How can you access Salesforce visualforce components values into a JavaScript?

<apex:page id="thePage">
<!-- A simple function for changing the font. -->
<script>
function changeFont(input, textid) {
if(input.checked) {
document.getElementById(textid).style.fontWeight = "bold";
}
else {
document.getElementById(textid).style.fontWeight = "normal";
}
}
</script>

<!-- This outputPanel calls the function, passing in the
checkbox itself, and the DOM ID of the target component. -->
<apex:outputPanel layout="block">
<label for="checkbox">Click this box to change text font:</label>
<input id="checkbox" type="checkbox"
onclick="changeFont(this,'{!$Component.thePanel}');"/>
</apex:outputPanel>

<!-- This outputPanel is the target, and contains
text that will be changed. -->
<apex:outputPanel id="thePanel" layout="block">
Change my font weight!
</apex:outputPanel>
</apex:page>