js 實時監聽textarea輸入

html:

<textarea class="area" name="" id="text1" maxlength="100" oninput="checkTxt()"></textarea>
<p class="notice">已輸入<span id="num">0</span>個字</p>

js:

function checkTxt() { 
    var obj = $('#text1'),
        len = obj.val().length;
    $('#num').html(len);
}