JavaScript get last day of Month - based on Year, Month and day-CodePudding

I need a JS function to get the last date of the month. as an example, I need to get the last Wednesday of this month.

I will pass Year, Month and day as a arguments.

getLastDayOfMonth('2022', '02', 'Wednesday');

function getLastDayOfMonth(year, month, day){

     // Output should be like this

       2022-02-23    

    (this is last Wednesday of the month, according to the arguments- Year, month and Day)
}

CodePudding user response：

Create a Date instance with the last day of the month then work backwards one day at a time until you match the day of the week you're after

const normaliseDay = day => day.trim().toLowerCase()
const dayIndex = new Map([
  ["sunday",    0],
  ["monday",    1],
  ["tuesday",   2],
  ["wednesday", 3],
  ["thursday",  4],
  ["friday",    5],
  ["saturday",  6],
])

const getLastDayOfMonth = (year, month, dow) => {
  const day = dayIndex.get(normaliseDay(dow))
  
  // init as last day of month
  const date = new Date(Date.UTC(parseFloat(year), parseFloat(month), 0))
  
  // work back one-day-at-a-time until we find the day of the week
  while (date.getDay() != day) {
    date.setDate(date.getDate() - 1)
  }
  
  return date
}

console.log(getLastDayOfMonth("2022", "02", "Wednesday"))

The numeric values are parsed as numbers so we don't run into problems with zero-padded octal numbers.