last-date-of-month

var lastDateOfMonth = require('last-date-of-month')
var assert = require('assert')

// January 31, 2016
assert(lastDateOfMonth(2016, 1) === 31)

// February 28, 2015
assert(lastDateOfMonth(2015, 2) === 28)

// February 29, 2016, a leap year
assert(lastDateOfMonth(2016, 2) === 29)