Enums

Numeric Enums

Enums define a set of named constants. Numeric enums default to sequential numbers starting from 0.

Basic Numeric Enum

enum Direction {
  Up,    // 0
  Down,  // 1
  Left,  // 2
  Right  // 3
}

let dir: Direction = Direction.Up;
console.log(dir); // 0

Custom Values

enum HttpStatus {
  OK = 200,
  NotFound = 404,
  ServerError = 500
}

function handleResponse(status: HttpStatus) {
  if (status === HttpStatus.OK) {
    console.log("Success");
  }
}

Computed Values

enum Color {
  Red,
  Green = Red + 1,
  Blue = Green * 2
}

// Red = 0, Green = 1, Blue = 2

Reverse Mapping

enum Direction {
  Up,
  Down,
  Left,
  Right
}

// Forward: number to string
let dirName = Direction[0]; // "Up"

// Reverse: string to number
let dirValue = Direction["Up"]; // 0

String Enums

String enums provide better readability and debugging experience.

Basic String Enum

enum Direction {
  Up = "UP",
  Down = "DOWN",
  Left = "LEFT",
  Right = "RIGHT"
}

let dir: Direction = Direction.Up;
console.log(dir); // "UP"

Benefits

• Better debugging output
• Self-documenting code
• No reverse mapping needed

Real-World Example

enum Status {
  Active = "ACTIVE",
  Inactive = "INACTIVE",
  Pending = "PENDING"
}

enum Priority {
  Low = "LOW",
  Medium = "MEDIUM",
  High = "HIGH"
}

type Task = {
  id: number;
  title: string;
  status: Status;
  priority: Priority;
};

const task: Task = {
  id: 1,
  title: "Learn TypeScript",
  status: Status.Active,
  priority: Priority.High
};

Const Enums

Const enums are fully inlined during compilation, resulting in better performance.

Basic Const Enum

const enum Direction {
  Up,
  Down,
  Left,
  Right
}

let dir = Direction.Up;
// Compiles to: let dir = 0;

String Const Enum

const enum Status {
  Active = "ACTIVE",
  Inactive = "INACTIVE"
}

let status = Status.Active;
// Compiles to: let status = "ACTIVE";

Benefits

• No generated JavaScript code
• Fully inlined at compile time
• Better performance
• Smaller bundle size

Limitations

• Cannot be used with dynamic values
• No reverse mapping
• Cannot be used in compute expressions

const enum Direction {
  Up,
  Down,
  Left,
  Right
}

// Cannot do this:
let dirKey = Direction[0]; // Error!

// Must use literal:
let dirKey = "Up";