Const Enums

A const enum is designed to be inlined by the TypeScript compiler instead of generating a normal runtime enum object. This can reduce generated JavaScript, but it has build-tool and library-distribution considerations.

In this page:

Basic Const Enum
Const Enum with Functions
Const Enum and Compilation
Const Enum Limitations
When to Use Const Enums

Syntax

typescript

const enum EnumName {
  Member1,
  Member2,
}
let value = EnumName.Member1;

Basic Const Enum

A const enum is declared with the const modifier and is normally inlined directly into its member value at compile time rather than generating a runtime enum object at all.

Example: Basic Const Enum

typescript

const enum Direction {
  Up,
  Down,
}
const d = Direction.Up;
console.log(d);

Const Enum with Functions

Const enum members work as typed function arguments the same way as normal enum members, but using them avoids creating an actual runtime enum object in the compiled output.

Example: Const Enum with Functions

typescript

const enum Color { Red, Green, Blue }
function paint(color: Color) {
  return `Painting with color ${color}`;
}
console.log(paint(Color.Green));

Const Enum and Compilation

Const enums are a compile-time-only feature — their members are substituted directly into the generated JavaScript, so there's no object you could inspect or iterate over at runtime the way you can with a regular enum.

Example: Const Enum and Compilation

typescript

const enum Size { Small, Medium, Large }
// At compile time, Size.Medium is inlined directly as the number 1.
console.log(Size.Medium);

Const Enum Limitations

Because const enums are meant to be inlined, code shouldn't rely on a runtime enum object existing — this becomes especially important when publishing a library, since some build tools handle const enums differently or disallow them entirely (isolatedModules).

Example: Const Enum Limitations

typescript

const enum Mode { Dev, Prod }
// No runtime object exists to iterate over -- Mode.Dev is just inlined as 0.
console.log(Mode.Dev);

When to Use Const Enums

Const enums suit internal applications where the team controls the whole toolchain and predictable inlining behavior is known, reducing the number of emitted enum objects in the final bundle.

Example: When to Use Const Enums

typescript

const enum Env {
  Development,
  Production,
}
console.log(Env.Production);

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Const Enums

Basic Const Enum

Const Enum with Functions

Const Enum and Compilation

Const Enum Limitations

When to Use Const Enums

Chapter Quiz — Complete all 6 topics to unlock

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