Is it valid to use Object.defineProperties with symbols?

Question

Let's take the following code:

var obj = {};
var x = Symbol();
Object.defineProperties(obj, {
  [x]: {
    value: true,
    writable: true
  },
  "property2": {
    value: "Hello",
    writable: false
  }
  // etc. etc.
});
console.log(obj[x])

Is this valid?

With the native Object.defineproperties code we get in the console.log true.

With the polyfill of zone.js

which is of the form of :

  Object.defineProperties = function (obj, props) {
    Object.keys(props).forEach(function (prop) {
      Object.defineProperty(obj, prop, props[prop]);
    });
    return obj;
  };

we get for the same code of console.log undefined.

This is because of the Object.keys function. I googled around and did not find in any place if this should be allowed or not.

Zumwalt

Syntactically it's valid, practically it is as any other object key is converted to a string, like Symbol() (that would be the exact key name in this case). If you really want to be able to key by an arbitrary object - use the ES2015 Map() – Jocose

Uh, you're not defining a property on the symbol here? – Bureaucratic

Answer 1

I googled around and did not find in any place if this should be allowed or not.

You can always check the spec, in this case § 19.1.2.3 Object.defineProperties(O, Properties).

It uses the OwnPropertyKeys internal method, which does indeed list all string and symbol keys of an object.

This is because of the Object.keys function

Correct. It should be Object.getOwnPropertyNames(props).concat(Object.getOwnPropertySymbols(props)) instead. You may want to file a bug with zone.js. I do however wonder why you'd need a polyfill for the ES5 Object.defineProperties function when you're working with ES6 symbols?

Bureaucratic

But the polyfill shouldn't be applied in an ES6 environment where Object.defineProperties is already natively defined? – Bureaucratic