title: Sum number to variable in for-statement returns NaN
body: <p>I want to sum every number to a global variable. To simplify the need and problem, if made this example:</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>let totaalWaarde;
  
  for (i = 5; i &lt;= 100; i += 5) {
    totaalWaarde += 1
    console.log(totaalWaarde)
  }
  
  console.log(totaalWaarde)</code></pre>
  </div>
  </div>
  </p>
  
  <p>Why does <strong>totaalWaarde</strong> returns NaN?
  If I assign the totaalWaarde in de for like totaalWaarde = 1, the totaalWaarde returns the number.</p>