Question

This is the client script function for an asp.net validation control.

<script type="text/javascript">
    function validateDateControl(sender, args) {

        var d = new Date(args.Value);

        args.IsValid = (Object.prototype.toString.call(d) === "[object Date]");

        if (!args.IsValid) {
            sender.innerText = "Client: Invalid date";
        }     

        return args.IsValid;
    }
</script>

Immediate Window Results:
d
NaN
Object.prototype.toString.call(d) === "[object Date]"
true

Why is the comparison evaluating True?

Answer 1

Because new Date('as;dlas;ld,as;dl,as') is still a Date object.

> var d = new Date('as;dlas;ld,as;dl,as')
  undefined
> d.toString()
  "Invalid Date"
> Object.prototype.toString.call(d)
  "[object Date]"

A better way to check date validity is to see that Date.getTime() does not return NaN:

function validateDateControl(sender, args) {

    var d = new Date(args.Value);

    args.IsValid = !isNaN(d.getTime());

    if (!args.IsValid) {
        sender.innerText = "Client: Invalid date";
    }     

    return args.IsValid;
}