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

Question

Say I have an array of objects:

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

I'm trying to draw a chart showing daily difference between the number of people, so I need the following array of objects:

[{ 
 '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.
},
...
];

I know I can achieve this with Array.push() and a for loop:

 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);

But is there a cleaner way to do this with map() , reduce() or any of those array functions?

Answer 1

You could use map() and do something like:

 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);

The map() takes a callback function that accepts three arguments:

1. currentValue - The current element being processed in the array;
2. index - The index of the current element being processed in the array:
3. array - The array map was called upon.

So you can use those parameters to achieve the same thing you've done with your for loop.

The reduce() is not really what you're looking for in this case because you want more than a single output value.

Answer 2

You can use the existing object as well if you want and use the same for the graph and any other place if needed.

 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);

Outputs:

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

Answer 3

You can use inline If Statement as follows:

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
    });
}