<script>
function sh(a) {
	if(a==0) {
		document.getElementById("sh").onclick = "sh(1)";
		document.getElementById("sh").innerHTML = "Show";
		document.getElementById("txt").style.display = "none";
	} else {
		document.getElementById("sh").onclick = "sh(0)";
		document.getElementById("sh").innerHTML = "Hide";
		document.getElementById("txt").style.display = "inline";
	}
}
</script>
<span id="sh" onclick="sh(0)" style="text-decoration: underline">Hide</span><br>
<textarea id="txt"></textarea>