//点加，小计 for (var i = 0; i < increment.length; i++) { increment[i].index = i; increment[i].onclick = function() { var flag = this.index; var q = quantity[flag]; var p = price[flag].textContent; var s = subtotal[flag]; var newvalue = parseInt(q.value) + 1; q.setAttribute("value", newvalue); s.textContent = (newvalue * parseFloat(p)).toFixed(2); calculateTotal(); }; } //点减，小计 for (var j = 0; j < decrement.length; j++) { decrement[j].index = j; decrement[j].onclick = function() { var flag = this.index; var q = quantity[flag]; var p = price[flag].textContent; var s = subtotal[flag]; if (parseInt(q.value) > 1) { var newvalue = parseInt(q.value) - 1; q.setAttribute("value", newvalue); s.textContent = (newvalue * parseFloat(p)).toFixed(2); calculateTotal(); } }; }根据此代码不点击按钮前设商品小计为一个商品的价格

如果不点击按钮，商品小计应该为商品单价乘以初始数量，可以在JavaScript代码中先获取每个商品的单价和数量输入框的初始值，然后计算商品小计，例如：

var price = document.getElementsByClassName("price");
var quantity = document.getElementsByClassName("quantity");
var subtotal = document.getElementsByClassName("subtotal");

for (var i = 0; i < price.length; i++) {
  var p = price[i].textContent;
  var q = quantity[i].value;
  var s = subtotal[i];
  s.textContent = (q * parseFloat(p)).toFixed(2);
}

calculateTotal(); // 计算总价

这样就可以在页面加载时根据每个商品的单价和初始数量计算出商品小计。

相关问题

for (var i = 0; i < a.length;i++)

This for loop is missing an increment statement for the variable i. It should be:

for (var i = 0; i < a.length; i++)

The increment statement (i++) will ensure that the loop continues to run until the condition (i < a.length) is no longer true.

var total=0.0; //加 //获取所有+按钮 var increment = document.getElementsByClassName("increase"); for (var i = 0; i < increment.length; i++) { //为a标签添加index属性，用于记录下标 increment[i].index = i; //点击+数量增加的功能函数 increment[i].onclick = function () { var flag = this.index; //获取当前按钮对应的数量框 var q = document.getElementsByClassName("quantity")[flag]; var newvalue = parseInt(q.value) + 1; q.setAttribute('value', newvalue); } } //减 //获取所有-按钮 var decrement=document.getElementsByClassName('decrease'); //点击-数量减少的功能函数 for(var j=0;j<decrement.length;j++) { decrement[j].index=j; decrement[j].onclick=function() { var flag=this.index; //获取当前a标签对应的那个数量框 var q=document.getElementsByClassName("quantity")[flag]; if(parseInt(q.value)>1) { var newvalue=parseInt(q.value)-1; q.setAttribute('value',newvalue); } } }能否根据这个加减写一个小计代码

可以，根据上述加减代码，可以在数量框的HTML代码中添加一个小计标签，然后在加减按钮的点击函数中，根据当前数量框的值和商品单价计算小计，并将小计的值设置为小计标签的文本内容。以下是示例代码：

HTML代码：

<div class="item">
  <span class="name">商品名称</span>
  <span class="price">商品单价</span>
  <div class="quantity-wrapper">
    <a href="#" class="decrease">-</a>
    <input type="text" class="quantity" value="1">
    <a href="#" class="increase">+</a>
  </div>
  <span class="subtotal">小计</span>
</div>

JavaScript代码：

var increment = document.getElementsByClassName("increase");
var decrement = document.getElementsByClassName("decrease");
var quantity = document.getElementsByClassName("quantity");
var price = document.getElementsByClassName("price");
var subtotal = document.getElementsByClassName("subtotal");

for (var i = 0; i < increment.length; i++) {
  increment[i].index = i;
  increment[i].onclick = function() {
    var flag = this.index;
    var q = quantity[flag];
    var p = price[flag].textContent;
    var s = subtotal[flag];
    var newvalue = parseInt(q.value) + 1;
    q.setAttribute("value", newvalue);
    s.textContent = (newvalue * parseFloat(p)).toFixed(2);
    calculateTotal();
  };
}

for (var j = 0; j < decrement.length; j++) {
  decrement[j].index = j;
  decrement[j].onclick = function() {
    var flag = this.index;
    var q = quantity[flag];
    var p = price[flag].textContent;
    var s = subtotal[flag];
    if (parseInt(q.value) > 1) {
      var newvalue = parseInt(q.value) - 1;
      q.setAttribute("value", newvalue);
      s.textContent = (newvalue * parseFloat(p)).toFixed(2);
      calculateTotal();
    }
  };
}

function calculateTotal() {
  var total = 0.0;
  for (var k = 0; k < subtotal.length; k++) {
    total += parseFloat(subtotal[k].textContent);
  }
  document.getElementById("total").textContent = total.toFixed(2);
}

这段代码实现了商品数量的加减以及每个商品的小计计算，同时还有一个总计的计算函数。注意，需要将每个商品的单价在HTML代码中设置为一个文本节点，例如`<span class="price">10.00</span>`，这样在JavaScript代码中才能通过`textContent`属性获取到它的值。小计和总计需要在计算时进行四舍五入，保留两位小数，可以使用`toFixed`方法实现。