title: Circle does not move right or down following arrow keypress
body: <p>I want my code to do the following:</p>
  <ol>
  <li>when the right arrow key is pressed, move the circle to the right</li>
  <li>when the down arrow key is pressed, move the circle to the bottom</li>
  </ol>
  <p>But instead it does the following:</p>
  <p>When one of these keys is pressed, it moves only once and than no more. What am I doing wrong?</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>document.onkeydown = function(event) {
    var circle = document.getElementById("circle");
  
    if (event.keyCode == 39) {
      circle.style.left += 100;
      console.log("right")
    } else if (event.keyCode == 40) {
      circle.style.top += 100;
      console.log("bottom")
    }
  }</code></pre>
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#circle {
    width: 50px;
    height: 50px;
    border-radius: 25px;
    background: red;
    position: absolute;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div id="circle"&gt;&lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>