在计算中使用输入字段的值，并将结果显示在另一个元素中

Question

我有一个购物车，其中包含每个产品的价格、数量和小计字段。数量字段可由用户更改，但其他字段是静态的。

有没有办法在用户更改数量时计算小计？我想将数量乘以价格，然后更新小计，而不需要用户提交表单。此外，在计算小计时，也应更新总计。

这里的视觉表示

我的代码是一个 Django 模板，看起来像这样，相关部分在循环中：

<div class="container">
<table id="cart" class="table table-hover table-condensed">
            <thead>
                <tr>
                    <th style="width:50%">Product</th>
                    <th style="width:10%">Price</th>
                    <th style="width:8%">Quantity</th>
                    <th style="width:22%" class="text-center">Subtotal</th>
                </tr>
            </thead>
{% for crops_ordered_names,crops_ordered_images,crops_ordered_cost,crops_ava in total %}
            <tbody>
                <tr>
                    <td data-th="Product">
                        <div class="row">
                            <div class="col-sm-2 hidden-xs"><img src="http://placehold.it/100x100" alt="..." class="img-responsive"/></div>
                            <div class="col-sm-10">
                                <h4 class="nomargin">{{crops_ordered_names}}</h4>
                                <p>Available amount: {{crops_ava}}</p>
                            </div>
                        </div>
                    </td>
                    <td data-th="Price" id="price">{{crops_ordered_cost}}</td>
                    <td data-th="Quantity">
                        <input type="number" class="form-control text-center" id="quan"  min="1" max="{{crops_ava}}">
                    </td>
                    <td data-th="Subtotal" class="text-center" id="subtotal"></td>
                    <td class="actions" data-th="">
                        <button class="btn btn-danger btn-sm"><i class="fa fa-trash-o"></i></button>
                    </td>
                </tr>
            </tbody>
{% endfor %}
            <tfoot>
                <tr class="visible-xs">
                    <td class="text-center"><strong>Total 1.99</strong></td>
                </tr>
                <tr>
                    <td colspan="2" class="hidden-xs"></td>
                    <td class="hidden-xs text-center" id="total"><strong>Total </strong></td>

                </tr>
            </tfoot>
        </table>

Answer 1

您需要做的第一件事是对您的 HTML 代码进行一些小改动。由于您将列出多个产品，每个产品都有自己的价格、数量和小计，您需要使用class 或数据属性而不是id，因为id 对整个文档是唯一的，并且ID 只能使用一次。相关代码（循环内）应如下所示：

<tbody>
  <tr>
    <td data-th="Product">
      <div class="row">
        <div class="col-sm-2 hidden-xs"><img src="http://placehold.it/100x100" alt="..." class="img-responsive"/></div>
        <div class="col-sm-10">
          <h4 class="nomargin">{{crops_ordered_names}}</h4>
          <p>Available amount: {{crops_ava}}</p>
        </div>
      </div>
    </td>
    <td data-th="Price" data-type="price">{{crops_ordered_cost}}</td>
    <td data-th="Quantity">
      <input type="number" class="form-control text-center" data-type="quan" min="1" max="{{crops_ava}}">
    </td>
    <td data-th="Subtotal" class="text-center" data-type="subtotal"></td>
    <td class="actions" data-th="">
      <button class="btn btn-danger btn-sm"><i class="fa fa-trash-o"></i></button>
    </td>
  </tr>
</tbody>

您还需要更新表格页脚，以便总单元格获得数据属性：

<td class="text-center" data-type="total"><strong>Total <span>0</span></strong></td>

（需要 span 元素，因此我们只能更新数字。）

下面的示例代码应该可以解决问题，当数量发生变化时更新小计。该代码可能无法在 Internet Explorer 中运行，因此请告诉我是否有此要求。我还添加了 cmets 以帮助阐明在哪里发生了什么。

// Find all quantity inputs, and the element where we display the total.
const quantityInputs = document.querySelectorAll('[data-type="quan"]');
const total = document.querySelector('[data-type="total"] span');

// For simplicity, we calculate the total in a separate function.
const calculateTotal = () => {
  // Find all the subtotals.
  const subtotals = document.querySelectorAll('[data-type="subtotal"]');
  let calculatedTotal = 0;

  // Loop through all the subtotals, and add them to the final total.
  Array.from(subtotals).forEach((subtotal) => {
    calculatedTotal += Number(subtotal.textContent);
  });

  // Then return the total.
  return calculatedTotal;
}

// As above, we use a separate function to calculate the subtotal for a product.
const calculateSubtotal = (product) => {
  // The input event will fire when the value of the quantity field is changed.
  product.addEventListener('input', () => {
    /*
     * With multiple prices and subtototals, we need to look for them going
     * upwards from the quantity field, using the parent elements (first the
     * table cell, then the table row).
     */
    const parentRow = product.parentNode.parentNode;

    // Then find the price and subtotal for this product.
    const price = parentRow.querySelector('[data-type="price"]');
    const subtotal = parentRow.querySelector('[data-type="subtotal"]');

    // And finally, update the subtotal and the total.
    subtotal.textContent = price.textContent * product.value;
    total.textContent = calculateTotal();
  });
}

// Loop through all the quantity inputs and wait for the subtotal to change.
Array.from(quantityInputs).forEach((element) => calculateSubtotal(element));