Javascript: isValidDate()

gistfile1

function isValidDate(value, userFormat) {

  // Set default format if format is not provided
  userFormat = userFormat || 'mm/dd/yyyy';

  // Find custom delimiter by excluding the
  // month, day and year characters
  var delimiter = /[^mdy]/.exec(userFormat)[0];

  // Create an array with month, day and year
  // so we know the format by index
  var theFormat = userFormat.split(delimiter);

  // Get the user date now that we know the delimiter
  var theDate = value.split(delimiter);

  function isDate(date, format) {
    var m, d, y, i = 0, len = format.length, f;
    for (i; i < len; i++) {
      f = format[i];
      if (/m/.test(f)) m = date[i];
      if (/d/.test(f)) d = date[i];
      if (/y/.test(f)) y = date[i];
    }
    return (
      m > 0 && m < 13 &&
      y && y.length === 4 &&
      d > 0 &&
      // Is it a valid day of the month?
      d <= (new Date(y, m, 0)).getDate()
    );
  }

  return isDate(theDate, theFormat);

}

$('button').click(function(){
    $('span').text(
      isValidDate( $('input').val() ) ? 
        'Congrats! It\'s a valid date' :
        'Sorry, not a valid date.'
    );
});

I like your usage of the regular expression to determine the delimiter. Elegant solution.

I propose an alternative implementation:

function isValidDate(value, userFormat) {
    return "I don't know."
}