Typescript 的枚举类型是对 JavaScript 标准数据类型的扩展对于熟悉C语言的开发者来说，枚举类型并不陌生，它是一系列数值集合，我们可以更很方便维护一组数据集。

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// 数字枚举
// 数字会有向下自增的功能 默认首项是0
enum Direction {
    Up = 1,
    Down,
    Left,
    Right
}
console.log(Direction.Down) // 2


// 字符串枚举
enum Direction {
  Up = 'Up',
  Down = 'Down',
  Left = 'Left',
  Right = 'RIGHT',
}

const value = 'Up'
if(value===Direction.Up){
  console.log('go Up!')
}

// 编译成js后

var Direction;

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

var value = 'Up';

if (value === Direction.Up)
  console.log('go Up!');


// 给枚举加上常量控制

const enum Direction {
  Up = 'Up',
  Down = 'Down',
  Left = 'Left',
  Right = 'RIGHT',
}

const value = 'Up'

if(value===Direction.Up){
  console.log('go Up!')
}

// 编译成js后

var value = 'Up';

if (value === "Up" /* Up */) {
  console.log('go Up!');
}

/** 可见，const常量有助于提高代码性能。**/