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@markthiessen
Last active August 9, 2024 13:52
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JavaScript - get weeks in a month as array of start and end days
//note: month is 0 based, just like Dates in js
function getWeeksInMonth(year, month) {
const weeks = [],
firstDate = new Date(year, month, 1),
lastDate = new Date(year, month + 1, 0),
numDays = lastDate.getDate();
let dayOfWeekCounter = firstDate.getDay();
for (let date = 1; date <= numDays; date++) {
if (dayOfWeekCounter === 0 || weeks.length === 0) {
weeks.push([]);
}
weeks[weeks.length - 1].push(date);
dayOfWeekCounter = (dayOfWeekCounter + 1) % 7;
}
return weeks
.filter((w) => !!w.length)
.map((w) => ({
start: w[0],
end: w[w.length - 1],
dates: w,
}));
}
exports.getWeeksInMonth = getWeeksInMonth;
const getWeeksInMonth = require("./getWeeksInMonth").getWeeksInMonth;
describe("getWeeksInMonth", () => {
test("June 2019 (starts on Saturday)", () => {
const weeks = getWeeksInMonth(2019, 5);
expect(weeks.length).toBe(6);
expect(weeks[0].start).toBe(1);
expect(weeks[0].end).toBe(1);
});
test("Feb 2021 (starts on Monday)", () => {
const weeks = getWeeksInMonth(2021, 1);
expect(weeks.length).toBe(5);
expect(weeks[0].start).toBe(1);
expect(weeks[0].end).toBe(6);
expect(weeks[4].start).toBe(28);
expect(weeks[4].end).toBe(28);
});
test("Aug 2021 (starts on Sunday)", () => {
const weeks = getWeeksInMonth(2021, 7);
expect(weeks.length).toBe(5);
expect(weeks[0].start).toBe(1);
expect(weeks[0].end).toBe(7);
expect(weeks[4].start).toBe(29);
expect(weeks[4].end).toBe(31);
});
});
@panchihpeng
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I used dayjs to write it. dayjs
It works !

const getWeekMonth = (year, month) => {
  const firstDay = dayjs(`${year}-${month}`).startOf('month').format('d')

  let startDay
  if (firstDay === 1) {
    startDay = dayjs(`${year}-${month}`).format('YYYY-MM-DD')
  } else {
    const diffDays = (8 - firstDay) % 7
    startDay = dayjs(`${year}-${month}`).startOf('month').add(diffDays, 'days').format('YYYY-MM-DD')
  }

  const lastDay = dayjs(`${year}-${month}`).endOf('month').format('d')

  let endDay
  if (lastDay === 0) {
    endDay = dayjs(`${year}-${month}`).endOf('month').format('YYYY-MM-DD')
  } else {
    const lastDiffDays = 6 - lastDay
    endDay = dayjs(`${year}-${month}`).startOf('month').add(1, 'month').add(lastDiffDays, 'days').format('YYYY-MM-DD')
  }

  const weeks = (dayjs(endDay).diff(dayjs(startDay), 'days') + 1) / 7

  const weekList = []
  for (let i = 1; i <= weeks; i++) {
    weekList.push({
      weekName: `第${i}周`,
      num: i,
      startDay,
      endDay: dayjs(startDay).add(6, 'days').format('YYYY-MM-DD'),
      range: `${dayjs(startDay).format('MM-DD')}~${dayjs(startDay).add(6, 'days').format('MM-DD')}`
    })
    startDay = dayjs(startDay).add(7, 'days').format('YYYY-MM-DD')
  }
  return weekList
}

@gktgautam
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gktgautam commented Jan 19, 2020

If you want to support starting with Monday,
I believe this TypeScript function should work best.

getWeeksInMonth(year: number, month: number) { const daysName = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
const weeks=[];      
const  lastDate = new Date(year, month + 1, 0); 

let start: number=0;
let end: number; 

for (let i = 1; i < lastDate.getDate()+1; i++) {         
  if (daysName[Number(new Date(year, month, i).getDay())] =="Sunday" || i == lastDate.getDate()) {
    end= i;
    weeks.push({
        start:start+1,
        end:end
      }); 
      start = i;           
   }
 }         
 console.log(weeks);  }`

