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>