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TypeScript-初级-04-枚举

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枚举

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,颜色限定为红绿蓝等。

简单的例子

枚举使用 enum 关键字来定义:

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

枚举成员会被赋值为从 0 开始递增的数字,同时也会对枚举值到枚举名进行反向映射:

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

console.log(Days["Sun"] === 0); // true
console.log(Days["Mon"] === 1); // true
console.log(Days["Tue"] === 2); // true
console.log(Days["Sat"] === 6); // true

console.log(Days[0] === "Sun"); // true
console.log(Days[1] === "Mon"); // true
console.log(Days[2] === "Tue"); // true
console.log(Days[6] === "Sat"); // true

事实上,上面的例子会被编译为:

var Days;
(function (Days) {
    Days[Days["Sun"] = 0] = "Sun";
    Days[Days["Mon"] = 1] = "Mon";
    Days[Days["Tue"] = 2] = "Tue";
    Days[Days["Wed"] = 3] = "Wed";
    Days[Days["Thu"] = 4] = "Thu";
    Days[Days["Fri"] = 5] = "Fri";
    Days[Days["Sat"] = 6] = "Sat";
})(Days || (Days = {}));

手动赋值

我们也可以给枚举项手动赋值:

enum Days {Sun = 7, Mon = 1, Tue, Wed, Thu, Fri, Sat};

console.log(Days["Sun"] === 7); // true
console.log(Days["Mon"] === 1); // true
console.log(Days["Tue"] === 2); // true
console.log(Days["Sat"] === 6); // true

上面的例子中,未手动赋值的枚举项会接着上一个枚举项递增。

如果未手动赋值的枚举项与手动赋值的重复了,TypeScript 是不会察觉到这一点的:

enum Days {Sun = 3, Mon = 1, Tue, Wed, Thu, Fri, Sat};

console.log(Days["Sun"] === 3); // true
console.log(Days["Wed"] === 3); // true
console.log(Days[3] === "Sun"); // false
console.log(Days[3] === "Wed"); // true

上面的例子中,递增到 3 的时候与前面的 Sun 的取值重复了,但是 TypeScript 并没有报错,导致 Days[3] 的值先是 "Sun",而后又被 "Wed" 覆盖了。编译的结果是:

var Days;
(function (Days) {
    Days[Days["Sun"] = 3] = "Sun";
    Days[Days["Mon"] = 1] = "Mon";
    Days[Days["Tue"] = 2] = "Tue";
    Days[Days["Wed"] = 3] = "Wed";
    Days[Days["Thu"] = 4] = "Thu";
    Days[Days["Fri"] = 5] = "Fri";
    Days[Days["Sat"] = 6] = "Sat";
})(Days || (Days = {}));

所以使用的时候需要注意,最好不要出现这种覆盖的情况。

手动赋值的枚举项可以不是数字,此时需要使用类型断言来让 tsc 无视类型检查 (编译出的 js 仍然是可用的):

enum Days {Sun = 7, Mon, Tue, Wed, Thu, Fri, Sat = <any>"S"};
var Days;
(function (Days) {
    Days[Days["Sun"] = 7] = "Sun";
    Days[Days["Mon"] = 8] = "Mon";
    Days[Days["Tue"] = 9] = "Tue";
    Days[Days["Wed"] = 10] = "Wed";
    Days[Days["Thu"] = 11] = "Thu";
    Days[Days["Fri"] = 12] = "Fri";
    Days[Days["Sat"] = "S"] = "Sat";
})(Days || (Days = {}));

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); // true
console.log(Days["Mon"] === 1.5); // true
console.log(Days["Tue"] === 2.5); // true
console.log(Days["Sat"] === 6.5); // true

常数项和计算所得项

枚举项有两种类型:常数项(constant member)和计算所得项(computed member)。

前面我们所举的例子都是常数项,一个典型的计算所得项的例子:

enum Color {Red, Green, Blue = "blue".length};

上面的例子中,"blue".length 就是一个计算所得项。

上面的例子不会报错,但是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 = "red".length, Green, Blue};

// index.ts(1,33): error TS1061: Enum member must have initializer.
// index.ts(1,40): error TS1061: Enum member must have initializer.

下面是常数项和计算所得项的完整定义,部分引用自[中文手册 - 枚举]:

当满足以下条件时,枚举成员被当作是常数:

所有其它情况的枚举成员被当作是需要计算得出的值。

常数枚举

常数枚举是使用 const enum 定义的枚举类型:

const enum Directions {
    Up,
    Down,
    Left,
    Right
}

let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

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。

上例的编译结果是:

var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];

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};

// index.ts(1,38): error TS2474: In 'const' enum declarations member initializer must be constant expression.

外部枚举

外部枚举(Ambient Enums)是使用 declare enum 定义的枚举类型:

declare enum Directions {
    Up,
    Down,
    Left,
    Right
}

let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

之前提到过,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。

上例的编译结果是:

var directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

外部枚举与声明语句一样,常出现在声明文件中。

同时使用 declareconst 也是可以的:

declare const enum Directions {
    Up,
    Down,
    Left,
    Right
}

let directions = [Directions.Up, Directions.Down, Directions.Left, Directions.Right];

编译结果:

var directions = [0 /* Up */, 1 /* Down */, 2 /* Left */, 3 /* Right */];

TypeScript 的枚举类型的概念[来源于 C#][C# Enum]。

标签:TypeScript,console,log,04,Sun,Days,枚举,Directions
来源: https://www.cnblogs.com/idspring/p/11784733.html