如何在 JavaScript 中计算没有 for 循环的对象数组中的值差异？

Question

假设我有一个对象数组：

const peopleArray = [
{
 'date': '3/1',
 'people': 0
},
{
 'date: '3/2',
 'people': 5
},
{
 'date: '3/3',
 'people': 8
},
...
];

我正在尝试绘制一个显示每日人数差异的图表，因此我需要以下对象数组：

[{ 
 'date': '3/1,
 'diff': 0 // Shows 0 the first day.
},
{
 'date': '3/2',
 'diff': 5 // 5 people joined.
},
{
 'date': '3/3',
 'diff': 3 // 3 more people joined.
},
...
];

我知道我可以使用Array.push()和 for 循环来实现这一点：

 const peopleArray = [{ date: '3/1', people: 0 }, { date: '3/2', people: 5 }, { date: '3/3', people: 8 } ]; let diffArray = []; for (i = 0; i < peopleArray.length; i++) { if (i === 0) { // Shows 0 the first day diffArray.push({ date: peopleArray[i].date, diff: 0 }); } else { diffArray.push({ date: peopleArray[i].date, diff: peopleArray[i].people - peopleArray[i - 1].people }); } } console.log(diffArray);

但是有没有更简洁的方法来使用map() 、 reduce()或任何这些数组函数？

Answer 1

您可以使用map()并执行以下操作：

 const peopleArray = [ { date: "3/1", people: 0, }, { date: "3/2", people: 5, }, { date: "3/3", people: 8, }, ]; let diffArray = peopleArray.map((o, i, a) => ({ date: o.date, diff: i > 0? o.people - a[i - 1].people: 0, })); console.log(diffArray);

map()接受一个回调 function，它接受三个 arguments：

1. currentValue - 数组中正在处理的当前元素；
2. index - 数组中正在处理的当前元素的索引：
3. array - 阵列 map 被调用。

因此，您可以使用这些参数来实现您对 for 循环所做的相同事情。

在这种情况下， reduce()并不是您真正需要的，因为您需要的不仅仅是一个 output 值。

Answer 2

如果需要，您也可以使用现有的 object，并将其用于图形和任何其他地方（如果需要）。

 const peopleArray = [{ date: '3/1', people: 0 }, { date: '3/2', people: 5 }, { date: '3/3', people: 8 } ]; var peopleArrayLength = peopleArray.length; if(peopleArrayLength > 0){ var peopleObject = peopleArray[0]; peopleObject.diff = 0; peopleArray[0] = peopleObject; for (i = 1; i < peopleArrayLength; i++) { peopleObject = peopleArray[i]; peopleObject.diff = peopleArray[i].people - peopleArray[i-1].people; peopleArray[i] = peopleObject; } } console.log(peopleArray);

输出：

[
  {
    "date": "3/1",
    "people": 0,
    "diff": 0
  },
  {
    "date": "3/2",
    "people": 5,
    "diff": 5
  },
  {
    "date": "3/3",
    "people": 8,
    "diff": 3
  }
]

Answer 3

您可以使用内联 If 语句，如下所示：

for (i = 0; i < peopleArray.length; i++) {
    var df=i!=0?(peopleArray[i].people - peopleArray[i - 1].people):0;
    diffArray.push({
      date: peopleArray[i].date,
      diff: df
    });
}