Knockout validation: Dynamic constraints

Question

I'm using Durandal, which in turn leverages off of Knockout.

I want to be able to Change validation lengths dynamically

Fiddle

The fiddle seems to be behaving slightly different than my "working" solution, but its still not doing what I'm wanting/expecting it to.

Viewmodel JS:

[Attempt 1]

define(function () {

   var self = this;

   self.userInfo = {       
        IdOrPassportNumber: ko.observable().extend({
            required: true,
            pattern: {
                message: 'A message',
                params: /some regex/
            }
        }),
        IdType: ko.observable()
    },

    self.isIdValid = ko.validatedObservable({ 
         IdOrPassportNumber: self.userInfo.IdOrPassportNumber 
    });

    self.userInfo.IdOrPassportNumber.subscribe(function (value) {
          if (isIdValid.isValid()) {
               console.log('YOLO!');
          }
    });

    self.userInfo.IdType.subscribe(function (value) {
        console.log(value);
        if (value === 'Passport') {
            self.userInfo.IdOrPassportNumber.extend({ maxLength: 15 });
        } else {
            self.userInfo.IdOrPassportNumber.extend({ maxLength: 13 });
        }
    });    

    var viewModel = {
        userInfo: self.userInfo
    };

    viewModel["errors"] = ko.validation.group(viewModel.userInfo);
    viewModel["errors"].showAllMessages();

    return viewModel;
});

What seems to be happening is that when i start typing i get the max & min validation of 13, but if i continue typing the validation changes to 15. I have tried another route of, setting the min & max length in the initial observable extend EG, just after the regex, and then setting the min and max length to use an observable, to no success.

[Attempt 2]

   self.userInfo = {       
       IdOrPassportNumber: ko.observable().extend({               
            maxLength: self.maxLength(), 
            minlength: self.maxLength()
       }),
       IdType: ko.observable()
   },

   self.maxLength = ko.observable();

   self.userInfo.IdType.subscribe(function (value) {

          if (value === 'Passport') {
             self.maxLength(15)
          } else {
              self.maxLength(3)
          }
    });

Answer 1

You were so close :-) You must provide the observable itself, not the unwrapped value. So just remove the () from maxLength() - the validation library will automatically unwrap it for you.

self.userInfo = {       
   IdOrPassportNumber: ko.observable().extend({               
        maxLength: self.maxLength, 
        minlength: self.maxLength
   }),
   IdType: ko.observable()
},

Here's another example with dynamic regex patterns.

    zipPostalPattern = ko.pureComputed(() => this.countryCode() === 'US' ? '^\\d{5}(?:[-\\s]\\d{4})?$' : '');
    zipOrPostal: KnockoutObservable<string> = ko.observable('').extend(
    {
        required: true,
        pattern: {
            message: 'This is not a valid postcode for the country',
            params: this.zipPostalPattern
        }
    });

or (if you don't want a message).

    zipPostalPattern = ko.pureComputed(function() { return this.countryCode() === 'US' ? '^\\d{5}(?:[-\\s]\\d{4})?$' : ''});
    zipOrPostal: KnockoutObservable<string> = ko.observable('').extend(
    {
        required: true,
        pattern:  self.zipPostalPattern
    });

Important: If you don't want a custom message don't just remove the message parameter and leave pattern = { params: this.zipPostalPattern } because it won't work. If you don't have a message you must set the Regex/string directly for the pattern parameter.

Or of course you can just define the computed observable in place (here it's ok to call countryCode() as a function because that's how computed's work)

    zipOrPostal: KnockoutObservable<string> = ko.observable('').extend(
    {
        required: true,
        pattern:  ko.pureComputed(function() { 
                     return self.countryCode() === 'US' ? '^\\d{5}(?:[-\\s]\\d{4})?$' : ''
                  })
    });