如何按Item分组统计订单数组中Amount总和？代码异常排查

现有包含订单对象的数组，需求是按Order.Item字段分组，统计对应Order.Amount的总和：

const sellingInfo = [
  {"OrderReceiverId":"6698eea782e24cc9d42eab7f","Order":{ "CustomerId":"669aa55079cce2e96f9a7e69","CustomerName":"Erigbemi","Date":"2024/07/19","TrackingId":"43035","Status":"Processing","Amount":{"$numberInt":"25000"},"Item":"Orange" }},
  {"OrderReceiverId":"6698eea782e24cc9d42eab7f","Order":{ "CustomerId":"669aa55079cce2e96f96t77","CustomerName":"James","Date":"2024/07/19","TrackingId":"42335","Status":"Processing","Amount":{"$numberInt":"15000"},"Item":"Tomatoes" }},
  {"OrderReceiverId":"6698eea782e24cc9d42eab7f","Order":{ "CustomerId":"669aa55079cce2e96f9a7e69","CustomerName":"Erigbemi","Date":"2024/07/19","TrackingId":"22235","Status":"Processing","Amount":{"$numberInt":"15000"},"Item":"Tomatoes" }}
];

预期输出：

{
  "Orange": 25000,
  "Tomatoes": 30000
}

尝试的JavaScript代码：

let itm = {};

sellingInfo.forEach(order => {
    const item = order.Order.Item;
    const amt = order.Order.Amount;
    
    if(!itm[item]){
        itm[item] = amt;
    }
    itm[item] += amt;
})

res.json(itm);

得到错误输出：

{
  "Orange": 50000,
  "Tomatoes": 45000
}

1. 数值提取错误：order.Order.Amount是包含$numberInt字段的对象，直接赋值或相加会导致类型异常，必须先提取字符串并转换为整数。
2. 逻辑重复累加：当前代码中，无论是否完成分组初始化，都会执行一次itm[item] += amt。以Orange为例，第一次循环初始化itm["Orange"] = 25000后，又立即执行累加操作，导致结果翻倍；Tomatoes的第三个订单则在已有30000的基础上再次累加15000，最终得到45000。

let itm = {};

sellingInfo.forEach(order => {
    const item = order.Order.Item;
    // 提取Amount的数值并转为整数
    const amt = parseInt(order.Order.Amount.$numberInt, 10);
    
    if (!itm[item]) {
        // 初始化分组总和为0
        itm[item] = 0;
    }
    // 仅执行一次累加
    itm[item] += amt;
})

res.json(itm);

也可以用更简洁的短路赋值逻辑：

let itm = {};

sellingInfo.forEach(order => {
    const item = order.Order.Item;
    const amt = parseInt(order.Order.Amount.$numberInt, 10);
    
    // 若分组不存在则初始化为0，再累加当前金额
    itm[item] = (itm[item] || 0) + amt;
})

res.json(itm);

执行后将得到正确的分组求和结果。

内容的提问来源于stack exchange，提问作者Osemu Aimiosior