Specifications

Specifications allow us to define business rules in our applications. It also lets us combine business rules to perform some boolean logic on our objets.

You can learn more about the Specification pattern here (opens new window).

Domeniere uses the Swindle Library (opens new window) to provide common patterns, including implementing Specifications.

Defining Specifications

We can define a specification by using the following command in our project root directory.

domeniere create specification <module-name>/path/to/specification-name

This will create a specification file inside the specifications subdirectory of our specified module.

Note: In order to create a specification, you must have created a module which will contiain the specification. You can learn more about Modules in the Modules section.

Below is an example definition of an RegistrationAgeSpecification class.

import { CompositeSpecification } from "@swindle/specification";

export class RegistrationAgeSpeciication extends CompositteSpecification {

    constructor() {
        //
    }

    public isSatisfiedBy(registration: Registration): boolean {
        return registration.age() >= 13;
    }
}

We define our specification's isSatisfiedBy() method to accept an instance of a Registration class and verifies that the registration meets the minimum allowed age to register. In our case, we check the age() property of our registration.

Using Specifications

Once we have defined our specification, we can use it in our application where we see fit. Below is an example of how we might use our specification.

const ageSpecification = new RegistrationAgeSpecification();
const isAllowedAge = ageSpecification.isSatisfiedBy(registration);

Chaining Specifications

We can also combine different specifications to perform a wide variety of boolena logic operations.

const ageSpecification = new RegistrationAgeSpecification();
const usernameSpecification = new RegistrationUsernameSpecification();
const agreementSpecification = new RegistrationAgreementSpecification();

canRegister = usernameSpecification
    .and(ageSpecification)
    .and(agreementSpecification)
    .isSatisfiedBy(registration);

Here, we use the and() method to chain our specifications together.

This, however, is not our only option. We can also use the or(), not(), andNot((), and orNot() methods as we see fit.