[Typescript] Restrict available operations on values using value objects

Value Objects are another pattern in Domain-driven Design that provide more structure around what you can and cannot do with a type.

In TypeScript we create Value Objects with classes that will define what operations can be performed to the value on the class itself. In this lesson we'll explore how to create a Value Object for our Money type.

 

declare const __brand_type__: unique symbol
type Branded<BrandType, BrandName> = BrandType & {
  readonly [__brand_type__]: BrandName
}
type Currency = "USD" | "EUR"
type Money = Branded<number, "MONEY">

class Money {
  constructor(
    private amount: Money,
    private currency: Currency
  ) {}
    
  add(another: Money) {
    if (this.currency != another.currency) {
      throw new Error(`Cannot add different currencies`);
    }
    return new Money(
      this.amount + another.amount,
      this.currency
    );
  }

  multiply(factor: number) {
    return new Money(this.amount * factor, this.currency);
  }
}

 

Due to definitly assign problem for constructor, you cannot restrict object creation by using constructor. One way to do it is using static from method.

1. change constructorto private
2. add static frommethod

declare const __brand_type__: unique symbol
type Currency = "USD" | "EUR"

class Money {
  private constructor(
    private amount: number,
    private currency: Currency
  ) {}
    
  static from(amount: number, currency: Currency) {
    if (amount <= 0) {
      throw new Error(`Cannot add different currencies`)
    }
    return new Money(
      this.amount + another.amount,
      this.currency
    )
  }
    
  add(another: Money) {
    if (this.currency != another.currency) {
      throw new Error(`Cannot add different currencies`);
    }
    return new Money(
      this.amount + another.amount,
      this.currency
    );
  }

  multiply(factor: number) {
    return new Money(this.amount * factor, this.currency);
  }
}