How to Merge TypeScript Enums With the Same Name?

When multiple enums of the same name exist in the same module, TypeScript merges them into a single enum automatically:

enum Direction {
  Up = "UP",
  Down = "DOWN",
}

enum Direction {
  Left = "LEFT",
  Right = "RIGHT",
}

const dir1: Direction = Direction.Up; // "UP"
const dir2: Direction = Direction.Left; // "LEFT"
// ...

This will compile into the following JavaScript code at runtime:

var Direction;
(function (Direction) {
  Direction["Up"] = "UP";
  Direction["Down"] = "DOWN";
})(Direction || (Direction = {}));
(function (Direction) {
  Direction["Left"] = "LEFT";
  Direction["Right"] = "RIGHT";
})(Direction || (Direction = {}));

If you declare multiple enums of the same name, then only one declaration can omit an initializer for its first enum element, otherwise a compile-time error will be thrown:

enum Color {
  Red,
  Green,
  Blue,
}

enum Color {
  // Error: In an enum with multiple declarations, only one declaration can omit an initializer for its first enum element.
  Yellow,
  Cyan,
}

enum Color {
  // Error: In an enum with multiple declarations, only one declaration can omit an initializer for its first enum element.
  Pink,
  Orange,
}

To avoid this error, it is necessary to ensure that only one of the merging enums is without an initializer:

enum Color {
  Red,
  Green,
  Blue,
}

enum Color {
  Yellow = 0,
  Cyan,
}

enum Color {
  Pink = 5,
  Orange,
}

const color1: Color = Color.Red; // 0
const color2: Color = Color.Green; // 1
// ...
const color3: Color = Color.Yellow; // 0
const color4: Color = Color.Cyan; // 1
// ...
const color5: Color = Color.Pink; // 5
const color6: Color = Color.Orange; // 6

As you can see in the example above, it is possible for merging enum members that have the same initializers.

This will compile into the following JavaScript code at runtime:

var Color;
(function (Color) {
  Color[Color["Red"] = 0] = "Red";
  Color[Color["Green"] = 1] = "Green";
  Color[Color["Blue"] = 2] = "Blue";
})(Color || (Color = {}));
(function (Color) {
  Color[Color["Yellow"] = 0] = "Yellow";
  Color[Color["Cyan"] = 1] = "Cyan";
})(Color || (Color = {}));
(function (Color) {
  Color[Color["Pink"] = 5] = "Pink";
  Color[Color["Orange"] = 6] = "Orange";
})(Color || (Color = {}));