Knockout minLength with empty string allowed

Question

I am new to knockout, and trying to workout how to use validations. We have the following piece of code -

 var MyObjectModel = function(myObject){
 var self = this;
 self.myNumber = ko.observable(myObject.number).trimmed();
 self.myNumber.extend({
 minLength: {
 params: 7,
 message: "My Number is too short."
 },
 maxLength: {
 params: 7,
 message: "My Number is too long."
 }
 });
 self.errors = ko.validation.group(self);
}

Now I would like to change the code so that either the length of myObject.number should be either exactly 7, or the field should be empty. How do I go about making this change ?

Any help would be appreciated. In case you think something is not clear, please let me know so I can elaborate more.

• nemesv: What is your problem with your current approach with minLength and minLength? What is not working exactly? Because it seems it is working fine: jsfiddle.net/X9mDy/1 it is only valid is you enter 7 digits long.
• Abhinav Manchanda: minLength and maxLength work fine for me. What I also want is to make the field optional. So if the user has put in any value, it should be exactly 7 characters long, else it should be empty. I tried required:false, but it does not help. Any idea where I could be going wrong ?

Answer 1

You can use a custom pattern to it:

var MyObjectModel = function(myObject){
 var self = this;
 self.myNumber = ko.observable(myObject.number).trimmed();
 self.myNumber.extend({
 pattern: {
 params: '^[0-9]{7}$|^$',
 message: "My Number must have 7 chars or stay empty."
 }
 });
 self.errors = ko.validation.group(self);
}