Constraints

Constraints function the same as validators, and in fact use the same functions but instead of reporting an error, they disallow a change that would not validate.

They are specified on the constraints key of a field in a form config object.

{
  name: {
    constraints: [hasLength(0, 10)];
  }
}

A couple things to note here:

Constraints are in general used more permissively than validators. For example a validator like hasLength(10, 10) could be paired with a constraint hasLength(0, 10) in order to prevent the user from typing after the tenth character, but displaying an error if they have less than 10 characters. hasLength(10, 10) as a constraint would be unlikely to be that useful as it would disallow any changes that make the string not length 10. This would make the only valid action pasting in a 10 character string.

Constraints are applied per set event. So if the set action contains a value that fails the constraint validation the state treats it as though it was never sent at all.

Constraints make some validators redundant. If you use onlyIntegers as a constraint then onlyIntegers is useless as a validator as it would never be able to display that error.

Remember that constraints are a UX consideration and should never be considered a security measure. Always validate on the backend where the user does not control the runtime.