控制多行文本的字数

 <textarea id="fhid_sp_note" cols="20" name="S4" rows="3" maxlength="240"  onkeydown="checklength(this);"></textarea>

function checklength(obj) {
        var max = obj.maxlength;
        if (max == null || max == "" || max == undefined) {
            return;
        }
        if (obj.value.length > max) {
            alert("最多只能输入:" + max+"个字符");
            obj.value = obj.value.substring(0, (max - 1));
            return;
        }
    }