title: js - array .reduce returns NaN instead of calculated value
body: <p>I am writing a simple reduce function:</p>
  
  <pre><code>var tiles = [
    {
      "date": "2017-11-26",
      "cloudCoverage": 92.2,
      "area": 12055600804
    },
    {
      "date": "2017-11-26",
      "cloudCoverage": 92.78,
      "area": 12055600804
    },
    {
      "date": "2017-11-26",
      "cloudCoverage": 97.29,
      "area": 11609327018
    }
  ]
  
  function selectDateTile(tiles) {
    return tiles.reduce((tile, currentPick) =&gt; {
      console.log('tileArea: ', tile.area, 'currentPick area: ', currentPick.area)
      return tile.area + currentPick.area
    });
  }
  
  console.log(selectDateTile(tiles))
  </code></pre>
  
  <p>I would expect it to log the actual values of the objects, and the sum of all the areas:</p>
  
  <blockquote>
    <p>"tileArea: " 12055600804 "currentPick area: " 12055600804 "tileArea: "
    12055600804 "currentPick area: " 24111201608 ...</p>
  </blockquote>
  
  <p>But the console output I am getting in jsbin is:</p>
  
  <blockquote>
    <p>"tileArea: " 12055600804 "currentPick area: " 12055600804 "tileArea: "
    undefined "currentPick area: " 11609327018 NaN</p>
  </blockquote>
  
  <p>What am I doing wrong?</p>
  
  <p><a href="https://jsbin.com/javivirijo/edit?html,js,console,output" rel="nofollow noreferrer">jsbin reference</a></p>