Why isn't ownKeys Proxy trap working with Object.keys()?

Question

In the documentation of the Proxy ownKeys trap on MDN it states that it will intercept Object.keys() calls:

This trap can intercept these operations:

Object.getOwnPropertyNames()

Object.getOwnPropertySymbols()

Object.keys()

Reflect.ownKeys()

However, from my tests it doesn't seem to work with Object.keys:

const proxy = new Proxy({}, {
  ownKeys() {
    console.log("called")
    return ["a", "b", "c"]
  }
})

console.log(Object.keys(proxy))

console.log(Object.getOwnPropertyNames(proxy))

console.log(Reflect.ownKeys(proxy))

FZs · Accepted Answer · 2020-12-17 12:17:49Z

The reason is simple: Object.keys returns only properties with the enumerable flag. To check for it, it calls the internal method [[GetOwnProperty]] for every property to get its descriptor. And here, as there’s no property, its descriptor is empty, no enumerable flag, so it’s skipped.

For Object.keys to return a property, we need it to either exist in the object, with the enumerable flag, or we can intercept calls to [[GetOwnProperty]] (the trap getOwnPropertyDescriptor does it), and return a descriptor with enumerable: true.

Here’s an example of that:

let user = { };

user = new Proxy(user, {
  ownKeys(target) { // called once to get a list of properties
    return ['a', 'b', 'c'];
  },

  getOwnPropertyDescriptor(target, prop) { // called for every property
    return {
      enumerable: true,
      configurable: true
      /* ...other flags, probable "value:..." */
    };
  }

});

console.log( Object.keys(user) ); // ['a', 'b', 'c']

Source

Bergi · Accepted Answer · 2020-12-17 11:54:35Z

2

Object.keys returns only the enumerable own properties of an object. Your proxy doesn't have such, or at least it doesn't report them in its getOwnPropertyDescriptor trap. It works with

const proxy = new Proxy({}, {
  ownKeys() {
    console.log("called ownKeys")
    return ["a", "b", "c"]
  },
  getOwnPropertyDescriptor(target, prop) {
    console.log(`called getOwnPropertyDescriptor(${prop})`);
    return { configurable: true, enumerable: true };
  } 
})

console.log(Object.keys(proxy))

console.log(Object.getOwnPropertyNames(proxy))

console.log(Reflect.ownKeys(proxy))

answered Dec 17, 2020 at 11:54

Bergi

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Comments

paliz · Accepted Answer · 2023-05-02 16:11:55Z

-2

const proxy = new Proxy({}, {
ownKeys: function () {
  console.log("called")
  return ["a", "b", "c"]
  }
 });

answered May 2, 2023 at 16:11

paliz

3632 silver badges5 bronze badges

2 Comments

Rohit Gupta Over a year ago

While possibly correct, a code-only answer helps the person who asked the question, it doesn't do them or future visitors any good. Please consider improving your answer.

paliz Over a year ago

my friend i test code and work's fine. what should i do to improve answer

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