Beginner's TypeScript #6

⭐ Constraining Value Types ⭐

We have a User interface below:

interface User {
  id: number;
  firstName: string;
  lastName: string;
  role: string;
}

We would not want to use a freeform string, because a User could have only a set number of roles: admin, user, or super-admin.

Consider this defaultUser:

export const defaultUser: User = {
  id: 1,
  firstName: "Uri",
  lastName: "Pilot",
  role: "I_SHOULD_NOT_BE_ALLOWED",
}

It is being defined with a role that is not one of our options.

We will update the User interface to restrict the role property to one of the set options.

The I_SHOULD_NOT_BE_ALLOWED role should cause an error.

👉 Solution:

The solution is to update role to be a Union type.

The syntax uses | to delineate the options for which values a key can be:

interface User {
  id: number;
  firstName: string;
  lastName: string;
  role: "admin" | "user" | "super-admin";
}

✍️ About Union Types:

Anything can be added to a union type. For example, we can make a new SuperAdmin type and add it to the role:

type SuperAdmin = "super-admin"

interface User {
  // ...other stuff
  role: "admin" | "user" | SuperAdmin;
}

Union types are everywhere within TypeScript. They give our code safety and allow us to be really sure of the types of data flowing through our app.

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