如何在不重复月份的情况下从日期数组中获取最多 8 个日期，这些日期的索引之间的间隔相同？

Question

For exemplo, in this array, I want to return max 8 elements without repeating month. Include the first element is optional.

['01-15', '01-16', '01-17', '01-18', '01-19', '01-20', '01-21', '01-22', '01-23', '01-24', '01-25', '01-26', '01-27', '01-28', '01-29', '01-30', '02-01', '01-02', '02-03', '02-04', '02-05', '02-06', '02-07', '02-08', '02-09', '02-10', '02-11', '02-12', '02-13', '02-14', '02-15', '02-16', '02-17', '02-18', '02-19', '02-20', '02-21', '02-22', '02-23', '02-24', '02-25', '02-26', '02-27', '02-28', '03-01', '03-02', '03-03', '03-04', '03-05', '03-06', '03-07', '03-08', '03-09', '03-10', '03-11', '03-12', '03-13', '03-14', '03-15']

would return

['01-15', '02-05', '03-01']

If by chance the array has a range of 12 months, this would return, for exemplo (I don't know the exactly result):

['01-20', '03-30', '05-01', '06-10', '07-20', '09-01', '11-01', '12-31'] // array size must be 8 and ensure that the months do not repeat

My code is:

const newArray = oldArray
.filter(function(date, index, arr){
  const delta = Math.floor(oldArray.length / 8);    
  return index % delta === delta - 1; // would be something here
});

Answer 1

This returns max 8 non-repeating months as you describe, but it doesn't output "02-05" but "02-01" as 2nd element. If that should be the case then extra criteria should be added to the description.

function filterEightMonths(array) {
  const map = {};
  let mapSize = 0;
  for (const date of array) {
    const [month, day] = date.split('-');
    if (!map[month]) {
      map[month] = date;
      mapSize++;
    }
    if (mapSize > 8) {
      break;
    }
  }
  return Object.values(map);
}

Answer 2

You could do something like this

function filterDateList(list) {
  const result = list.reduce((previousValue, currentValue) => {
    if (
      !previousValue.find(
        (el) => el.split("-")[0] === currentValue.split("-")[0]
      )
    ) {
      previousValue.push(currentValue);
    }
    return previousValue;
  }, []);
  return result.slice(0, 8);
}

const dates = ['01-15', '01-16', '01-17', '01-18', '01-19', '01-20', '01-21', '01-22', '01-23', '01-24', '01-25', '01-26', '01-27', '01-28', '01-29', '01-30', '02-01', '01-02', '02-03', '02-04', '02-05', '02-06', '02-07', '02-08', '02-09', '02-10', '02-11', '02-12', '02-13', '02-14', '02-15', '02-16', '02-17', '02-18', '02-19', '02-20', '02-21', '02-22', '02-23', '02-24', '02-25', '02-26', '02-27', '02-28', '03-01', '03-02', '03-03', '03-04', '03-05', '03-06', '03-07', '03-08', '03-09', '03-10', '03-11', '03-12', '03-13', '03-14', '03-15'];

console.log(filterDateList(dates));