使用对象的键值获取新数组中的最后一个元素？答案

【问题标题】：Get last element inside new array with key value from Object?使用对象的键值获取新数组中的最后一个元素？
【发布时间】：2020-05-01 01:03:17
【问题描述】：

我有一个对象，其中每个键都有一个由多个对象组成的数组。我只想从每个数组中获取最后一项。

我尝试只将最后一个元素推送到新数组中，但问题是现在它没有说明哪个对象属于哪个类。

let data = {
        "classA": [
            {
                date: '01-01',
                present: 49,
                absent: 14
            },
            {
                date: '02-01',
                present: 39,
                absent: 24
            },
            {
                date: '03-01',
                present: 35,
                absent: 28
            }
        ],
        "classB": [
            {
                date: '01-01',
                present: 49,
                absent: 14
            },
            {
                date: '02-01',
                present: 39,
                absent: 24
            },
            {
                date: '03-01',
                present: 35,
                absent: 28
            }
        ],
        "classC": [
            {
                date: '01-01',
                present: 49,
                absent: 14
            },
            {
                date: '02-01',
                present: 39,
                absent: 24
            },
            {
                date: '03-01',
                present: 35,
                absent: 28
            }
        ]
    }

    let newData = [];

    for (let [key, value] of Object.entries(data)) {
        newData.push(value[value.length - 1]);
    }

    console.log(newData);

有没有办法在每个对象中推送类名并得到类似的结果。

[
  {
    "class": "ClassA",
    "date": "03-01",
    "present": 35,
    "absent": 28
  },
  {
    "class": "ClassB",
    "date": "03-01",
    "present": 35,
    "absent": 28
  },
  {
    "class": "ClassC",
    "date": "03-01",
    "present": 35,
    "absent": 28
  }
]

【问题讨论】：

标签： javascript arrays object

【解决方案1】：

一种方法是使用Object.assign

newData.push(Object.assign({class:key}, value[value.length -1]))

但是，更现代的方法，使用spread in object literals 语法使其（我认为）更易于阅读

newData.push({class:key, ...value[value.length - 1]});

let data = {"classA":[{"date":"01-01","present":49,"absent":14},{"date":"02-01","present":39,"absent":24},{"date":"03-01","present":35,"absent":28}],"classB":[{"date":"01-01","present":49,"absent":14},{"date":"02-01","present":39,"absent":24},{"date":"03-01","present":35,"absent":28}],"classC":[{"date":"01-01","present":49,"absent":14},{"date":"02-01","present":39,"absent":24},{"date":"03-01","present":35,"absent":28}]}

    let newData = [];

    for (let [key, value] of Object.entries(data)) {
        newData.push({class:key, ...value[value.length - 1]});
    }

    console.log(newData);

【讨论】：

正如我要回答的那样。
@D.Pardal - 实际上我使用{class: key, ...value[value.length - 1]} 更改了它以获得更简洁的代码
感谢@Phil - 添加解释后，我将使其更具可读性
不用担心。我只是想强调有趣的部分。我们都知道数据是什么样子的?
谢谢@JaromandaX。我已经摆弄了一个小时，终于决定在这里发帖。 :)

【解决方案2】：

let data = {
        "classA": [
            {
                date: '01-01',
                present: 49,
                absent: 14
            },
            {
                date: '02-01',
                present: 39,
                absent: 24
            },
            {
                date: '03-01',
                present: 35,
                absent: 28
            }
        ],
        "classB": [
            {
                date: '01-01',
                present: 49,
                absent: 14
            },
            {
                date: '02-01',
                present: 39,
                absent: 24
            },
            {
                date: '03-01',
                present: 35,
                absent: 28
            }
        ],
        "classC": [
            {
                date: '01-01',
                present: 49,
                absent: 14
            },
            {
                date: '02-01',
                present: 39,
                absent: 24
            },
            {
                date: '03-01',
                present: 35,
                absent: 28
            }
        ]
    }

    let newData = [];

    for (let [key, value] of Object.entries(data)) {
        newData.push(value[value.length - 1]);
newData[newData.length-1].class=key 
    }

    console.log(newData);

【讨论】：

【解决方案3】：

你也可以这样做

let result = Object.entries(data).map(data => {
    return { class: data[0], ...data[1][data[1].length - 1] };
    });

【讨论】：