const enum

In most cases, enums are a perfectly valid solution. However sometimes requirements are tighter. To avoid paying the cost of extra generated code and additional indirection when accessing enum values, it’s possible to use const enums. Const enums are defined using the const modifier on our enums:

const enum Enum {
  A = 1,
  B = A * 2,
}

Const enums can only use constant enum expressions and unlike regular enums they are completely removed during compilation. Const enum members are inlined at use sites. This is possible since const enums cannot have computed members.

const enum Direction {
  Up,
  Down,
  Left,
  Right,
}
 
let directions = [
  Direction.Up,
  Direction.Down,
  Direction.Left,
  Direction.Right,
];

in generated code will become

"use strict";
let directions = [
    0 /* Direction.Up */,
    1 /* Direction.Down */,
    2 /* Direction.Left */,
    3 /* Direction.Right */,
];

Object vs Enums

In modern TypeScript, you may not need an enum when an object with as const could suffice:

const enum EDirection {
  Up,
  Down,
  Left,
  Right,
}
 
const ODirection = {
  Up: 0,
  Down: 1,
  Left: 2,
  Right: 3,
} as const;
 
EDirection.Up; //(enum member) EDirection.Up = 0
ODirection.Up; //(property) Up: 0
 
// Using the enum as a parameter
function walk(dir: EDirection) {}
 
// It requires an extra line to pull out the values
type Direction = typeof ODirection[keyof typeof ODirection];
function run(dir: Direction) {}
 
walk(EDirection.Left);
run(ODirection.Right);