<div class="form-group">
    <label class="col-sm-3 control-label no-padding-right" for="form-field-5"> 內容</label>
    <div class="col-sm-9">
        <textarea class="col-sm-8" id="form-field-5" placeholder="請輸入內容..."></textarea>
    </div>
</div>

jQuery.extend({
    textareaAutosize_dc: function() {
        $("textarea").on("keyup", function(e) {
            var currentEnterCount = $(this).val().split("/n").length;
            var lineHeight = Number($(this).css("line-height").replace("px", ""));
            var enterCount = $(this).attr("enterCount");
            if (currentEnterCount < enterCount && enterCount != undefined) {
                //每行減掉固定行高
                $(this).height($(this).height() - lineHeight);
            } else if (currentEnterCount > enterCount) {
                //每行加入固定行高
                $(this).height($(this).height() + lineHeight);
                $(this).attr("enterCount", currentEnterCount);
            }
            //記錄當前行高
            $(this).attr("enterCount", currentEnterCount);
        });
    }
});
//調用自動高度
$.textareaAutosize_dc();