javascript - 使用Lodash用两个条件对总数进行计数

我无法弄清楚如何根据位置和唯一的序列号对设备总数进行计数。

{
"dates": [
    {
        "date": "Sep 1, 2014",
        "devices": [
            {
                "model": "Canon",
                "location": "Chicago",
                "serialNum": "abc123"
            },
            {
                "model": "Canon",
                "location": "Chicago",
                "serialNum": "xyz456"
            },
            {
                "model": "HP",
                "location": "New York",
                "serialNum": "123456"
            },
            {
                "model": "Brother",
                "location": "Chicago",
                "serialNum": "DEF777"
            }
        ]
    },
    {
        "date": "Sep 2, 2014",
        "devices": [
            {
                "model": "Canon",
                "location": "Chicago",
                "serialNum": "abc123"
            },
            {
                "model": "Canon",
                "location": "Chicago",
                "serialNum": "xyz456"
            }
        ]
    },
    {
        "date": "Sep 3, 2014",
        "devices": [
            {
                "model": "Canon",
                "location": "Chicago",
                "serialNum": "xyz456"
            },
            {
                "model": "Canon",
                "location": "Chicago",
                "serialNum": "stu789"
            },
            {
                "model": "Epson",
                "location": "NewYork",
                "serialNum": "123456"
            },
            {
                "model": "Epson",
                "location": "NewYork",
                "serialNum": "555555"
            },
            {
                "model": "HP",
                "location": "NewYork",
                "serialNum": "987654"
            }
        ]
    }
]

}

我想记录每个位置的唯一设备总数

Chicago - 4
New York - 3

最佳答案

它是在具有lodash链语法的单个表达式中。简短的版本是将所有设备归入一个庞大的列表，然后按位置将它们分组，然后针对每个位置消除重复的ID，然后对设备进行计数。

_(dates) //Begin chain
    .pluck("devices") //get the device list for each date
    .flatten() //combine all device lists into a master list
    .groupBy("location") //group into an object of {"location": [devices]} pairs
    .mapValues(function (devicesForLocation) { //replaces [devices] with a count of number of unique serial numbers
        return _(devicesForLocation) //Begin chain
            .pluck("serialNum") //get the serial number for each device
            .uniq() //remove the duplicate serial numbers
        .value() //End chain
        .length; // get the count of unique serial numbers
    }) // result is an object of {"location": countOfUniqueDevices} pairs
.value() //End chain

最终结果是以下形式的对象：{“ New York”：1，“ NewYork”：3，“ Chicago”：4}，尽管您显然可以添加另一个语句以字符串形式将其打印出来。

我鼓励您逐步执行此步骤，以查看每个步骤的结果并了解其工作情况。

关于javascript - 使用Lodash用两个条件对总数进行计数，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/25677207/