How can I type a typescript interface property using a predefined object?

Question

• I have a typescript interface called Customer.
• I want one property of that interface, orderAuthorizationStatus, to be only one of three strings: 'unauthorized', 'waitingForAuthorization' or 'authorized'
• At the same time I want to store these three strings somewhere, so I can use them for comparisons, translations etc., and not change them 10 different times should I decide to change the wording.
• I want to use the stored strings, for typing my interface property, I currently have this solution:

export const OrderAuthorizationStates = {
  unauthorized: "unauthorized",
  waitingForAuthorization: "waitingForAuthorization",
  authorized: "authorized"
}

export interface Customer {
  id: number;
  name: string;
  orderAuthorizationStatus: "unauthorized" | "waitingForAuthorization" | "authorized"
}

This works and gives me the errors i want:

let customer: Customer;

if (customer.orderAuthorizationStatus === 'asdf') {
 somecode
}

Error: "This condition will always return 'false' since the types '"unauthorized" | "waitingForAuthorization" | "authorized"' and '"asdf"' have no overlap"
I can also use it like this:

let customer: Customer;

if (customer.orderAuthorizationStatus === OrderAuthorizationStates.authorized) {
 somecode
}

What I cannot solve is, how to use the objectproperties of my OrderAuthorizationStates object to type the interfaceproperty of my customer interface:

export interface Customer {
  id: number;
  name: string;
  orderAuthorizationStatus: OrderAuthorizationStates.unauthorized | OrderAuthorizationStates.waitingforAuthorization | OrderAuthorizationStates.authorized
}

Error: Cannot find namespace 'OrderAuthorizationStates'

Help would be much appreciated!

Answer 1

String enums do almost exactly what you want:

enum OrderAuthorizationStates {
    Unauthorized = "Unauthorized",
    WaitingForAuthorization = "WaitingForAuthorization",
    Authorized = "Authorized"
}

interface Customer {
    id: number;
    name: string;
    orderAuthorizationStatus: OrderAuthorizationStates;
}

let customer: Customer = { id: 1, name: "RichN", orderAuthorizationStatus: OrderAuthorizationStates.Unauthorized};
if (customer.orderAuthorizationStatus === "Unauthorized") {
    console.log("Customer is unauthorized");
}

// Authorize it!
customer.orderAuthorizationStatus = OrderAuthorizationStates.Authorized;

if (customer.orderAuthorizationStatus === OrderAuthorizationStates.Authorized) {
    console.log("Customer is now authorized");
}

// Output:
// Customer is unauthorized
// Customer is now authorized

In addition, if you do if (customer.orderAuthorizationStatus === 'asdf') you get the error 'This condition will always return 'false' since the types 'OrderAuthorizationStates' and '"asdf"' have no overlap.', which is what you want.