code></pre>
  
  <p>I have counted how many years the person lived: </p>
  
  <pre><code>const oldest = person.map(x =&gt; x.passed - x.year);
  </code></pre>
  
  <p>Got new array with the years for every person.</p>
  
  <p>Now I would like to push this calculated year as a new property age to each person object in this array.</p>
  
  <p>Can you help me out?</p>