Type aliases

In simple terms, a type alias creates a new name for a type. To define a type alias, we use the type keyword, followed by the alias name, followed by the type that we want to alias.

We'll explore this with the following example:

1. Let's create a type alias for the getTotal method in the OrderDetail interface we have been working with. Let's try this in the TypeScript playground:

type GetTotal = (discount: number) => number;

interface OrderDetail {
  product: Product;
  quantity: number;
  getTotal: GetTotal;
}

Nothing changes with objects that implement this interface – it is purely a way we can structure our code. It arguably makes the code a little more readable.

1. Type aliases can also define the shape of an object. We could use a type alias for our Product and OrderDetail types that we previously defined with an interface:

type Product = {
  name: string;
  unitPrice: number;
};

type OrderDetail = {
  product: Product;
  quantity: number;
  getTotal: (discount: number) => number;
};

1. We use these types in exactly the same way as we used our interface-based types:

const table: Product = {
  name: "Table",
  unitPrice: 500
};

const orderDetail: OrderDetail = {
  product: table,
  quantity: 1,
  getTotal(discount: number): number {
    const priceWithoutDiscount = this.product.unitPrice * this.quantity;
    const discountAmount = priceWithoutDiscount * discount;
    return priceWithoutDiscount - discountAmount;
  } 
};

So, type aliases seem very similar to interfaces. What is the difference between a type alias and an interface? The main difference is that type aliases can't be extended or implemented from like you can with interfaces. So, for a simple structure that doesn't require inheritance, should we use an interface or should we use a type alias? There isn't strong reasoning to prefer either approach. However, we should be consistent with whichever approach we choose to improve the readability of our code.