1 定义

枚举 ( Enum ) 类型常用于限定取值在一定范围的场景，类似于 type 关键字约束基础字面量类型值的范围（详见）。
举例：一周的值被限定为七天

// 注意声明方式
enum Week {Sun, Mon, Tue, Wed, Thu, Fri, Sat};
console.log('enum Week =>', Week);

2 理解

2.1 原理

上述例子编译后结果为：

// 注意声明方式
var Week;
(function (Week) {
    Week[Week["Sun"] = 0] = "Sun";
    Week[Week["Mon"] = 1] = "Mon";
    Week[Week["Tue"] = 2] = "Tue";
    Week[Week["Wed"] = 3] = "Wed";
    Week[Week["Thu"] = 4] = "Thu";
    Week[Week["Fri"] = 5] = "Fri";
    Week[Week["Sat"] = 6] = "Sat";
})(Week || (Week = {}));
;
console.log('enum Week =>', Week);

即：枚举成员会以枚举值（默认从 0 开始，顺序递增 1）和枚举名互为键值赋值给枚举对象。
最终

console.log('enum Week =>', Week);
enum Week => {
  '0': 'Sun',
  '1': 'Mon',
  '2': 'Tue',
  '3': 'Wed',
  '4': 'Thu',
  '5': 'Fri',
  '6': 'Sat',
  Sun: 0,
  Mon: 1,
  Tue: 2,
  Wed: 3,
  Thu: 4,
  Fri: 5,
  Sat: 6
}

2.2 枚举值手动赋值

可以对枚举值进行手动赋予初始值，如：

// 注意声明方式
enum Week {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat};
console.log('enum Week =>', Week);

则未赋予初始值的会接着前一个枚举值顺序递增 1 ，如上述编译后：

// 注意声明方式
var Week;
(function (Week) {
    Week[Week["Sun"] = 7] = "Sun";
    Week[Week["Mon"] = 1] = "Mon";
    Week[Week["Tue"] = 2] = "Tue";
    Week[Week["Wed"] = 3] = "Wed";
    Week[Week["Thu"] = 4] = "Thu";
    Week[Week["Fri"] = 5] = "Fri";
    Week[Week["Sat"] = 6] = "Sat";
})(Week || (Week = {}));
;
console.log('enum Week =>', Week);

❗注意：如果手动赋予初始值与顺序递增值重复，会产生值被覆盖的情况：

enum Week {Sun = 3, Mon = 1, Tue, Wed, Thu, Fri, Sat};
console.log('enum Week =>', Week);

编译结果

// 注意声明方式
var Week;
(function (Week) {
    Week[Week["Sun"] = 3] = "Sun";
    Week[Week["Mon"] = 1] = "Mon";
    Week[Week["Tue"] = 2] = "Tue";
    Week[Week["Wed"] = 3] = "Wed";
    Week[Week["Thu"] = 4] = "Thu";
    Week[Week["Fri"] = 5] = "Fri";
    Week[Week["Sat"] = 6] = "Sat";
})(Week || (Week = {}));
;
console.log('enum Week =>', Week);

最终打印后发现：Week[3] = "Sun" 被 Week[3] = "Wed" 覆盖。

console.log('enum Week =>', Week);
enum Week => {
  '1': 'Mon', 
  '2': 'Tue', 
  '3': 'Wed', // 这里被覆盖了
  '4': 'Thu', 
  '5': 'Fri', 
  '6': 'Sat', 
  Sun: 3,
  Mon: 1,
  Tue: 2,
  Wed: 3,
  Thu: 4,
  Fri: 5,
  Sat: 6
}

手动赋值的其他情况：

手动赋予初始值可以为负数和小数，后续未赋值的仍会以顺序递增 1 ；

手动赋予初始值可以为字符串，但后续不能含有未赋值的枚举值；

// ERROR: Week.eight 枚举成员必须具有初始化表达式。
enum Week {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat = 'sex', eight, };
console.log('enum Week =>', Week);

// 注意声明方式
enum Week {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat = 'sex', eight = 'eight'};
console.log('enum Week =>', Week);

编译后

var Week;
(function (Week) {
  Week[Week["Sun"] = 7] = "Sun";
  Week[Week["Mon"] = 1] = "Mon";
  Week[Week["Tue"] = 2] = "Tue";
  Week[Week["Wed"] = 3] = "Wed";
  Week[Week["Thu"] = 4] = "Thu";
  Week[Week["Fri"] = 5] = "Fri";
  Week["Sat"] = "sex";
  Week["eight"] = "eight";
})(Week || (Week = {}));
;
console.log('enum Week =>', Week);

3 常数项与计算所得项

枚举值有两种类型：常数项 ( constant member ) 和计算所得项 ( computed member ) ；
一般情况下都是常数项居多，符合常数项的要求有：

枚举值为数值，未被初始化的可通过前一项顺序递增（第一项默认为 0）；
枚举值为带括号的常数表达式；
枚举值为 +, -, ~ 一元运算符的常数表达式或 +, -, *, /, %, &, |, <<, >> 等二元运算符的常数表达式；

其他情况为计算所得项，如前面的字符串值，或其他表达式。

// Sat 与 eight 为 计算所得项
enum Week {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat = 'sex', eight = 'eight'.length};
console.log('enum Week =>', Week);

与字符串值一样，计算所得项后续不能含有未赋值的枚举值。

4 常数枚举与外部枚举

常数枚举是使用 const enum 定义的枚举，它与普通枚举的区别是：它会在编译阶段被删除，且不能包括计算所得项；

// ERROR: 'eight'.length 含字符串值成员的枚举中不允许使用计算值。
const enum Week {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat = 'sex', eight = 'eight'.length};
console.log('enum Week =>', Week);

// 常数枚举，编译后被删除
const enum Week {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat};
const weekend = [Week.Sat, Week.Sun];
console.log('weekend => ', weekend);

编译后

;
var weekend = [6 /* Sat */, 7 /* Sun */];
console.log('weekend => ', weekend);

外部枚举是使用 declare enum 声明的枚举，前面知道 declare 声明的仅能用于编译时类型检查，编译后会被删除，外部枚举同声明语句一样，常用于声明文件 .d.ts 中；

// 外部枚举，与常数枚举不同它能使用计算所得项
declare enum Week {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat, eight = "eight"};
const weekend = [Week.Sat, Week.Sun];
console.log('weekend => ', weekend);

编译后

;
var weekend = [Week.Sat, Week.Sun]; // ReferenceError: Week is not defined
console.log('weekend => ', weekend);

也能同时使用常数枚举和外部枚举，如 declare const ... ，编译结果与常数枚举 const 一样；

TypeScript