<textarea id="Remark" class="form-control" maxlength="150" oninput="TextareaFontLength('Remark')"  onchange="this.value = this.value.substring(0, 150);TextareaFontLength('Remark')" onkeydown="this.value = this.value.substring(0, 150);TextareaFontLength('Remark')" onkeyup="this.value = this.value.substring(0, 150);TextareaFontLength('Remark')"
 
style="height:100px;"></textarea>
                <div style="text-align:right">
                    <span id="span_Font_Length" style="font-size: 14px;">0</span>
                    <span style="font-size: 14px;">/</span>
                    <span style="font-size: 14px;"
 
>150</span>
                </div>

jq部分：

   <script type="text/javascript">
  
        //TextArea 實時顯示字數
        function TextareaFontLength(id) {
            var strTeatArea = $("[id$='" + id + "']").val().length;
            $("[id$='span_Font_Length']").text(strTeatArea);
        }

    
 
</script>