AsyncValidator

The AsyncValidator<TModel> generic class is an extension of Validator<TModel> that has additional async rules available (most notably .mustAsync and .setAsyncValidator).

import { AsyncValidator } from 'fluentvalidation-ts';

Defining an async validator for a model of type TModel works exactly the same as defining a standard validator - all you have to do is define a class which extends AsyncValidator<TModel> (as opposed to Validator<TModel>) and specify some rules in the constructor using the .ruleFor and .ruleForEach methods.

type FormModel = { username: string };

class FormValidator extends AsyncValidator<FormModel> {
  constructor() {
    super();

    this.ruleFor('username').mustAsync(async (username) =>
       await api.usernameIsAvailable(username);
    )
    .withMessage('This username is already taken');
  }
}

To actually validate an instance of your model, simply create an instance of your validator and pass your model to the .validateAsync method. As the name suggests this method is asynchronous, so be sure to await the result or use Promise callback methods (i.e. .then and .catch).

Note that the synchronous .validate method is not available on an instance of AsyncValidator, you must always use the .validateAsync method.

const formValidator = new FormValidator();

const validResult = await formValidator.validateAsync({
  username: 'ajp_dev123',
});
// ✔ {}

const invalidResult = await formValidator.validateAsync({
  username: 'ajp_dev',
});
// ❌ { username: 'This username is already taken' }

A call to .validateAsync returns a Promise that resolves to an object of type ValidationErrors<TModel>, which describes the validity of the given value.