How can I split an array of objects and extract specific keys while adding 0s to fill up to a specific length?

My array of objects looks like this:

[{"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "03.07.", "beginYear": "2023", "end": "10.07.", "endYear": "2023", "timestamp": 1688335200000}}, {"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "26.06.", "beginYear": "2023", "end": "03.07.", "endYear": "2023", "timestamp": 1687730400000}}, {"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "19.06.", "beginYear": "2023", "end": "26.06.", "endYear": "2023", "timestamp": 1687125600000}}, {"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "12.06.", "beginYear": "2023", "end": "19.06.", "endYear": "2023", "timestamp": 1686520800000}}, {"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "05.06.", "beginYear": "2023", "end": "12.06.", "endYear": "2023", "timestamp": 1685916000000}}, {"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "29.05.", "beginYear": "2023", "end": "05.06.", "endYear": "2023", "timestamp": 1685311200000}}]

I want to map through the array, checking if the data key contains more than 4 numbers and if so, I want to extract the first 4 numbers (each data key is the same in all objects) and put it into a new object key "data" and the date key should look like the first item of the original array. The rest of the data key (in this example [4,2] should go into a new array that will be filled up with 0s until the data's length is 4. The date key of this should contain the date in 4 weeks starting from the first date.

I want to modify it so the result will be like this:

[{
    data: [ 5, 2, 7, 2 ],
    date: {
      begin: '29.05.',
      beginYear: '2023',
      end: '05.06.',
      endYear: '2023',
      timestamp: 1685311200000
    }
  },
{
    data: [4, 2, 0, 0 ],
    date: {
      begin: '26.06.',
      beginYear: '2023',
      end: '03.07.',
      endYear: '2023',
      timestamp: 1687730400000
    }
  }
]

The logic should also applicable if the data key contains more numbers

I tried it several times but somehow it doesn't as planned.

const fourStack = []

    const firstFourItems = result.slice(0, 4)
    const restItems = result.slice(4)

    const firstItem = firstFourItems[0]
    const newData = firstFourItems.map((item) => item.data.slice(0, 4))
    const newObj = {
      data: newData,
      date: {
        begin: firstItem.date.begin,
        beginYear: firstItem.date.beginYear,
        end: firstItem.date.end,
        endYear: firstItem.date.endYear,
        timestamp: firstItem.date.timestamp
      }
    }

    fourStack.push(newObj)

    restItems.forEach((item) => {
      fourStack.push({
        data: item.data.slice(0, 4),
        date: {
          begin: newObj.date.begin,
          beginYear: newObj.date.beginYear,
          end: newObj.date.end,
          endYear: newObj.date.endYear,
          timestamp: newObj.date.timestamp
        }
      })
    })

It gives me, not the result I want. Instead, it gives me this:

[{"data": [[Array], [Array], [Array], [Array]], "date": {"begin": "03.07.", "beginYear": "2023", "end": "10.07.", "endYear": "2023", "timestamp": 1688335200000}}, {"data": [5, 2, 7, 2], "date": {"begin": "03.07.", "beginYear": "2023", "end": "10.07.", "endYear": "2023", "timestamp": 1688335200000}}, {"data": [5, 2, 7, 2], "date": {"begin": "03.07.", "beginYear": "2023", "end": "10.07.", "endYear": "2023", "timestamp": 1688335200000}}]

newData shouldn't be a map, but a slice of the data of firstItem. You're getting all the data of all items in firsFourItems by doing a map.

You're pushing the same date in your forEach and the data you're using is still the first four of the original data key. It should use a slice of the initial data and filled with 0 until the length is 4.

