isBefore returns null if first argument is not a valid date

[rustybailey]

In the isBefore() method, before it does the comparison, it converts both arguments to a date using the toDate() helper. If the argument passed into toDate() is not a valid date, it returns null. However, this means that when we get to the comparison line (https://github.com/chriso/validator.js/blob/master/validator.js#L483), if the original variable is null, that line evaluates to null.

Is this the intended functionality? Shouldn't all validation functions return a boolean? If so, shouldn't we convert those toDate() variables to booleans so that the line would be something like return !!original && !!comparison && original < comparison;?

If this is actually a bug, just let me know and I can open a PR. If it is not a bug, I can find a way to work around it.

[chriso]

Yeah I'd say that's a bug and return !!original && !!comparison && original < comparison would be a reasonable fix.