从lodash中的map（）返回对象数组上groupBy（）的结果将导致[object]

Question

I am trying to do two groupBy on an array of objects ( essentially I am trying to chain two groupBy)

let lodash = require('lodash');
var characters = [
  { 'name': 'barney', 'age': 42,  'pet': 'dog' },
  { 'name': 'fred',   'age': 35,  'pet': 'dog' },
  { 'name': 'barney', 'age': 42,  'pet': 'cat' },
  { 'name': 'fred',   'age': 35,  'pet': 'goldfish' }
];

var groupByAge = lodash.groupBy(characters, 'age');

//console.log(groupByAge);

var groupByAgeAndPet = lodash.map(groupByAge, function(value){
  return lodash.groupBy(value, 'pet');

});

console.log(groupByAgeAndPet);

The output, however, is:

[ { dog: [ [Object] ], goldfish: [ [Object] ] }, { dog: [ [Object] ], cat: [ [Object] ] } ]

If I do a console.log() within the map, I will get the result I want.

What should I be using instead of lodash.map() or do I need to apply some processing before returning from map.

repl.it link to the code: https://repl.it/@Kun_XinXin/OptimalUnkemptTrapezoids

Here's the expected return:

{
   35: { dog: [ { name: 'barney', age: 42, pet: 'dog' } ],
  cat: [ { name: 'barney', age: 42, pet: 'cat' } ] },
   42:{ dog: [ { name: 'fred', age: 35, pet: 'dog' } ],
  goldfish: [ { name: 'fred', age: 35, pet: 'goldfish' } ] }
}

Answer 1

With native javascript and array reduce function the same output can be achieved.Create a function and pass a parameter by which the objects will be grouped. For example in the below function you can pass name , age , pet so the object will be grouped by this key name.

 var characters = [{ 'name': 'barney', 'age': 42, 'pet': 'dog' }, { 'name': 'fred', 'age': 35, 'pet': 'dog' }, { 'name': 'barney', 'age': 42, 'pet': 'cat' }, { 'name': 'fred', 'age': 35, 'pet': 'goldfish' } ]; // creating a function which accepts an parameter which will be used to // group the objects function groupBy(keyName) { return characters.reduce(function(acc, curr, currIndex) { // checking if the object have any key by which the object will be grouped if (!acc.hasOwnProperty(curr[keyName])) { // if not create a key acc[curr[keyName]] = []; acc[curr[keyName]].push(curr) } // else just push the value acc[curr[keyName]].push(curr); return acc; }, {}) } // replace pet with name or age to group result by that key console.log(groupBy('pet')) 

Answer 2

In case anyone come across this while using console.log to debug such cases, it is actually working; it's just that somehow the object contain inside is shortened to [object]

If you access the object directly using console.log, it does work.

let lodash = require('lodash');
var characters = [
  { 'name': 'barney', 'age': 42,  'pet': 'dog' },
  { 'name': 'fred',   'age': 35,  'pet': 'dog' },
  { 'name': 'barney', 'age': 42,  'pet': 'cat' },
  { 'name': 'fred',   'age': 35,  'pet': 'goldfish' }
];

var groupByAge = lodash.groupBy(characters, 'age');

//console.log(groupByAge);

var groupByAgeAndPet = lodash.mapValues(groupByAge, function(value){
  return lodash.groupBy(value, 'pet');

});

// this will display the actual object instead of the string 
console.log(groupByAgeAndPet[35]);