Enums

TypeScript · Reference cheat sheet

📋 Overview

Enums name a fixed set of related constants. Prefer string enums or union literals for most APIs; numeric enums are useful for bit flags and interop with C-style APIs. Const enums inline members at compile time.

🔧 Core concepts

• Union alternative — type Status = "ok" | "err" is often enough (TS 5.x).
• as const object — const Status = \{ Ok: "ok" \} as const + typeof / keyof.

💡 Examples

enum Direction {
  Up,      // 0
  Down,    // 1
  Left = 10,
  Right,   // 11
}

Direction.Up;       // 0
Direction[0];       // "Up" (numeric reverse map)

enum Color {
  Red = "RED",
  Blue = "BLUE",
}

function paint(c: Color) {
  return c === Color.Red;
}

const enum Flag {
  Read = 1 << 0,
  Write = 1 << 1,
}
const mode = Flag.Read | Flag.Write; // inlined numbers

// Prefer unions for public APIs
type Role = "admin" | "user" | "guest";

const Http = {
  Ok: 200,
  NotFound: 404,
} as const;
type HttpCode = (typeof Http)[keyof typeof Http]; // 200 | 404

// IsolatedModules / erasableSyntaxOnly (TS 5.x): prefer
// `const enum` carefully or use unions — classic enums emit JS.

⚠️ Pitfalls

• Numeric enums are not a closed set at runtime — any number can be assigned unless you narrow.
• Reverse mapping only exists for numeric enums; Color["RED"] is undefined for string enums.
• const enum + isolatedModules / Babel can break if the enum isn’t preserved.
• Prefer string unions over enums when you don’t need a runtime object.
• Declaring members without initializers after a string member is an error.

🔗 Related

• literal_types.md
• const_assertions.md
• union_types.md
• keyof_typeof.md
• namespaces.md