ES6只读枚举可以将值映射到名称 [英] ES6 read-only enums that can map value to name

问题描述

我想在JS中定义类似枚举的结构，但有两个要求：

I would like to define an enum-like structure in JS, but have two requirements:

1. 这些值是只读的，即，没有用户可以分配给他们。


2. 可以将值（0，1，2，...）映射回名称（如Java的名称方法）

1. The values be read-only, i.e. no users can assign to them.
2. The values (0, 1, 2, ...) can be mapped back into the names (as with Java's name method)

我知道创建这样的枚举的方法通常符合一个要求，而不是两者。

The methods I know to create enums like this typically meet one requirement or the other, not both.

我试过：

const MyEnum = {
  a: 0,
  b: 1,
  c: 2
};

枚举本身是常量，但值仍然是可变的，我无法将值映射回

The enum itself is constant, but the values are still mutable and I can't map values back to names efficiently.

在Typescript中编写枚举时，它会输出：

When writing an enum in Typescript, it outputs:

var MyEnum;
(function (MyEnum) {
    MyEnum[MyEnum["a"] = 0] = "a";
    MyEnum[MyEnum["b"] = 1] = "b";
    MyEnum[MyEnum["c"] = 2] = "c";
})(MyEnum || (MyEnum = {}));

这可以映射两种方式，但仍然没有固定值。

This can map both ways, but still doesn't have constant values.

我发现符合这两个要求的唯一选项是在类上使用getter：

The only option I've found that meets both requirements would be using getters on a class:

class MyEnum {
  get a() {
    return 0;
  }
  ...
}

此方法极大地限制合法的名称，并且有很多的开销，特别是在浏览器不能很好地嵌入getter（或不能）。

This method dramatically restricts the legal names and has a lot of overhead, especially in browsers that don't inline getters well (or can't).

@Shmiddty建议冻结一个对象：

const MyEnum = Object.freeze({
  a: 0,
  b: 1,
  c: 2
});

这满足了常规的要求，但并没有提供将值映射到名称的好方法

This meets the constant requirement well, but doesn't provide a great way to map values back to names.

我可以编写一个帮助器来构建反向映射，如：

I could write a helper that builds the reverse mapping like:

function reverseEnum(enum) {
  Object.keys(enum).forEach(k => {
    enum[enum[k]] = k;
  });
}

但是，生成反向映射的任何类型的编程解决方案将遇到问题，如果原始对象被冻结或以其他方式实际上是不变的。

But any kind of programmatic solution to generate the reverse mapping will run into problems if the original object is frozen or otherwise actually constant.

JS中有没有一个干净简洁的解决方案？

Is there a clean, concise solution to this in JS?

推荐答案

我将使用地图，以便您的枚举值可以是任何类型，而不是将其强制为字符串。

I'd use a Map so that your enum values can be any type, rather than having them coerced into strings.

function Enum(obj){
    const keysByValue = new Map();
    const EnumLookup = value => keysByValue.get(value);

    for (const key of Object.keys(obj)){
        EnumLookup[key] = obj[key];
        keysByValue.set(EnumLookup[key], key);
    }

    // Return a function with all your enum properties attached.
    // Calling the function with the value will return the key.
    return Object.freeze(EnumLookup);
}

如果您的枚举是字符串，我也可能会更改一行：

If your enum is all strings, I'd also probably change one line to:

EnumLookup[key] = Symbol(obj[key]);

以确保枚举值正确使用。只使用一个字符串，你不能保证一些代码没有简单地传递一个正常的字符串，恰好与一个枚举值相同。如果您的值始终是字符串或符号，您还可以将Map简单地替换为简单对象。

to ensure that the enum values are being used properly. Using just a string, you have no guarantee that some code hasn't simply passed a normal string that happens to be the same as one of your enum values. If your values are always strings or symbols, you could also swap out the Map for a simple object.

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