Inspect emitted JavaScript before replacing enums with literal unions

If a state only needs to be constrained to "ready" or "waiting" in the type system, a literal union can express that. If code must also refer to a named value such as State.Ready at runtime, it needs a value object as well.

Separate those requirements when comparing an enum, a literal union, and an as const object. A regular enum emits JavaScript; it is more than another spelling for a type.

Emit the same states in four forms

The experiment ran on September 11, 2026, with macOS arm64, Node.js 24.15.0, and TypeScript 7.0.2. The build used strict: true, target: ES2022, module: ESNext, and moduleResolution: Bundler.

export enum NumericState { Ready, Waiting }
export enum StringState { Ready = "ready", Waiting = "waiting" }
export type State = "ready" | "waiting";
export const StateValue = { Ready: "ready", Waiting: "waiting" } as const;
export const selected: State = "ready";
console.log(JSON.stringify({ numeric: NumericState, string: StringState, object: StateValue, selected }));

The log contained the following values. The emitted JavaScript was inspected as well.

Definition	What remained at runtime
Numeric enum	Ready: 0 and Waiting: 1, plus reverse mappings 0: "Ready" and 1: "Waiting"
String enum	Ready: "ready" and Waiting: "waiting"
Literal union State	The type disappeared; the value "ready" assigned to selected remained
as const object	Ready: "ready" and Waiting: "waiting"

The numeric enum's reverse mapping, and its absence from the string enum, match the official enums reference. Reverse lookup is a runtime capability of the numeric enum. If the application does not use it, it need not be a selection requirement.

Derive the value union from an object

export const StateValue = { Ready: "ready", Waiting: "waiting" } as const;
export type State = typeof StateValue[keyof typeof StateValue];
export const value: State = "ready";
console.log(value, Object.isFrozen(StateValue));

StateValue holds the named values, and the object supplies the allowed value type. Assigning "ready" to State passed, and the log printed ready false. Object.isFrozen was false because as const does not freeze objects at runtime.

The string enum did not accept a direct assignment merely because the string matched a member's value:

enum State { Ready = "ready", Waiting = "waiting" }
const value: State = "ready";
export {};

This produced TS2322. A string received externally cannot simply be treated as an enum value. A literal union does not establish that an arbitrary, unchecked string is safe either; external input still needs runtime validation.

Whether const enum disappears also depends on configuration

The following input was compiled with only isolatedModules changed:

const enum State { Ready = "ready", Waiting = "waiting" }
export const value = State.Ready;
console.log(value);

With isolatedModules: false, the value was inlined in the emitted JavaScript:

export const value = "ready" /* State.Ready */;
console.log(value);

With isolatedModules: true, code constructing the State object and accessing State.Ready remained. Both versions printed ready. Do not expect every build pipeline to erase the object just because the declaration says const enum.

This comparison defines and uses the enum in the same source file. It does not test ambient const enums in a published library's .d.ts files or mismatched versions between a definition and its consumers.

Type stripping makes the syntax constraint visible

With erasableSyntaxOnly: true, both the regular enum and const enum produced TS1294. The literal union and as const object example passed. This option avoids syntax that needs transformation into JavaScript. The official configuration reference lists the affected constructs.

Directly running a .ts file containing the regular enum on Node.js 24.15.0, without transformation options, also produced ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX. Node.js TypeScript support distinguishes execution through type stripping from features that require syntax transformation.

A literal union is a candidate when only a type is needed. Deriving a union from an object also provides named runtime values. If existing code depends on enum reverse lookup or a public enum API, inspect those consumers before changing it. This output comparison does not establish a bundle-size or execution-speed advantage.

The T05 cases in the comparison inputs and the compiler and runtime results preserve the details. Compare alternatives under the runtime requirements and transformation settings the application will actually use.