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Typescript学习笔记-枚举

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使用枚举我们可以定义一些带名字的常量。 使用枚举可以清晰地表达意图或创建一组有区别的用例。 TypeScript支持数字的和基于字符串的枚举。

数字枚举类型

会自动填充，如果为声明，默认值为0，也可以手动赋值，后续未声明的则自动递增

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// ts
enum Week {
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
}

console.log(Week[0] === 'Monday')
console.log(Week.Monday)

// javascript
var Week;
(function (Week) {
    Week[Week["Monday"] = 0] = "Monday";
    Week[Week["Tuesday"] = 1] = "Tuesday";
    Week[Week["Wednesday"] = 2] = "Wednesday";
    Week[Week["Thursday"] = 3] = "Thursday";
    Week[Week["Friday"] = 4] = "Friday";
    Week[Week["Saturday"] = 5] = "Saturday";
    Week[Week["Sunday"] = 6] = "Sunday";
})(Week || (Week = {}));
console.log(Week[0] === 'Monday'); // true
console.log(Week.Monday) // 0

// ts
enum Week {
    Monday = 1,
    Tuesday,
    Wednesday,
    Thursday,
    Friday = 8,
    Saturday,
    Sunday
}

console.log(Week[0] === 'Monday')
// javascript
var Week;
(function (Week) {
    Week[Week["Monday"] = 1] = "Monday";
    Week[Week["Tuesday"] = 2] = "Tuesday";
    Week[Week["Wednesday"] = 3] = "Wednesday";
    Week[Week["Thursday"] = 4] = "Thursday";
    Week[Week["Friday"] = 8] = "Friday";
    Week[Week["Saturday"] = 9] = "Saturday";
    Week[Week["Sunday"] = 10] = "Sunday";
})(Week || (Week = {}));
console.log(Week[0] === 'Monday'); // false

常数项和计算所得项

枚举项有两种类型：常数项和计算所得项。

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

blue’.length为计算所得项。

如果紧接着计算所得项继续添加枚举项，会因为无法获得初始值而报错。

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enum Color {
    Red,
    Green,
    Blue = 'blue'.length,
    Grey // Enum member must have initializer.
}

字符串枚举

在一个字符串枚举里，每个成员都必须用字符串字面量，或另外一个字符串枚举成员进行初始化。
由于字符串枚举没有自增长的行为，字符串枚举可以很好的序列化。

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 enum Direction {
    Up = "UP",
    Down = "DOWN",
    Left = "LEFT",
    Right = "RIGHT",
}