const ingredientId = {
  salad: 'Salad',
  carotte: 'Carotte',
};

class Recipe {
  constructor(ingredientList = {}) {
    this.ingredients = ingredientList;
  }

  getAmount(ingredientId) {
    const amount = this.ingredients[ingredientId];
    if (amount) return amount;
    return 0;
  }
}

const i = new Recipe({ salad: 2, carotte: 4 });
const amount = i.getAmount(ingredientId.salad); // Expected to be 2
console.log(`Amount = ${amount}`);