how to group by and total sum array of object in javascript?

Question

After I use reduce() to group the objects in an array by month-year:

let groupByMonth;
if (newlistTaskEvaluation) {
    groupDataByMonth = newlistTaskEvaluation.reduce((groups, item) => {
     groups[item.time] = [...groups[item.time] || [], item];
      return groups;
     }, {});
}

I have an array of event objects formatted by group month-year as follows:

groupByMonth = {
    '7-2020': [ //july
        {
            time: "7-2020",
            task: [
                { code: "p1", value: 123 },
                { code: "p2", value: 234 },
            ]
        },
        {
            time: "7-2020",
            task: [
                { code: "p1", value: 345 },
                { code: "p2", value: 456 },
            ]
        },
    ],
    '8-2020': [ //august
        {
            time: "8-2020",
            task: [
                { code: "p1", value: 567 },
                { code: "p2", value: 678 },
            ]
        },
        {
            time: "8-2020",
            task: [
                { code: "p1", value: 789 },
                { code: "p2", value: 999 },
            ]
        },
    ]
}

How to group object of Array by key 'code', time and total sum by value?

Expected result:

output = [
    {
        time: "7-2020", //total month 7-2020
        task: [
            { code: "p1", valueSum: 468 }, // 123 + 345
            { code: "p2", valueSum: 690 }, // 234 +456
        ] 
    },
    {
        time: "8-2020",
        task: [
            { code: "p1", valueSum: 1356 }, // 567 + 789
            { code: "p2", valueSum: 1677 }, // 999 +678
        ]
    }
]

Please help me.

@NinaScholz code here: let groupDataByMonth; if (newlistTaskEvaluation) { groupByMonth = newlistTaskEvaluation.reduce((groups, item) => { groups[item.time] = [...groups[item.time] || [], item]; return groups; }, {}); } — minhminh
– minhminh, Commented Aug 7, 2020 at 7:16

Xetera · Accepted Answer · 2020-08-07 07:19:39Z

2

You could try something like this

const output = Object.entries(groupByMonth).map(([time, datapoints]) => {
  const codes = {}
  const allTasks = datapoints.flatMap(point => point.task)
  for (const { code, value } of allTasks) {
    codes[code] = (codes[code] || 0) + value
  }
  return {
    time,
    tasks: Object.entries(codes).map(([code, value]) => ({ code, value }))
  }
}

Though one downside is that the time complexity isn't perfect because of the structure of the data

answered Aug 7, 2020 at 7:19

Xetera

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1 Comment

minhminh Over a year ago

I tried quickly to use it and got the right result. Thank you <3

Paul Zaslavskij · Accepted Answer · 2020-08-07 07:26:20Z

2

    const groupByMonth = {
  "7-2020": [
    //july
    {
      time: "7-2020",
      task: [
        { code: "p1", value: 123 },
        { code: "p2", value: 234 },
      ],
    },
    {
      time: "7-2020",
      task: [
        { code: "p1", value: 345 },
        { code: "p2", value: 456 },
      ],
    },
  ],
  "8-2020": [
    //august
    {
      time: "8-2020",
      task: [
        { code: "p1", value: 567 },
        { code: "p2", value: 678 },
      ],
    },
    {
      time: "8-2020",
      task: [
        { code: "p1", value: 789 },
        { code: "p2", value: 999 },
      ],
    },
  ],
};

const result = Object.keys(groupByMonth).reduce((arr, key)=>{
  const task = (groupByMonth[key]).reduce((acc, rec) => {
    return [...acc, rec.task]
  }, [])
    const newObj = {time: key, task}
    return [...arr, newObj]
}, [])

console.log(JSON.stringify(result, 2, 2))

You can get object's keys and when you iterate over them find out inner objects, then, in every object use another oner reduce to get tasks in that data-type what you need. for iterations i suddenly use reduce() method as a swiss-knife to working with arrays.

answered Aug 7, 2020 at 7:26

Paul Zaslavskij

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3 Comments

Deykun Over a year ago

This will be faster .reduce((acc, rec) => { acc.push(rec.task); return acc; }, []). Spreading arrays and objects is recommended but you don't have to waste time on acc because he is only used inside reducer, and spreading the acc array is most costly than adding that one item to it.

Paul Zaslavskij Over a year ago

thanks! it's my well-known problem - writing more code then conditions really need according task

minhminh Over a year ago

@Paul Zaslavskij i want the output looks like this: output = [ { time: "7-2020", //total month 7-2020 task: [ { code: "p1", valueSum: 468 }, // 123 + 345 { code: "p2", valueSum: 690 }, // 234 +456 ] }, { time: "8-2020", task: [ { code: "p1", valueSum: 1356 }, // 567 + 789 { code: "p2", valueSum: 1677 }, // 999 +678 ] } ]

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