在数组中查找对象的数量会改变数组本身

Question

我有一个关于在数组中查找对象计数的问题。 我的数组是这样的：

completePositions: any = [
    {
      totalPositions: [
        { position: "CNA", count: 2 },
        { position: "LVN", count: 5 },
        { position: "RNA", count: 8 }
      ]
    },
    {
      totalPositions: [
        { position: "CNA", count: 13 },
        { position: "LVN", count: 11 },
        { position: "RNA", count: 15 }
      ]
    }
  ];

我试图在 totalPositions 数组中找到每个位置的总数，这样我的最终数组将如下所示：

totalPositionsCount = [
        { position: "CNA", count: 15 },
        { position: "LVN", count: 16 },
        { position: "RNA", count: 23 }
  ];

我创建了一个函数来对“totalPositons”数组中的每个位置求和并将总和推送到totalPositionsCount。我的函数完成了这项工作，我得到了总数，但它改变了父数组“completePositions”，并且“completePositions”的第一个“totalPositions”被“totalPositionsCount”数组替换。我尝试创建“completePositions”的备份并将其用于计算，但两个数组第一个“totalPositions”仍然被“totalPositionsCount”数组替换。

我总结位置的函数是：

codeToShowTotalResources() {
    for (let totPos of this.completePositions) {
        for (let eachPos of totPos.totalPositions) {
            console.log("eachPos", eachPos);

            let postCountIndex = this.totalPositionsCount.findIndex(
                pos => pos.position == eachPos.position
            );

            if (postCountIndex != -1) {
                console.log("Already exists, sum up positions");
                let positionCount = this.totalPositionsCount[postCountIndex].count + eachPos.count;
                this.totalPositionsCount[postCountIndex].count = positionCount;
            } else {
                console.log("Add it for first time");
                this.totalPositionsCount.push(eachPos);
            }
        }

    }
}

如果我用this.totalPositionsCount[postCountIndex] = { position: eachPos.position, count: positionCount }; 替换this.totalPositionsCount[postCountIndex].count = positionCount;，那么它工作得很好。我想知道我做错了什么，或者它应该以这种方式工作。

我在stackblitz 中打开了一个示例项目 任何帮助都会很棒:)

Answer 1

当您这样做时：this.totalPositionsCount.push(eachPos)，eachPos 变量代表来自completePositions 的条目。所以现在this.totalPositionsCount 和this.completePositions 引用了同一个对象。如果您修改该对象，则更改会显示在两个位置，因为它是同一个对象。

这是使用reduce 和forEach 的替代方法：

const completePositions = [
  {
    totalPositions: [
      {position: "CNA", count: 2},
      {position: "LVN", count: 5},
      {position: "RNA", count: 8}
    ]
  },
  {
    totalPositions: [
      {position: "CNA", count: 13},
      {position: "LVN", count: 11},
      {position: "RNA", count: 15}
    ]
  }
];

const totals = completePositions
  .map(x => x.totalPositions)
  .reduce((acc, totalPositions) => {
    totalPositions.forEach(({position, count}) => {
      acc[position] = (acc[position] || 0) + count;
    })
    return acc;
  }, {})
  
console.log(totals);