Union of Literals as an Alternative

The Modern Alternative

A union of string literals gives the same safety with no runtime code. Combine it with an as const array when you also need the list of values at runtime.

Example

(typeof STATUSES)[number] means "the type of any element of this array", so the type is generated from the array and the two can never get out of sync.

const STATUSES = ["PLACED", "SHIPPED", "DELIVERED"] as const;
type OrderStatus = (typeof STATUSES)[number]; // "PLACED" | "SHIPPED" | "DELIVERED"

function isOrderStatus(value: string): value is OrderStatus {
  return (STATUSES as readonly string[]).includes(value);
}

function canCancel(status: OrderStatus): boolean {
  return status === "PLACED";
}

console.log(canCancel("PLACED")); // plain strings work
console.log(STATUSES.join(" -> "));
const fromApi = "SHIPPED";
if (isOrderStatus(fromApi)) console.log("valid status:", fromApi);

Output

true
PLACED -> SHIPPED -> DELIVERED
valid status: SHIPPED

Which One to Choose

	enum	const enum	literal union
Runtime object	yes	no	no (add an `as const` array if needed)
Accepts plain strings	no	no	yes
Works with isolatedModules	yes	risky	yes
Reverse mapping	numeric only	no	no
Common in	Angular, NestJS, older code	libraries	React, Next.js, modern code

Tip: Both styles are fine in interviews. Know that enums produce runtime code and literal unions do not.