title: Javascript adding a number to a textbox with a button
body: <p>Maybe a dumb question but i am trying to grab a number that is in a textbox and add 1 to it by pressing a button. </p>
  
  <p>Here is the html</p>
  
  <pre><code>&lt;input type="text" id="txt_invoer" value="1"&gt;
  &lt;button onclick="countUp()"&gt;+&lt;/button&gt;
  &lt;button onclick="countDown()"&gt;-&lt;/button&gt;
  </code></pre>
  
  <p>Here is the javascript</p>
  
  <pre><code>&lt;script&gt;
      function countUp() {
          var i = parseInt(document.getElementById('txt_invoer').value);
          var iResult = i++;
          document.getElementById('txt_invoer').innerHTML = 
          iResult.toString();
      }
  &lt;/script&gt;
  </code></pre>
  
  <p>I hope im not being too dumb...
  Thanks in advance</p>