Find next instance of a given weekday (ie. Monday) with moment.js

The trick here isn't in using Moment to go to a particular day from today. It's generalizing it, so you can use it with any day, regardless of where you are in the week.

First you need to know where you are in the week: moment().day(), or the slightly more predictable (in spite of locale) moment().isoWeekday(). Critically, these methods return an integer, which makes it easy to use comparison operators to determine where you are in the week, relative to your targets.

Use that to know if today's day is smaller or bigger than the day you want. If it's smaller/equal, you can simply use this week's instance of Monday or Thursday...

const dayINeed = 4; // for Thursday
const today = moment().isoWeekday();

if (today <= dayINeed) { 
  return moment().isoWeekday(dayINeed);
}

But, if today is bigger than the day we want, you want to use the same day of next week: "the monday of next week", regardless of where you are in the current week. In a nutshell, you want to first go into next week, using moment().add(1, 'weeks'). Once you're in next week, you can select the day you want, using moment().day(1).

Together:

const dayINeed = 4; // for Thursday
const today = moment().isoWeekday();

// if we haven't yet passed the day of the week that I need:
if (today <= dayINeed) { 
  // then just give me this week's instance of that day
  return moment().isoWeekday(dayINeed);
} else {
  // otherwise, give me *next week's* instance of that same day
  return moment().add(1, 'weeks').isoWeekday(dayINeed);
}

See also https://stackoverflow.com/a/27305748/800457

---

EDIT: other commenters have pointed out that the OP wanted something more specific than this: the next of an array of values ("the next Monday or Thursday"), not merely the next instance of some arbitrary day. OK, cool.

The general solution is the beginning of the total solution. Instead of comparing for a single day, we're comparing to an array of days: [1,4]:

const daysINeed = [1,4]; // Monday, Thursday
// we will assume the days are in order for this demo, but inputs should be sanitized and sorted

function isThisInFuture(targetDayNum) {
  // param: positive integer for weekday
  // returns: matching moment or false
  const todayNum = moment().isoWeekday();  

  if (todayNum <= targetDayNum) { 
    return moment().isoWeekday(targetDayNum);
  }
  return false;
}

function findNextInstanceInDaysArray(daysArray) {
    // iterate the array of days and find all possible matches
    const tests = daysINeed.map(isThisInFuture);

    // select the first matching day of this week, ignoring subsequent ones, by finding the first moment object
    const thisWeek = tests.find((sample) => {return sample instanceof moment});

    // but if there are none, we'll return the first valid day of next week (again, assuming the days are sorted)
    return thisWeek || moment().add(1, 'weeks').isoWeekday(daysINeed[0]);;
}
findNextInstanceInDaysArray(daysINeed);

I'll note that some later posters provided a very lean solution that hard-codes an array of valid numeric values. If you always expect to search the same days, and don't need to generalize for other searches, that'll be the more computationally efficient solution, although not the easiest to read, and impossible to extend.

The following can be used to get any next weekday date from now (or any date)

var weekDayToFind = moment().day('Monday').weekday(); //change to searched day name

var searchDate = moment(); //now or change to any date
while (searchDate.weekday() !== weekDayToFind){ 
  searchDate.add(1, 'day'); 
}

moment().day() will give you a number referring to the day_of_week.

What's even better: moment().day(1 + 7) and moment().day(4 + 7) will give you next Monday, next Thursday respectively.

See more: http://momentjs.com/docs/#/get-set/day/

get the next monday using moment

moment().startOf('isoWeek').add(1, 'week');