TypeScript Enums

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Numeric enums

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enum Direction {
  UP,
  DOWN,
  LEFT,
  RIGHT,
}

function move(length: number, direction: Direction): void {
  console.log(`move: ${direction} ${length}meters.`)
}

move(10, Direction.UP)

String enums

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enum Direction {
  UP = "UP",
  DOWN = "DOWN",
  LEFT = "LEFT",
  RIGHT = "RIGHT",
}

Heterogeneous enums

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enum E {
  yes,
  no = "no",
}

Computed members

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enum E {
  None,
  Read = 1 << 1,
  Write = 1 << 2,
  ReadWrite = Read | Write,
  L = "1234".length,
}

Union enums and enum member types

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enum Kind {
  Square,
  Circle,
}

interface Circle {
  kind: Kind.Circle
  radius: number
}

interface Square {
  kind: Kind.Square
  length: number
}

let c: Circle = { kind: Kind.Circle, radius: 5 }

Enums at runtime

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enum E {
  X,
  Y,
  Z,
}

function f(obj: { X: number }) {
  return obj.X
}

f(E) // work

Enums at compile time

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enum LogLevel {
  ERROR,
  WARN,
  INFO,
  DEBUG,
}

type LogLevelString = keyof typeof LogLevel

function print(key: LogLevelString, message: string): void {
  const num = LogLevel[key]
  if (num <= LogLevel.WARN) {
    console.log(`${key}-${num}-${message}`)
  }
}

print("WARN", "warning")

Reverse mappings

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enum E {
  A,
}

let a = E.A
let getName = E[a] // 'A'