Consider:

for(var k = 0; k < result[0].data.length / 4; ++k){
    newstack.push({
        data:Array.from({...result[0].data.slice(4 * k),length:4},(v,i)=>v??0),
        date: result[k*4].date // Edit: since you want the date sorted, sort the dates then get the [k*4] of the sorted array
    })
}

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let arr = [{
  "data": [5, 2, 7, 2, 4, 2],
  "date": {
    "begin": "03.07.",
    "beginYear": "2023",
    "end": "10.07.",
    "endYear": "2023",
    "timestamp": 1688335200000
  }
}, {
  "data": [5, 2, 7, 2, 4, 2],
  "date": {
    "begin": "26.06.",
    "beginYear": "2023",
    "end": "03.07.",
    "endYear": "2023",
    "timestamp": 1687730400000
  }
}, {
  "data": [5, 2],
  "date": {
    "begin": "19.06.",
    "beginYear": "2023",
    "end": "26.06.",
    "endYear": "2023",
    "timestamp": 1687125600000
  }
}, {
  "data": [5, 2, 7, 2, 4, 2],
  "date": {
    "begin": "12.06.",
    "beginYear": "2023",
    "end": "19.06.",
    "endYear": "2023",
    "timestamp": 1686520800000
  }
}, {
  "data": [5, 2, 7, 2, 4, 2],
  "date": {
    "begin": "05.06.",
    "beginYear": "2023",
    "end": "12.06.",
    "endYear": "2023",
    "timestamp": 1685916000000
  }
}, {
  "data": [5],
  "date": {
    "begin": "29.05.",
    "beginYear": "2023",
    "end": "05.06.",
    "endYear": "2023",
    "timestamp": 1685311200000
  }
}];

arr.map((e) => {
  if (e.data.length > 4) {
    e.data = e.data.slice(0, 4);
  } else {
    let length = 4 - e.data.length;
    for (let i = 0; i < length; i++) {
      e.data.push(0);
    }
  }

})
console.log(arr);

<!-- end snippet -->

Here is a configurable solution. You can specify data items per entry (dataLength), change the difference between begin and end dates within each entry (daysDelay), and set the difference between begin dates of subsequent entries obtained from the same initial entry (daysOffset).

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let source = [
{"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "03.07.", "beginYear": "2023", "end": "10.07.", "endYear": "2023", "timestamp": 1688335200000}},
{"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "26.06.", "beginYear": "2023", "end": "03.07.", "endYear": "2023", "timestamp": 1687730400000}},
{"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "19.06.", "beginYear": "2023", "end": "26.06.", "endYear": "2023", "timestamp": 1687125600000}},
{"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "12.06.", "beginYear": "2023", "end": "19.06.", "endYear": "2023", "timestamp": 1686520800000}},
{"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "05.06.", "beginYear": "2023", "end": "12.06.", "endYear": "2023", "timestamp": 1685916000000}},
{"data": [5, 2, 7, 2, 4, 2], "date": {"begin": "29.05.", "beginYear": "2023", "end": "05.06.", "endYear": "2023", "timestamp": 1685311200000}}
];
function processArray (source, dataLength, daysDelay, daysOffset) {
function getTimezoneOffset (obj) {
let year  = +obj.beginYear;
let month = +obj.begin.slice(3, 5) - 1;
let day   = +obj.begin.slice(0, 2);
return Date.UTC(year, month, day) - obj.timestamp;
}
function getDDMM (date) {
let DD = (date.getUTCDate() + 100).toString().slice(1);
let MM = (date.getUTCMonth() + 101).toString().slice(1);
return `${DD}.${MM}.`;
}
const result = [];
const msDay = 1000 * 60 * 60 * 24;
const msDaysDelay = daysDelay * msDay;
const msDaysOffset = daysOffset * msDay;
source.forEach(({data, date}) => {
let ts = date.timestamp;
for (let dataOffset = 0; dataOffset < data.length; dataOffset += dataLength) {
let msTimezoneOffset = getTimezoneOffset(date);
let dateCurrent = new Date(ts + msTimezoneOffset);
let dateNext = new Date(ts + msTimezoneOffset + msDaysDelay);
result.push({
data: data.slice(dataOffset, dataOffset + dataLength),
date: {
begin: getDDMM(dateCurrent),
beginYear: dateCurrent.getUTCFullYear(),
end: getDDMM(dateNext),
endYear: dateNext.getUTCFullYear(),
timestamp: ts
}
});
ts += msDaysOffset;
}
let lastData = result[result.length - 1].data;
while (lastData.length < dataLength) {
lastData.push(0);
}
});
return result;
}
console.log(processArray(source, 4, 7, 28));

<!-- end snippet -->