Why object property became undefined when using setInterval
<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>