Enum vs Union Types
In this page:
enum EnumName { A = "a", B = "b" }
type UnionName = "a" | "b";Enum and Union Basics
Both enums and union types can restrict a value to a known set of options, but they work fundamentally differently once you get past the type-checking stage and into actual runtime behavior.
Example: Enum and Union Basics
enum StatusEnum { Pending, Done }
type StatusUnion = "pending" | "done";
const a: StatusEnum = StatusEnum.Pending;
const b: StatusUnion = "pending";
console.log(a, b);Runtime Differences
A normal enum emits a real JavaScript object at runtime that you could inspect or log, while a union type is a purely compile-time construct that disappears entirely once TypeScript compiles down to JavaScript.
Example: Runtime Differences
enum StatusEnum { Pending, Done }
console.log(StatusEnum); // a real object exists at runtime
type StatusUnion = "pending" | "done";
const b: StatusUnion = "pending"; // no runtime trace of the type itself
console.log(b);Function Parameters
Both approaches restrict function arguments to a known set of values equally well at the type-checking level — the difference only shows up in what, if anything, exists once the code actually runs.
Example: Function Parameters
enum StatusEnum { Pending, Done }
type StatusUnion = "pending" | "done";
function handleEnum(s: StatusEnum) { return s; }
function handleUnion(s: StatusUnion) { return s; }
console.log(handleEnum(StatusEnum.Done), handleUnion("done"));When Unions Are Simpler
A literal union (like "pending" | "done" | "failed") is often the simpler choice for a small, fixed set of string values when you don't need a runtime object or any enum-specific behavior like reverse mapping.
Example: When Unions Are Simpler
type Status = "pending" | "done" | "failed";
function label(status: Status) {
return `Status: ${status}`;
}
console.log(label("pending"));When Enums Are Useful
Enums earn their keep when you need named members backed by a shared runtime object, or when numeric values genuinely matter to how the application behaves — situations a union type alone can't provide.
Example: When Enums Are Useful
enum HttpStatus {
Ok = 200,
NotFound = 404,
}
function isSuccess(status: HttpStatus) {
return status === HttpStatus.Ok;
}
console.log(isSuccess(HttpStatus.Ok));- Expecting a union type to exist at runtime, when it is erased and cannot be iterated.
- Assigning a plain string like
"pending"to a string enum type, which is not allowed. - Choosing an enum when a simple union would do, adding extra JavaScript output for no benefit.
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