@brunolkatz
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I make this for get array of weeks:

function getWeeksStartAndEndInMonth(_month: number, _year: number, returnDate: boolean = false) {
    const lastDay = new Date(_year, _month + 1, 0);
    let weeks: Array<{start: number, end: number}> = [];
    let _days = [];
    for (let _day = 1; _day <= lastDay.getDate(); _day++){
        let _thisDay = new Date(_year, _month, _day);
        _days.push(returnDate ? _thisDay : _day);
        if(_thisDay.getDay() === 6){
            weeks.push({start: _days[0], end: _days[_days.length-1]});
            _days = [];
        }else if(_thisDay.getDate() === lastDay.getDate()){
            weeks.push({start: _days[0], end: _days[_days.length-1]});
        }
    }
    return weeks;
}

for(let i = 0; i <= 11; i++){
    let _month = getWeeksStartAndEndInMonth(i,2020);
    console.log(_month);
}

@bedirhanyildirim
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bedirhanyildirim commented Nov 27, 2020

If the week starts with 'Monday' as index 1:

// note: month is in range 0-11
function getFullWeeksStartAndEndInMonth (month, year) {
    let weeks = [],
        firstDate = new Date(year, month, 1),
        lastDate = new Date(year, month + 1, 0),
        numDays = lastDate.getDate()

    let start = 1
    let end
    if (firstDate.getDay() === 1) {
        end = 7
    } else if (firstDate.getDay() === 0) {
        let preMonthEndDay = new Date(year, month, 0)
        start = preMonthEndDay.getDate() - 6 + 1
        end = 1
    } else {
        let preMonthEndDay = new Date(year, month, 0)
        start = preMonthEndDay.getDate() + 1 - firstDate.getDay() + 1
        end = 7 - firstDate.getDay() + 1
        weeks.push({
            start: start,
            end: end
        })
        start = end + 1
        end = end + 7
    }
    while (start <= numDays) {
        weeks.push({
            start: start,
            end: end
        });
        start = end + 1;
        end = end + 7;
        end = start === 1 && end === 8 ? 1 : end;
        if (end > numDays && start <= numDays) {
            end = end - numDays
            weeks.push({
                start: start,
                end: end
            })
            break
        }
    }
    return weeks
}

@Sagargalande
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Sagargalande commented Feb 1, 2021

This code will start first day of month

Week start day Monday and end day Sunday

Last week end is month end day

function getWeeksStartAndEndInMonth(month, year) {
  let weeks = [],
      firstDate = new Date(year, month, 1),
      lastDate = new Date(year, month , 0),
      numDays = lastDate.getDate();

   start = 1;
  let end = GetfirstWeekEnd(month,year).getDate();
  while (start <= numDays) {
      weeks.push({start: start, end: end});
      start = end + 1;
      end = end + 7;
      end = start === 1 && end === 8 ? 1 : end;
      if (end > numDays) {
          end = numDays;
      }
  }
  return weeks;
  }

 function GetfirstWeekEnd(month,year) {
      var firstDay= new Date(year + "-" + month + "-" + 1);
      var first = firstDay;
      const lastDay = new Date(year, month, 0);
      const daysInMonth = lastDay.getDate();
      let dayOfWeek = firstDay.getDay();
      var endday="";
      var startCount = dayOfWeek;
      if (startCount != 0) {
           endday = new Date( firstDay.setDate( firstDay.getDate() + (7-dayOfWeek)));
      }
      else {
          endday = new Date(firstDay.setDate( firstDay.getDate() + 1));

      }
      return endday;
  } 
  
  function endFirstWeek(firstDate, firstDay) {
  if (! firstDay) {
      return 7 - firstDate.getDay();
  }
  if (firstDate.getDay() < firstDay) {
      return firstDay - firstDate.getDay();
  } else {
      return 7 - firstDate.getDay() + firstDay;
  }
}

