javascript - 如何在JavaScript中添加需要递归计算的新属性

Question

我想使用映射动态地将新属性添加到数组中的所有项目。但是在将其添加到每个项目时，该属性的值是之前所有项目新添加的属性的总和

考虑这个例子:

let persons = [{
"name": "A",
 "salary": 2
}, {
"name": "B",
 "salary": 5
},{
"name":"C",
"salary":12
}];

我想返回:

[{
    "name": "A",
     "salary": 2,
     "sumSalary":2
    }, {
    "name": "B",
     "salary": 5,
     "sumSalary":7
    },{
    "name":"C",
    "salary":12,
    "sumSalary":19
   }];

我已经尝试过这个:

let mutatedPersons = persons.map((currentValue, index, mutatedPersons) => ({
  ...currentValue,
    sumSalary: currentValue.name === 'A' ? currentValue.salary : mutatedPersons[index - 1].sumSalary + currentValue.salary
}))

但我一直收到这个:

[
  0: {name: "A", salary: 2, sumSalary: 2}
  1: {name: "B", salary: 5, sumSalary: NaN}
  2: {name: "C", salary: 12, sumSalary: NaN}
]

Answer 1

您引用的 mutatedPersons 是原始数组(请参阅 map's parameters )，而不是更新后的数组，更新后的数组在 map 结束之前实际上并不存在。您可以将前一个总和缓存在外部变量 (prevSumSalary) 中，并将其用作新总和的基础:

const persons = [{ name: "A", salary: 2 }, { name: "B", salary: 5 }, { name: "C", salary: 12 }]

let prevSumSalary = 0;
const mutatedPersons = persons.map((currentValue, index) => ({
  ...currentValue,
    sumSalary: (prevSumSalary = prevSumSalary + currentValue.salary)
}))

console.log(mutatedPersons);

另一个选择是使用 Array.reduce() ，因为您可以访问累积值:

const persons = [{ name: "A", salary: 2 }, { name: "B", salary: 5 }, { name: "C", salary: 12 }]

const mutatedPersons = persons.reduce((r, currentValue) => [...r, 
  ({
    ...currentValue,
      sumSalary: currentValue.salary + (r.length && r[r.length - 1].sumSalary)
  })
], [])

console.log(mutatedPersons);