title: Why object property became undefined when using setInterval
body: <p>As below code, I make an object named "test", and give it properties and method.</p>
  
  <p>The property came from its argument.</p>
  
  <p>And I try to call the method every 2 sec after onload, and the result shows undefined.</p>
  
  <p>But if I only call the method not using setInterval(), like this</p>
  
  <pre><code>window.onload = function() {
     giveword.showWord();
  }
  </code></pre>
  
  <p>I'll be able to show the text "Hi".. Why is that?</p>
  
  <pre><code>var giveword = new test("Hi");
  
  function test(word) {
      this.word = word;
  }
  
  test.prototype.showWord = function() {
      document.getElementById("msg_box").innerHTML = this.word;
  }
  
  window.onload = function() {
      setInterval(giveword.showWord, 2000);
  }
  </code></pre>
  
  <p>Thanks for help...</p>