JS 判断字串字节数，并截取长度

var matchwords;

        function notifyTextLength() {

            var inputNum = document.getElementById("txtTitle").value.replace(/[^\x00-\xff]/g, "**").length; //得到输入的字节数

            if (inputNum <= 200) {

                matchWords = document.getElementById("txtTitle").value.length;

                document.getElementById("inputedWord").innerHTML = inputNum + "字节," + matchWords + "字符";

                document.getElementById("inputtingWord").innerHTML = (200 - inputNum) + "字母,"+(Math.round(((200-inputNum)/2)-0.5))+"汉字";

            }

            if (inputNum > 200) {

                document.getElementById("txtTitle").value = document.getElementById("txtTitle").value.substring(0, matchWords);

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 //如果超过200字节，就截取到200字节

             }

        }

<input id="txtTitle" type="text" class="inputText" runat="server" onpropertychange="notifyTextLength();" />

当前已经输入<span id="inputedWord" style="color:red"></span> 还可以输入<span id="inputtingWord" style="color:Red;"></span>