ES6 read-only enums that can map value to name

Question

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

1. The values be read-only, i.e. no users can assign to them.
2. The values (0, 1

Answer 1

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);
}

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

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

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.