Javascript 如何计算不同类型文档的平均值

我有一份这样的文件

{
  type: 'vehicles',
  vin: 1234, 
  companyId: 123,
  _id: 321
}

{
  type: 'company',
  name: 'x company',
  _id: 123
}
{
  type: 'company',
  name: 'y company',
  _id: 333
}

key: ["averageVehiclesPerCompany"]

我正在尝试设计map reduce函数，以获得每个公司的平均车辆数（allVehicle/allCompany），我希望通过调用这样的键来获得值

{
  type: 'vehicles',
  vin: 1234, 
  companyId: 123,
  _id: 321
}

{
  type: 'company',
  name: 'x company',
  _id: 123
}
{
  type: 'company',
  name: 'y company',
  _id: 333
}

key: ["averageVehiclesPerCompany"]

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下面是计算平均值的完美解决方案

function calcAverage(companies) {
    var totalVehicle = 0;
    Object.keys(companies).map(function(key, index) {
        var vehicle = 0;
        if (!!companies[key].vin) {
            vehicle = companies[key].vin;
        }
        totalVehicle += vehicle;
    });
    return parseFloat(totalVehicle / companies.length).toFixed(2);
}

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map reduce我的意思是Couchdb map reduce函数，我可以用它在Couchdb中创建视图吗