How to validate date if is the last day of the month with javascript?

function isLastDayOfMonth(date){
    return date.getDate() == new Date(date.getFullYear(),date.getMonth()+1,0).getDate();
}

If you want to avoid reinventing the wheel and having extra code to maintain and test, I highly suggest to use an external library like Moment.js or date-fns.

I personally advocate for date-fns as it is not as bulky as Moment and relies on Date primitives rather than Moment objects.

Your function of interest is named isLastDayOfMonth and can be found here