javascript空白验证还是不行解决方案

javascript空白验证还是不行
这是个餐馆网站，菜单可以选点的菜的数量，然后放进购物车里面。

PHP code

<!--

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-->
while(...){
echo "<form name='shopping_cart_form' method='post' action='include/shopping_cart_form.php' onsubmit='return dosubmit();'>";
                echo "<div id='item_details' style=''>
                        <div id='item_label'>
                            <div id='dish_price'>
                                <a style='font-size:30pt; margin-top:20px; float:left;'>\$</a>
                                <a style='font-size:50pt; margin-top:40px; float:left;'>$prices[0]</a>
                                <a style='font-size:10pt; margin-top:40px; float:left;'>.</a>
                                <a style='font-size:20pt; margin-top:30px; float:left;'>$prices[1]</a>
                            </div>
                            <div id='dish_img'><img  src='css_and_img/$dish_img'   style="max-width:90%" border=none alt=" javascript空白验证还是不行解决方案 " ></div>
                        </div>
                        <div id='item_info'>    
                            <div id='dish_name'>$dish_name</div>
                            <div id='dish_descr'>Description: $dish_descr</div>
                            <div id='add_to_function'>  Qty:
                                <input class='txt_qty' type='text' name='textbox_qty' id='textbox_qty' />
                                <input class='add_button' type='submit' name='btn_add' id='btn_add' value='Add to Cart' />
            
                                <input type='hidden' name='shopID' value='$shop_id'>
                                <input type='hidden' name='destID' value='$dest_id'>
                                <input type='hidden' name='catID' value='$cat_id'>
                                <input type='hidden' name='dishID' value='$dish_id'>
                                <input type='hidden' name='dishName' value='$dish_name'>
                                <input type='hidden' name='dishPrice' value='$dish_price'>
                            </div>
            
                        </div>
                    </div>";
                echo "</br>";
                echo "</form>";

}

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JScript code

function dosubmit(fmt) {
  if(fmt.textbox_qty.value == '') {
    fmt.textbox_qty.focus();
    alert('不能为空');
    return false;
  }
  return true;
}
<br><font color="#e78608">------解决方案--------------------</font><br>function isEmpty()<br>{<br>   function isEmpty() {<br>这里看到函数命名两次。是手误还是什么？ <div class="clear"></div>

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