isSame() function in moment.js or Date Validation

Docs – Is Same

Check if a moment is the same as another moment.

moment('2010-10-20').isSame('2010-10-20'); // true

If you want to limit the granularity to a unit other than milliseconds,
pass the units as the second parameter.

moment('2010-10-20').isSame('2009-12-31', 'year'); // false
moment('2010-10-20').isSame('2010-01-01', 'year'); // true
moment('2010-10-20').isSame('2010-12-31', 'year'); // true
moment('2010-10-20').isSame('2011-01-01', 'year'); // false

Your code

var x=moment("28-02-1999","DD-MM-YYYY"); // working
x.isSame("28-02-1999"); // comparing x to an unrecognizable string

If you try moment("28-02-1999"), you get an invalid date. So comparing x to an invalid date string returns false.

To fix it, either use the default date format (ISO 8601):

var x = moment("28-02-1999","DD-MM-YYYY");
x.isSame("1999-02-28"); // YYYY-MM-DD

Or pass isSame a moment object.

var x = moment("28-02-1999","DD-MM-YYYY");
x.isSame( moment("28-02-1999","DD-MM-YYYY") );