Define Knockout validation rule that takes an observable parameter using typescript - validation

Question

I have defined a validation rule like this
ko.validation.rules["studentValidation"] = {
validator: (val: any, params: any) => {
return (this.IsInRequiredRangeForStudent(params.DateOfBirth) && val === false);
}
}
IsInRequiredRangeForStudent = (dateOfBirth: any) () => {
//my implementation
}
Here is my ViewModel class, where i consume and apply this rule on an observable
this.isStudent = ko.observable<boolean>(isStudent).extend({
studentValidation: {
message: "Invalid student option!",
params: {
DateOfBirth: this.dateOfBirth()
}
}
});
In my validation rule implementation, I always get params.DateOfBirth as null. What I am doing wrong here?

Answer 1

params.DateOfBirth can be null for several reasons. But firstly I would check one scenario. There is a chance that when you extending isStudent observable, you define validation params assigning value of dateOfBirth observable. But the value is evaluated at the moment of assigning, I don't see the rest of your code but it's highly possible that dateOfBirth observable is null at the moment of assigning to params. So every further check of params.DateOfBirth may return NULL value.
Please try following:
this.isStudent = ko.observable<boolean>(isStudent).extend({
studentValidation: {
message: "Invalid student option!",
params: {
DateOfBirth: this.dateOfBirth
}
}
});
and this:
ko.validation.rules["studentValidation"] = {
validator: (val: any, params: any) => {
return (this.IsInRequiredRangeForStudent(params.DateOfBirth()) && val === false);
}
}
What it changes? It defines params.DateOfBirth as function (not a value), so you can evaluate its value on every validation call.