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Value Objects - DDD w/ TypeScript

Apr 7th, 2019 / 7 min read / Share / Edit on GitHub
Value Objects are one of the primary components of Domain-Driven Design. Here's a simple Value Object class in TypeScript.

This is part of the Domain-Driven Design w/ TypeScript & Node.js course. Check it out if you liked this post.

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:

domain/user.ts
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.

services/createUserService.ts
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?

services/editUserService.ts
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.

domain/user.ts
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.

domain/name.ts
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.

domain/user.ts
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.

shared/domain/valueObject.ts
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

This is part of the Domain-Driven Design w/ TypeScript & Node.js course. Check it out if you liked this post.



Discussion

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4 Comments

Submit
Elliott
3 months ago

Yo your articles are actually the bomb, helping me out so much with my research project right now!

Khalil Stemmler
3 months ago

Woo! Happy to hear that, man :)

Matteo
3 months ago

Really nice article. I have a question about the instantiation of the Name class, would you just inject it into the User class? Ex: User.create(Name.create(name)) or is there another DDD pattern to delegate its instantiation? Thanks

Khalil Stemmler
3 months ago

Thanks Matteo!


Good question. For the most part, yes! But you want to make sure that your Value Objects are valid. That can be remedied by using the Result<T> class + a private constructor + a factory method so that you never pass in an invalid Value Object to an Entity.


With respect to delegation, to reconstruct Domain entities and transform them between layers, I use the Data Mapper pattern. I use it to:


  • Create instances of a domain model from persistence (toDomain)
  • Map a Domain model to the persistence representation (toPersistence).
  • Map a Domain model to a DTO (toDTO).


That's a good separation of concerns, specifically delegating a construct to be responsible for converting domain objects to the appropriate representation.


You'll often find that you'll need to create Domain Objects from HTTP controller requests as well in order to pass input values to application layer Use Cases.


For a Use Case like Create User (UserEmail, Name, Password), try the `Result<T>.combine([])` method to check your objects are correct before passing them into your Use Case.

Maxim
2 months ago

Thanks for great explanation. What about case when we need to determine equality of passed and persisted passwords? Should we encapsulate hashing inside Password VO and check equality between recounsiled from persistence and passed passwords?

Khalil Stemmler
2 months ago

Absolutely. Check out an example here: https://github.com/stemmlerjs/ddd-forum/blob/master/src/modules/users/domain/userPassword.ts


And when you're reconstituting the `UserPassword` from persistence and turning it into a domain object again, you can signal to the value object that it's hashed.


```typescript

const userPasswordOrError = UserPassword.create({ value: raw.user_password, hashed: true });

```

Another example here: https://github.com/stemmlerjs/ddd-forum/blob/master/src/modules/users/mappers/userMap.ts



Rahul
8 days ago

Great stuff in here! I'm going through all your content to get up to speed on DDD and implement parts of it into my project. I'm currently using TypeORM which allows me to define entities like this:


```ts

@Entity()

export class User {

@PrimaryGeneratedColumn()

id!: number;


@Column("text")

@IsName()

name!: string;

}

```


In the above example, you can define a validator function called `IsName` directly on the entity. Would you recommend still extracting this validation into its own class?

Khalil Stemmler
5 days ago

Hey Rahul!


Ah, that's pretty cool :)


Well, if it was possible using TypeORM, I'd recommend you create a new type/class called `Name` and locate all the validation logic for `Name` there instead using a factory method, because:


  • Validation rules for `name` belong to name and not `User` (Single responsibility principle).
  • You might end up needing to create a `Name` elsewhere in your code, let's say in order to create a default `ProfileHandle`, like ProfileHandle.create(name: Name). If thats the case, we'd have to duplicate the validation rules for `name` in both `ProfileHandle` AND `User`. So, try to encapsulate the validation rules for `name` as close to the `Name` (class) as possible.


I hope that was helpful!


Stay in touch!



About the author

Khalil Stemmler,
Developer Advocate @ Apollo GraphQL ⚡

Khalil is a software developer, writer, and musician. He frequently publishes articles about Domain-Driven Design, software design and Advanced TypeScript & Node.js best practices for large-scale applications.



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