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how to group date array by month in javascript

https://stackoverflow.com/questions/14446511/most-efficient-method-to-groupby-on-an-array-of-objects

function groupBy(list, keyGetter) {
    const map = new Map();
    list.forEach((item) => {
         const key = keyGetter(item);
         const collection = map.get(key);
         if (!collection) {
             map.set(key, [item]);
         } else {
             collection.push(item);
         }
    });
    return map;
}

// example usage

const pets = [
    {type:"Dog", name:"Spot"},
    {type:"Cat", name:"Tiger"},
    {type:"Dog", name:"Rover"}, 
    {type:"Cat", name:"Leo"}
];
    
const grouped = groupBy(pets, pet => pet.type);
    
console.log(grouped.get("Dog")); 
//  [{type:"Dog", name:"Spot"}, {type:"Dog", name:"Rover"}]

console.log(grouped.get("Cat"));
//  [{type:"Cat", name:"Tiger"}, {type:"Cat", name:"Leo"}]
    

https://stackoverflow.com/questions/41888837/js-group-by-month-of-date-values-objects-in-an-array

const dates = [
    {date: "2017-01-01", num: "2"},
    {date: "2017-01-02", num: "3"},
    {date: "2017-02-04", num: "6"},
    {date: "2017-02-05", num: "15"}
],

let groupKey = 0;

groups = dates.reduce(function (r, o) {
        var m = o.date.split(('-'))[1];
        (r[m])? r[m].data.push(o) : r[m] = {group: String(groupKey++), data: [o]};
        return r;
    }, {});

const result = Object.keys(groups).map(k => groups[k]);

console.log(result);

OK

const json = [
  {
    "id": 191701,
    "productId": 13602,
    "activityEventId": 1623852,
    "ticketCategoryId": 5246618,
    "ticketGroupId": 11798619,
    "start": 1585670400000,
    "end": 1585670400000,
    "ticketsNumber": 100,
    "lowPrice": 127.00,
    "status": 0,
    "priceLowest": false,
    "hasTicket": true,
    "availableNumbers": [
      1,
      2,
      3,
      4,
      5,
      6
    ],
    order: "C"
  },
  {
    "id": 191837,
    "productId": 13602,
    "activityEventId": 1623896,
    "ticketCategoryId": 5246754,
    "ticketGroupId": 11798755,
    "start": 1585497600000,
    "end": 1585497600000,
    "ticketsNumber": 100,
    "lowPrice": 127.00,
    "status": 0,
    "priceLowest": false,
    "hasTicket": true,
    "availableNumbers": [
      1,
      2,
      3,
      4,
      5,
      6
    ],
    order: "A"
  },
  {
    "id": 191812,
    "productId": 13602,
    "activityEventId": 1623891,
    "ticketCategoryId": 5246729,
    "ticketGroupId": 11798730,
    "start": 1585584000000,
    "end": 1585584000000,
    "ticketsNumber": 100,
    "lowPrice": 127.00,
    "status": 0,
    "priceLowest": false,
    "hasTicket": true,
    "availableNumbers": [
      1,
      2,
      3,
      4,
      5,
      6
    ],
    order: "B"
  },
];

const sortArray = json.sort((a, b) => a.start > b.start ? 1 : -1);
// asc order 升序

const calendarArray = sortArray.map(obj => {
  const date = new Date(obj.start);
  const year  = date.getFullYear();
  const month  = date.getMonth() + 1;
  return {
    year,
    month,
    ...obj,
  };
});


// group date by month / year


const groupArrayByKey = (items, key) => items.reduce(
  (result, item) => ({
    ...result,
    [item[key]]: [
      ...(result[item[key]] || []),
      item,
    ],
  }),
  {},
);


const groupArray = groupArrayByKey(calendarArray, `month`);

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