枚举总结

枚举

• 概述：枚举（Enum）类型用于取值被限定在一定范围内的场景，比如一周只能有七天。

enum Days {Sun, Mon, Tue, Wed, Thu, Fri, Sat};

1.给枚举项手动赋值：

enum Days {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat};
 
console.log(Days["Sun"] === 7);  
console.log(Days["Mon"] === 1);  
 
// 当然，手动赋值的枚举项也可以为小数或负数，此时后续未手动赋值的项的递增步长仍为 1
 
enum Days {Sun = 7, Mon = 1.5, Tue, Wed, Thu, Fri, Sat};
 
console.log(Days["Sun"] === 7);  
console.log(Days["Mon"] === 1.5); 

2.常数项和计算所得项

• 概述：枚举项有两种类型：常数项和计算所得项

（1）一个典型的计算所得项的例子（如果紧接在计算所得项后面的是未手动赋值的项，那么它就会因为无法获得初始值而报错）

enum Color {Red, Green, Blue = "blue".length};  // 正确
enum Color {Red = "red".length, Green, Blue};  // 错误

3.常数枚举

• 概述：常数枚举是使用 const enum 定义的枚举类型

（1）常数枚举与普通枚举的区别是，它会在编译阶段被删除，并且不能包含计算成员

const enum Directions { Up, Down, Left, Right }
let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];
 
//编译成js后
var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];

（2）假如包含了计算成员，则会在编译阶段报错

const enum Color {Red, Green, Blue = "blue".length}; // error: In 'const' enum declarations member initializer must be constant expression

4.外部枚举

• 概述：外部枚举是使用 declare enum 定义的枚举类型

（1）declare 定义的类型只会用于编译时的检查，编译结果中会被删除

declare enum Directions { Up, Down, Left, Right }
let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];
 
//编译成js后
var directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

（2）外部枚举和常数枚举同时使用

declare const enum Directions { Up, Down, Left, Right }
let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];
 
//编译成js后
var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];