JavaScript date : How to get date for first, mid and last day of a month?

var firstDayStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth(), 1);

var midDayStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth(), 15);

var midDayPlusOneStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth(), 16);

var lastDayStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth() + 1, 0);