Typescript type/object's conditional property feature

Imagine having a recipe ingredient type structured like this

export type RecipeIngredient = {
  name: string;
  amount: Number | string;
  unit: "grams" | "milliliters" | "custom";
};

To illustrate

const apples: RecipeIngredient = { name: 'apples', amount: 200, unit: 'grams' }

This setup works well, but the requirement is to allow the amount to be a string only when the unit is considered custom. Is this achievable?

The ideal scenario

const vanillaExtract: RecipeIngredient = { name: 'vanilla extract', amount: 'a dash', unit: 'custom' }

What should be considered incorrect

const milk: RecipeIngredient = { name: 'milk', amount: '500ml', unit: 'milliliters' }

Answer №1

declare type Ingredient = {
  name: string;
  amount: Number;
  unit: "grams" | "mililitres";
} | {
  name: string;
  amount: string;
  unit: "custom";
};

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