document.getElementById not working at FBJS

I'm developing apps at Facebook using FBML and FBJS. When I tried to use document.getElementById to read tag, it did 开发者_如何学Pythonnot reply the correct value. Can we really use this command on FBML to get data?

Here is detail of my code :

<input type="hidden" value="123" id="number"/>
<a href="#" onclick="new Dialog().showMessage('Dialog', document.getElementById('number').value);return false"/>

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I found the answer. We can access the value by :

<input type="hidden" value="123" id="number"/> <a href="#" onclick="new Dialog().showMessage('Dialog', document.getElementById('number').getValue());return false"/>