動態顯示輸入框placeholder文字

html

<input type="text" name="" id="jgChatInput" class="duihIntI" />

js

<script>
	/*輸入框依次顯示文字*/
	var userIput = document.querySelector(".duihIntI");
	var i = 0;
	var s = "在此輸入問題可直接對話";
	var clock = setInterval(function() {
		i++;
		if(i > 12) {
			i = 0;
		}
		var p = s.slice(0, i);
		userIput.setAttribute("placeholder", p);
	}, 600)
	userIput.addEventListener("focus", function() {
		clearInterval(clock)
		userIput.setAttribute("placeholder", "")
	})
	/*依次顯示文字結束*/
</script>