Describing Shapes›Core· 30 min read

Enums: Named Sets of Constants

An enum gives friendly names to a fixed group of related values, so your code reads clearly.

What you will learn

Define and use a numeric enum
Use a string enum for readable values
Choose between an enum and a literal union

Names instead of mystery values

Code is full of fixed sets: directions, roles, statuses. Instead of remembering "0 means North", an enum lets you use a clear name like Direction.North.

A numeric enum — values start at 0

enum Direction {
  North,    // 0
  East,     // 1
  South,    // 2
  West      // 3
}

let heading: Direction = Direction.East;
console.log(heading);
console.log(Direction[1]);   // look up the name

Note: Output: 1 East By default enum members get numbers starting at 0, so Direction.East is 1. You can also look up the name from the number with Direction[1].

String enums — readable everywhere

Numbers can be cryptic in logs and databases. A string enum stores a readable word for each member:

A string enum keeps values human-readable

enum Status {
  Active = 'ACTIVE',
  Paused = 'PAUSED',
  Closed = 'CLOSED'
}

let s: Status = Status.Active;
console.log('Account is ' + s);

Note: Output: Account is ACTIVE Now the value is the clear string 'ACTIVE' instead of a number. This is easier to read in logs and safer to store.

Using an enum in real code

Enums are most useful as the type of a parameter. The function below only accepts a valid Status, and the editor will suggest the members for you while you type:

An enum used as a typed function parameter

enum Status {
  Active = 'ACTIVE',
  Paused = 'PAUSED',
  Closed = 'CLOSED'
}

function message(status: Status): string {
  switch (status) {
    case Status.Active: return 'Your account is live.';
    case Status.Paused: return 'Your account is on hold.';
    case Status.Closed: return 'Your account is closed.';
  }
}

console.log(message(Status.Paused));

Note: Output: Your account is on hold. The parameter type Status means you cannot pass a random string by mistake — only Status.Active, Status.Paused or Status.Closed. Passing 'ACTIVE' as a plain string would be rejected.

Enum or literal union?

A literal union (from earlier) can do a similar job. Here is how to choose:

Want	Pick
A simple set of allowed strings	Literal union — lighter, no extra JS
A named group you reference as `X.Member`	Enum
Reverse lookup (name from number)	Numeric enum
Smallest compiled output	Literal union

Tip: Beginners often start with literal unions ('active' | 'paused') because they are simpler and add no code at runtime. Reach for enums when you want a named, grouped set you refer to as Status.Active.

Q. In enum Direction { North, East }, what is the value of Direction.East?

Answer: Numeric enums start counting at 0, so North is 0 and East is 1. Use a string enum if you want readable values instead.

✍️ Practice

Create a numeric enum Day { Mon, Tue, Wed } and print the value of Day.Wed.
Create a string enum Role { Admin = 'ADMIN', User = 'USER' } and log a role.

🏠 Homework

Make a string enum for traffic lights (Red, Yellow, Green) and a function that returns advice for each colour.