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

This does a pretty good job, IMHO.

function Enum(a){
  let i = Object
    .keys(a)
    .reduce((o,k)=>(o[a[k]]=k,o),{});

  return Object.freeze(
    Object.keys(a).reduce(
      (o,k)=>(o[k]=a[k],o), v=>i[v]
    )
  );
} // y u so terse?

const FOO = Enum({
  a: 0,
  b: 1,
  c: "banana"
});

console.log(FOO.a, FOO.b, FOO.c);            // 0 1 banana
console.log(FOO(0), FOO(1), FOO("banana"));  // a b c

try {
  FOO.a = "nope";
}
catch (e){
  console.log(e);
}