JAVASCRIPT

onclick remove textarea value

<textarea id='comments' rows='4' cols='4'></textarea>
<input type="button" value="Clear" onclick="javascript:document.getElementById('comments').value='';">

onclick remove textarea value

<input type="button" value="Clear" onclick="javascript: functionName();" >
you just need to set the onclick event, call your desired function on this onclick event.

function functionName()
{
    $("#output").val("");
}
Above function will set the value of text area to empty string.