db.collection.mapReduce(
   function() {emit(key,value)},//map 函数
   function(key,values) {return reduced}, //reduce 函数
   <collection>//out collection
)
//第三个参数也可以传入更多选项：
{
    finalize: function(key, reduced){return finalized}
    out: <collection>|{inline:true},
    query: <document>,
    sort: <document>,
    limit: <number>,
}

db.runCommand({
     mapReduce: <collection>,
     map: <function>,
     reduce: <function>,
     finalize: <function>,
     out: <output>,
     query: <document>,
     sort: <document>,
     limit: <number>,
     scope: <document>,
     jsMode: <boolean>,
     verbose: <boolean>,
     bypassDocumentValidation: <boolean>,
     collation: <document>
})

1.{
    "name" : "姓名1",
    "gender" : "男",
    "city" : "城市a",
    "province" : "省份A"
}
2.{
    "name" : "姓名2",
    "gender" : "女",
    "city" : "城市b",
    "province" : "省份B"
}
3...

db.person.mapReduce(
    function(){//map
        var key = {province:this.province, city:this.city}
        var value = {total: 1}
        if (this.gender == '男'){
            value.male = 1
        }else if (this.gender == '女'){
            value.female = 1
        }else{
            value.unknown = 1
        }
        emit(key, value)
    },
    function(key, values){//reduce
        var value = {}//累计各性别的数量
        values.forEach(function(item){
            //item里可能是单个值，也可能已经是累计值
            for (var k in item){
                value[k] = (value[k]||0) + item[k]
            }
        })
        return value//reduce返回值不支持数组，若需多值请用对象格式
    },{
        query:{},//指定过滤源数据的查询条件
        sort:{province:1, city:1},//按key排序可减少reduce的次数，加快执行速度
        finalize: function(key, rValue){
            for (var k in rValue){
                if (k !== 'total'){//数量转为比例
                    rValue[k] = rValue[k]/rValue.total
                }
            }
            return rValue
        },
        out:{inline:true},
    }
).find()

[
    {
        "_id" : {
            "province" : "省份A",
            "city" : "城市a"
        },
        "value" : {
            "total" : 40.0,
            "male" : 0.425,
            "female" : 0.575
        }
    },
    {
        "_id" : {
            "province" : "省份A",
            "city" : "城市b"
        },
        "value" : {
            "total" : 1.0,
            "male" : 1.0
        }
    },
    {
        "_id" : {
            "province" : "省份A",
            "city" : "城市c"
        },
        "value" : {
            "total" : 150.0,
            "male" : 0.526,
            "female" : 0.473
        }
    },
    ...
]

//已知:
typeof undefined === 'undefined'
//你可能会以为:
typeof null === 'null'//但，不是这样
//真相是:
typeof null === 'object'//null是指空对象，空对象也是对象！

//已知：
typeof {} === 'object'
typeof function(){} === 'function'
//你可能会以为:
typeof [] === 'array'//但，不是这样的
//真相是：
typeof [] === 'object'//数组也是对象，基本类型外的除了函数都是对象。

0==''//true
0=='0'//true
0==false//true
[1,2]=='1,2'//true
null==undefined//true
!null && !undefined//true
//注意：null和undefined既不等于true也不等于false
null==true || null==false//false
undefined==true || undefined==false//false

null===null//true
undefined===undefined//true
null===undefined//false
0==='0'//false
0===-0//true
NaN===NaN//false?--这个比较有性格

Objecct.is(NaN,NaN)//true
Objecct.is(0,-0)//false

var obj1 = {foo: "foo", bar: "bar"};
var obj2 = {foo: "foo2", bar: "bar2"};
var copy1 = {...obj1}; // Object {foo: "foo", bar: "bar"}
var copy2 = Object.assign({}, obj); //Object {foo: "foo2", bar: "bar2"}
var copySpread = {...obj1, ...obj2}; //Object {foo: "foo2", bar: "bar2"}
var copyAssign = Object.assign({}, obj2, obj1); //Object {foo: "foo", bar: "bar"}

var obj = { a: 0, b: { c: 0 } };
var copy = JSON.parse(JSON.stringify(obj));

function deepClone(obj) {
  var copy;
  // Handle the 3 simple types, and null or undefined
  if (null == obj || "object" != typeof obj) return obj;
  // Handle Date
  if (obj instanceof Date) {
    copy = new Date();
    copy.setTime(obj.getTime());
    return copy;
  }
  // Handle Array
  if (obj instanceof Array) {
    copy = [];
    for (var i = 0, len = obj.length; i < len; i++) {
        copy[i] = deepClone(obj[i]);
    }
    return copy;
  }
  // Handle Function
  if (obj instanceof Function) {
    copy = function() {
      return obj.apply(this, arguments);
    }
    return copy;
  }
  // Handle Object
  if (obj instanceof Object) {
      copy = {};
      for (var attr in obj) {
          if (obj.hasOwnProperty(attr)) copy[attr] = deepClone(obj[attr]);
      }
      return copy;
  }
  throw new Error("Unable to copy obj as type isn't supported " + obj.constructor.name);
}