How to prevent JavaScript Date Constructor from accepting invalid dates?

If you can count on the string input being formatted as digits (no weekday or month names), you can look at the input before creating a Date object.

function validDate(s){
    //check for day-month order:
    var ddmm= new Date('12/6/2009').getMonth()=== 5;

    //arrange month,day, and year digits:

    var A= s.split(/\D+/).slice(0, 3), 
    month= ddmm? A[1]: A[0], 
    day= ddmm? A[0]: A[1], 
    y= A.pop(), 

    //figure february for given year:

    feb= y%4== 0 && (y%100 || y%400== 0)? 29: 28, 

    // set maximum days per month:

    mdays= [0, 31, feb, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

    //if the string is a valid calendar date, return a date object.
    //else return NaN (or throw an Error):

    return mdays[parseInt(month, 10)]-A[1]>= 0? new Date(s): NaN;
}

validDate('02/29/2015')

/* returned value: (Number) NaN */

It seems that you cannot force a failure on illegal dates. The MDN docs claim, the observed behaviour should only happen when the constructor is called with more than 1 argument, but this condition does not appear to hold (at least it doesn't on chrome 40).

However, you can re-convert the Date and compare it with the original string:

var s = '02/31/2015';
var d = new Date(s)
var s_re = d.toLocaleDateString('en-US', { year: 'numeric', month: '2-digit', day: '2-digit' } );
if (s === s_re) {
    // ok
}

You can't set a JavaScript Date object to an invalid date.

Nevertheless you may want to check if a date is invalid.