9、枚举

enum Direction1 {
  up = 1, // 1
  down, // 2
  left, // 3
  right // 4
}
console.log(Direction1.up)
console.log(Direction1[0])
enum Direction2 {
  up = 'UP',
  down = 'DOWN',
  left = 'LEFT',
  right = 'RIGHT'
}
const value = 'UP'
if (value === Direction2.up) {
  console.log('go up')
}
const enum Direction3 {   // 不会编译
  up = 'UP',
  down = 'DOWN',
  left = 'LEFT',
  right = 'RIGHT'
}
console.log('UP' === Direction3.up) // 常量枚举 直接翻译成结果 UP

常量枚举：不会把枚举编译成javascript代码
计算枚举：