How to initialize a property of a mapped typed in Typescript

Question

Let's say I want to model a Fruit Shop in Typescript: I have a union type of string literals:

type Fruit = "apple" | "banana" | "cherry"

I have a fruit shop with a price list and stock register:

interface FruitShop1 {
  priceList: {
    apple: number,
    banana: number,
    cherry: number
  }
  stock: {
    apple: number,
    banana: number,
    cherry: number
  }
}

Since I will want to add lots of fruit later and I don't want to duplicate the properties manually on the priceList and stock properties, so I modified the FruitShop1 interface to use a mapped types based off Fruit:

interface FruitShop2 {
  priceList: { [fruit in Fruit]: number }
  stock: { [fruit in Fruit]: number }
}

I create my class implemeting FruitShop:

class FruitShopImpl implements FruitShop2 {
  priceList: { apple: number; banana: number; cherry: number; };
  stock: { apple: number; banana: number; cherry: number; };
}

However, I need to initialize pricelist and stock. How do I do this?

What I'd like is for every property in priceList be initialized to 1 and every property in stock to be initialized to 0.

I've tried interating over every property in a constructor, but this produces errors I think because priceList isn't initialized so it has no properties to iterate over. This feels like a chicken-and-egg catch-22 problem. Is there a solution, without manually initializing each property individually?

Answer 1

I've gone for a similar solution as suggested by @soffyo, but instead of using an array I'm using an enum backed by enums. Here's my code:

enum Fruits {
  Apple = "apple",
  Banana = "banana",
  Cherry = "cherry"
}

interface IFruitShop {
  prices: { [fruit in Fruits]: number }
  stock: { [fruit in Fruits]: number }
}

class FruitShop implements IFruitShop {
  prices: IFruitShop['prices'];
  stock: IFruitShop['stock'];

  constructor() {
    this.prices = {} as { [fruit in Fruits]: number };
    this.stock = {} as { [fruit in Fruits]: number };
    Object.keys(Fruits).forEach(f => {
      this.prices[f as Fruits] = 1;
      this.stock[f as Fruits] = 0;
    });
  }
}

Typscript Playground

I think string enum could be an advantage for me in my actual project.