javascript - Why can ES6 Symbol properties be made enumerable by Object.defineProperty?

Question

In ES6 properties can be defined as symbol properties:

var symbol = Symbol();
var object = {};
object[symbol] = 'value';

MDN defines enumerable properties as 'those which can be iterated by a for..in loop' (1). Symbol properties are never iterated by a for...in loop, therefore they can be considered non-enumerable (2).

Does it make any sense, then, that you can do this:

Object.defineProperty(object, symbol, {
    value: 'value',
    enumerable: true
});

and that querying object for it's descriptor does indeed confirm that this property is enumerable:

Object.getOwnPropertyDescriptor(object, symbol)
// -> { enumerable: true }

Why? What use is this?

(1) https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties

(2) for...in uses [[Enumerate]], which only includes string keys. Probably the definition on MDN should be changed now that we have symbol properties.

Answer 1

Yes, there's a reason for allowing Symbol properties to be enumerable: Object.assign:

let s1 = Symbol();
let s2 = Symbol();
let s3 = Symbol();
let original = {};
original[s1] = "value1";                // Enumerable
Object.defineProperty(original, s2, {   // Enumerable
  enumerable: true,
  value: "value2"
});
Object.defineProperty(original, s3, {   // Non-enumerable
  value: "value3"
});
let copy = {};
Object.assign(copy, original);
console.log("copy[s1] is " + copy[s1]); // value1, because it was enumerable
console.log("copy[s2] is " + copy[s2]); // value2, because it was enumerable
console.log("copy[s3] is " + copy[s3]); // undefined, because it wasn't enumerable

Live Copy on Babel's REPL.

Just for clarity:

MDN defines enumerable properties as 'those which can be iterated by a for..in loop' (1).

That's simply wrong for ES6 (ES2015). It was a reasonable, if simplistic, definition in ES5 and earlier, no it's no longer even simplistically correct because of Symbols. I've fixed the article.

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This is a CW because it was the outgrowth of the comments on the question.