Get all the weeks in a given month and year using Moment.js

weeks_in_month.coffee

# year and month are variables
year = 2015
month = 7 # August (0 indexed)
startDate = moment([year, month])

# Get the first and last day of the month
firstDay = moment(startDate).startOf('month')
endDay = moment(startDate).endOf('month')

# Create a range for the month we can iterate through
monthRange = moment.range(firstDay, endDay)

# Get all the weeks during the current month
weeks = []
monthRange.by('days', (moment)->
    if moment.week() not in weeks
        weeks.push(moment.week())
)

# Create a range for each week
calendar = []
for week in weeks
    # Create a range for that week between 1st and 7th day
    firstWeekDay = moment().week(week).day(1)
    lastWeekDay = moment().week(week).day(7)
    weekRange = moment.range(firstWeekDay, lastWeekDay)
    
    # Add to the calendar
    calendar.push(weekRange)

console.log calendar

Ok, changing firstWeekDay to 0 and lastWeekDay to 6 (week starts at Sunday) fixes the problem:

const firstWeekDay = moment([year, month]).week(week).day(0);
const lastWeekDay = moment([year, month]).week(week).day(6);