The data type is expanding to encompass the entire enumeration

Question

The data type is expanding to encompass the entire enumeration

Within the provided code snippet, if the 'as' keyword is omitted in each action, the inferred type for method widens to any of the Kind types. Is there a way to prevent having to repeat 'Kind.PAYPAL as Kind.PAYPAL'?

enum Kind {
    CASH = 'CASH',
    PAYPAL = 'PAYPAL',
    CREDIT = 'CREDIT'
}

const Cash = () => ({
    kind: Kind.CASH as Kind.CASH,
});

const PayPal = (email: string) => ({
    kind: Kind.PAYPAL as Kind.PAYPAL,
    email
});

const CreditCard = (payload: { cardNumber: string, cvv: string }) => ({
    kind: Kind.CREDIT as Kind.CREDIT,
    payload
});

type PaymentMethod = ReturnType<
    typeof Cash
    | typeof PayPal
    | typeof CreditCard
>;

function describePaymentMethod(method: PaymentMethod): string {
  switch (method.kind) {
    case Kind.CASH:
      // Here, method represents type Cash
      return "Cash";

    case Kind.PAYPAL:
      // Here, method signifies type PayPal
      return `PayPal (${method.email})`;

    case Kind.CREDIT:
      // Here, method denotes type CreditCard
      return `Credit card (${method.payload.cardNumber})`;
  }
}

typescript

Answer 1

Answer №1

If you are utilizing TypeScript 3.4 or later, you have the option to utilize const assertions in the following manner:

const Cash = () => ({
  kind: Kind.CASH as const
});

const PayPal = (email: string) => ({
  kind: Kind.PAYPAL as const,
  email
});

const CreditCard = (payload: { cardNumber: string; cvv: string }) => ({
  kind: Kind.CREDIT as const,
  payload
});

This approach allows for the kind property type to be interpreted as narrowly as possible.

For scenarios prior to TS3.4, or if you prefer not to use a const assertion, you can rely on TypeScript's heuristics for determining when to widen literals, like the helper function shown below:

const lit = <T extends string | number | boolean | void | null | {}>(x: T) => x;

By using this function, you can ensure that x is returned as a literal. Here is how you would implement it:

const Cash = () => ({
  kind: lit(Kind.CASH)
});

const PayPal = (email: string) => ({
  kind: lit(Kind.PAYPAL),
  email
});

const CreditCard = (payload: { cardNumber: string; cvv: string }) => ({
  kind: lit(Kind.CREDIT),
  payload
});

Both methods are viable options depending on your preference. Best of luck with your implementation!

Answer 2

If you are utilizing TypeScript 3.4 or later, you have the option to utilize const assertions in the following manner:

const Cash = () => ({
  kind: Kind.CASH as const
});

const PayPal = (email: string) => ({
  kind: Kind.PAYPAL as const,
  email
});

const CreditCard = (payload: { cardNumber: string; cvv: string }) => ({
  kind: Kind.CREDIT as const,
  payload
});

This approach allows for the kind property type to be interpreted as narrowly as possible.

For scenarios prior to TS3.4, or if you prefer not to use a const assertion, you can rely on TypeScript's heuristics for determining when to widen literals, like the helper function shown below:

const lit = <T extends string | number | boolean | void | null | {}>(x: T) => x;

By using this function, you can ensure that x is returned as a literal. Here is how you would implement it:

const Cash = () => ({
  kind: lit(Kind.CASH)
});

const PayPal = (email: string) => ({
  kind: lit(Kind.PAYPAL),
  email
});

const CreditCard = (payload: { cardNumber: string; cvv: string }) => ({
  kind: lit(Kind.CREDIT),
  payload
});

Both methods are viable options depending on your preference. Best of luck with your implementation!

The data type is expanding to encompass the entire enumeration

Answer №1

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