enum枚举

1. 枚举自动从 0 开始递增
2. 常量枚举，只有常量值能够使用常量枚举
3. 计算值枚举 ```typescript enum Season{ Spring, Summer, Autumn, Winter };

let season:Season=Season.Spring;

enum Season1{ Spring=1, Summer, Autumn, Winter }

enum Direction { Up, Down, Left, Right }

Direction.Up // 0 Direction[0] // Up

// 编译成 js的结果 var Direction; (function(Direction) { Direction[Direction[‘Up’] = 0] = ‘Up’; Direction[Direction[‘Down’] = 1] = ‘Down’ Direction[Direction[‘Left’] = 2] = ‘Left’ Direction[Direction[‘Right’] = 3] = ‘Right’ })(Direction || (Direction = {}))

Direction.Up // 0 Direction[0] // ‘Up’

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## 常量枚举
常量枚举提升性能，直接转义成结果<br />computed计算值不能用常量枚举
```typescript
const enum Direction {
  Up = 'UP', 
  Down = 'DOWN', 
  Left = 'LEFT', 
  Right = 'RIGHT',
}
const value = 'UP'
if (value === Direction.Up) {
  console.log('is Up')
}
// 编译的结果
var value = 'UP'
if (value === 'UP') {
    console.log('is Up')
}