### ### ### ###

Input :
getWeeksStartAndEndInMonth(1, 2021)
here is output
0: {start: 1, end: 3}
1: {start: 4, end: 10}
2: {start: 11, end: 17}
3: {start: 18, end: 24}
4: {start: 25, end: 31}

@pereira-a
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If you want a function to return a month's week based on business days this shall work:

function getWeeksInMonth(year: number, month: number) {
        const weeks = [],
            firstDate = new Date(year, month, 1),
            lastDate = new Date(year, month + 1, 0),
            numDays = lastDate.getDate();
        
        let offset = 0;

        if(firstDate.getDay() === 0) offset = 1;
        else if(firstDate.getDay() === 6) offset = 2;

        let start = 1 + offset;
        let end: number;

        if(offset === 0) end = 5 - firstDate.getDay() + 1;
        else end = 5 - start + 1 + (offset * 2);

        while(start <= numDays) {
            weeks.push({start: start, end: end});
            start = end + 3;
            end = end + 7;
            end = start === 1 && end === 8 ? 1 : end;
            if(end > numDays) end = numDays;
        }

        return weeks;
}

Results for Oct 2020:
image

Results for Feb 2020:
image

Results for March 2020:
image

@suryavi
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suryavi commented Jul 19, 2022

getWeekOfMonth
Screenshot (219)

@dllsystem
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dllsystem commented Jan 24, 2024

/**

  • It takes a year and a month as arguments, and returns an array of objects, each object containing
  • the start and end dates of the week, and an array of all the dates in that week
  • @param year - The year of the month you want to get the weeks for.
  • @param month - The month you want to get the weeks for.
  • @returns An array of objects.
    */

function getWeeksInMonth(year, month) {
const weeks = []

let firstDate = new Date(year, month, 1)
let lastDate = new Date(year, month + 1, 0)
let numDays = lastDate.getDate()

let dayOfWeekCounter = firstDate.getDay()

for (let date = 1; date <= numDays; date++) {
if (dayOfWeekCounter === 0 || weeks.length === 0) {
weeks.push([])
}
weeks[weeks.length - 1].push(date)
dayOfWeekCounter = (dayOfWeekCounter + 1) % 7
}

/* This is to add the last week of the previous month to the first week of the current month. */
if (weeks[0].length < 7) {
const beforeIndex1 = addMonth(year, month - 1, 1)
const indexRefactor = [...beforeIndex1, ...weeks[0]]
weeks[0] = indexRefactor
}

/* This is to add the first week of the next month to the last week of the current month. */
if (weeks[weeks.length - 1].length < 7) {
const afterIndex1 = addMonth(year, month + 1, 0)
const indexRefactor = [...weeks[weeks.length - 1], ...afterIndex1]
weeks[weeks.length - 1] = indexRefactor
}

return weeks
.filter((w) => !!w.length)
.map((w) => ({
start: w[0],
end: w[w.length - 1],
dates: w,
}))
}

/**

  • It takes a year, month, and flag, and returns the first or last week of the month, depending on the flag
  • @param year - the year of the month you want to get
  • @param month - The month you want to get the first or last week of.
  • @param flag - 0 for first week, 1 for last week, 2 for all weeks
  • @returns An array of arrays.
    */

const addMonth = (year, month, flag) => {
const weeks = [],
firstDate = new Date(year, month, 1),
lastDate = new Date(year, month + 1, 0),
numDays = lastDate.getDate()

let dayOfWeekCounter = firstDate.getDay()
for (let date = 1; date <= numDays; date++) {
if (dayOfWeekCounter === 0 || weeks.length === 0) {
weeks.push([])
}
weeks[weeks.length - 1].push(date)
dayOfWeekCounter = (dayOfWeekCounter + 1) % 7
}
if (flag === 0) {
return weeks[0]
}
if (flag === 1) {
return weeks[weeks.length - 1]
}
return []
}

console.table(getWeeksInMonth(2024, 0))

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