Get which day of week a month starts on from given input of month and year

Question

I have some JS code which stacks div's that are representing days in a month in a larger div that is actually a calendar. Users can freely select a month and year. I need to build a calendar for that month.

For example: if this month is starting with Saturday then I need to first build 5 empty blocks and then start filling the calendar with blocks that have a number of day inside them.

How can I calculate this number for the given input of month and year?

Answer 1

You can get the day of the week by using the getDay function of the Date object.

To get the first of the month create a new Date object :

 var year = "2012"; var month = "12"; var day = new Date(year + "-" + month + "-01").getDay(); // 6 - Saturday console.log(day);

Since you count from 1 and Monday is the first day of the week you'll also have to do this:

day = (day===0) ? 7 : day

Answer 2

ES6 way:

const firstDayInMonthIndex = (
  monthIndex = new Date().getMonth(), 
  year = new Date().getFullYear()
) => (
  new Date(`${year}-${monthIndex + 1}-01`).getDay()
)