Value Objects - DDD w/ TypeScript

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Also from the Domain-Driven Design with TypeScript article series.

In Domain-Driven Design, Value Objects are one of two primitive concepts that help us to create rich and encapsulated domain models.

Those concepts are Entities and Value Objects.

Value Objects are best understood by understanding how it's different from an Entity. Their main difference is in how we determine identity between two Value Objects and how we determine identity between two Entities.

Entity Identity

We use an Entity to model a domain concept when we care about the model's identity and being able to distinguish that identity from other instances of the model.

The way that we determine that identity helps us determine whether it's an Entity or a Value Object.

A common example is modeling a user.

In this example, we'd say that a User is an Entity because the way that we determine the difference between two different instances of a User is through it's Unique Identifier.

The Unique Identifier we use here is either a randomly-generated UUID or an auto-incremented SQL id that becomes a Primary Key that we can use for lookup from some persistence technology.

Value Objects

With Value Objects, we establish identity through the structural equality of two instances.

Structural Equality

Structural equality means that two objects have the same content. This is different from referential equality / identity which means that the two objects are the same.

To identify two Value Objects from each other, we look at the actual contents of the objects and compare based on that.

For example, there might be a Name property on the User Entity.

How can we tell if two Names are the same?

It's pretty much like comparing two strings, right?

"Nick Cave" === "Nick Cave" // true

"Kim Gordon" === "Nick Cave" // false

This is easy.

Our User entity could look like this:

interface UserProps {
  name: string
}

class User extends Entity<UserProps> {
  
  get name (): string {
    return this.props.name;
  }

  constructor (props: UserProps) {
    super(props);
  }
}

This is OK, but it could be better. Lemme ask a question:

What if we wanted to limit the length of a user's name. Let's say that it can be no longer than 100 characters, and it must be at least 2 characters.

A naive approach would be to write some validation logic before we create an instance of this User, maybe in a service.

class CreateUserService {
  public static createUser (name: string) : User{
    if (name === undefined || name === null || name.length <= 2 || name.length > 100) {
      throw new Error('User must be greater than 2 chars and less than 100.')
    } else {
      return new User(name)
    }
  }
}

This isn't ideal. What if we wanted to handle Editing a user's name?

class EditUserService {
  public static editUserName (user: User, name: string) : void {
    if (name === undefined || name === null || name.length <= 2 || name.length > 100) {
      throw new Error('User must be greater than 2 chars and less than 100.')
    } else {
      user.name = name;
      // save
    }
  }
}

1. This isn't really the right place to be doing this.
2. We've just repeated the same validation logic.

This is actually how a lot of projects start to spin out of scope. We end up putting too much domain logic and validation into the services, and the models themselves don't accurately encapsulate the domain logic.

We call this an Anemic Domain Model.

We introduce value object classes to strictly represent a type and encapsulate the validation rules of that type.

Value Objects

We had this before, a basic class for our User entity wiith a string-ly typed name property.

interface UserProps {
  name: string
}

class User extends Entity<UserProps> {

  get name (): string {
    return this.props.name;
  }

  constructor (props: UserProps) {
    super(props);
  }
}

If we were to create a class for the name property, we could co-locate all of the validation logic for a name in that class itself.

The upper bound (max length), the lower bound (min length), in addition to any algorithm that we wanted to implement in order to strip out whitespace, remove bad characters, etc- it could all go in here.

Using a static factory method and a private constructor, we can ensure that the preconditions that must be satisfied in order to create a valid name.

interface NameProps {
  value: string
}

class Name extends ValueObject<NameProps> {

  get value (): string {
    return this.props.value;
  }
  
  // Can't use the `new` keyword from outside the scope of the class.
  private constuctor (props: NameProps) {
    super(props);
  }

  public static create (name: string) : Name {
    if (name === undefined || name === null || name.length <= 2 || name.length > 100) {
      throw new Error('User must be greater than 2 chars and less than 100.')
    } else {
      return new Name({ value: name })
    }
  }
}

Then, in the User class, we'll update the name attribute in UserProps to be of type Name instead of string.

interface UserProps {
  name: Name;
}

class User extends Entity<UserProps> {

  get name (): Name {
    return this.props.name;
  }

  private constructor (props: UserProps) {
    super(props);
    this.name = props.name;
  }

  public static create (props: IUser) {
    if (props.name === null || props.name === undefined) {
      throw new Error('Must provide a name for the user');
    } else {
      return new User(props);
    }
  }
}

We apply the static factory method here as well.

Value Object class

Here's an example of a Value Object class.

import { shallowEqual } from "shallow-equal-object";

interface ValueObjectProps {
  [index: string]: any;
}

/**
 * @desc ValueObjects are objects that we determine their
 * equality through their structrual property.
 */

export abstract class ValueObject<T extends ValueObjectProps> {
  public readonly props: T;

  constructor (props: T) {
    this.props = Object.freeze(props);
  }

  public equals (vo?: ValueObject<T>) : boolean {
    if (vo === null || vo === undefined) {
      return false;
    }
    if (vo.props === undefined) {
      return false;
    }
    return shallowEqual(this.props, vo.props)
  }
}

Check out the equals method. Notice that we use shallowEquals in order to determine equality. This is one way to accomplish structural equality.

When it makes sense, subclasses of this Value Object base class can also be extended to include convenience methods like greaterThan(vo?: ValueObject<T>) or lessThan(vo?: ValueObject<T>). It wouldn't make sense in this example, but it might if we were talking about something like LoggingLevels or BusinessRatings.

In future articles, we'll talk about:

• entity design
• better error handling technique for object creation
• moving anemic code out of services and into domain models
• writing DTOs to create data contracts

This is part of the Domain-Driven Design with TypeScript series. If this was useful to you, let me know in the comments & subscribe to the newsletter to get notified when new articles come out. Cheers!

More in this series so far..

An Introduction to Domain-Driven Design - DDD w/ TypeScript

We cover this topic in The Software Essentialist online course. Check it out if you liked this post.