How can I get the 4 Mondays of a month with js?

问题

I'm building a chart where the x-axis should be the four weeks of a month. I would like to display only the four Mondays of that month.

I already have the currentMonth and the currentYear variables, and I know how to get the first day of the month. All I need is to get the four Mondays of a month in an array. And all of this in the same JavaScript file.

I'm pretty lost within my programming logic, and I've seen plenty of solutions that don't fit my use case.

Right now, I have:

var date = new Date();
var currentYear = date.getFullYear();
var currentMonth = date.getMonth();
var firstDayofMonth = new Date(currentYear, currentMonth, 1);
var firstWeekDay = firstDayofMonth.getDay();

but I would like to have something like this:

var myDates = [
    new Date(firstMonday),
    new Date(secondMonday),
    new Date(thirdMonday),
    new Date(fourthMonday),
];

回答1:

The following function will return all Mondays for the current month:

function getMondays() {
    var d = new Date(),
        month = d.getMonth(),
        mondays = [];

    d.setDate(1);

    // Get the first Monday in the month
    while (d.getDay() !== 1) {
        d.setDate(d.getDate() + 1);
    }

    // Get all the other Mondays in the month
    while (d.getMonth() === month) {
        mondays.push(new Date(d.getTime()));
        d.setDate(d.getDate() + 7);
    }

    return mondays;
}

回答2:

This would return the ~~fourth~~ last monday of month [m] in year [y]

function lastmonday(y,m) {
 var dat = new Date(y+'/'+m+'/1')
    ,currentmonth = m
    ,firstmonday = false;
  while (currentmonth === m){
    firstmonday = dat.getDay() === 1 || firstmonday;
    dat.setDate(dat.getDate()+(firstmonday ? 7 : 1));
    currentmonth = dat.getMonth()+1;
  }
  dat.setDate(dat.getDate()-7);
  return dat;
}
// usage 
lastmonday(2012,3); //=>Mon Mar 26 2012 00:00:00 GMT+0200
lastmonday(2012,2)  //=>Mon Feb 27 2012 00:00:00 GMT+0100
lastmonday(1997,1)  //=>Mon Jan 27 1997 00:00:00 GMT+0100
lastmonday(2012,4)  //=>Mon Apr 30 2012 00:00:00 GMT+0200

To be more generic, this will deliver the last any weekday of a month:

function lastDayOfMonth(y,m,dy) {
 var  days = {sun:0,mon:1,tue:2,wed:3,thu:4,fri:5,sat:6}
     ,dat = new Date(y+'/'+m+'/1')
     ,currentmonth = m
     ,firstday = false;
  while (currentmonth === m){
    firstday = dat.getDay() === days[dy] || firstday;
    dat.setDate(dat.getDate()+(firstday ? 7 : 1));
    currentmonth = dat.getMonth()+1 ;
  }
  dat.setDate(dat.getDate()-7);
  return dat;
 }
// usage 
lastDayOfMonth(2012,2,'tue'); //=>Tue Feb 28 2012 00:00:00 GMT+0100
lastDayOfMonth(1943,5,'fri'); //=>Fri May 28 1943 00:00:00 GMT+0200

来源：https://stackoverflow.com/questions/9481158/how-can-i-get-the-4-mondays-of-a-month-with-js

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