.ruleFor

The .ruleFor method on the Validator class is used to build up rule chains for properties on your model.

To get started, simply call this.ruleFor in the constructor of your validator and pass in the name of a property on your model (note that this is strongly typed, you'll get a compilation error if you pass the name of a property that doesn't exist on the model).

The result of this call is a rule chain builder that exposes all the relevant built-in validation rules for the property you specified.

import { Validator } from 'fluentvalidation-ts';

type FormModel = {
  name: string;
  isEmployed: boolean;
  jobTitle: string | null;
};

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

    // Returns a rule chain builder for the 'name' property
    this.ruleFor('name');
  }
}

To add a validation rule to the target property, simply call the relevant method on the rule chain builder (passing in any parameters as necessary). The result of such a call is again the rule chain builder, so you can specify multiple rules in a single call to .ruleFor.

// The result of adding a rule is again the rule chain builder,
// so you can add multiple rules in a single call
this.ruleFor('name')
  .notEmpty()
  .maxLength(100);

After adding a rule to the chain you also gain access to a number of configuration methods which allow you to do things like specify what error should be used if the validation rule fails, and conditions under which the rules should/shouldn't run.

this.ruleFor('name')
  .notEmpty()
  .maxLength(100);

// You can specify a custom error message for each rule in the chain,
// and provide a condition to determine when the rules should run
this.ruleFor('jobTitle')
  .notEmpty()
  .withMessage('Please enter a Job Title')
  .maxLength(100)
  .withMessage('Please enter no more than 100 characters')
  .when(formModel => formModel.isEmployed);

// You can also provide a condition to determine when certain rules
// should not run
this.ruleFor('jobTitle')
  .equal('')
  .withMessage('You cannot enter a Job Title if you are not employed')
  .unless(formModel => formModel.isEmployed);

As the above example illustrates, you can make several calls to .ruleFor for the same property. It doesn't matter how many rule chains you define for a particular property, and you don't have to define any at all if you don't need to.