javascript - Why isn't ownKeys Proxy trap working with Object.keys()? - Stack Overflow

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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))

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))

• javascript
• ecmascript-6
• es6-proxy

Share Improve this question Follow edited May 2, 2023 at 15:57 sdgluck asked Dec 17, 2020 at 11:38 sdglucksdgluck 27.3k1212 gold badges8181 silver badges9595 bronze badges

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3 Answers 3

Sorted by: Reset to default 9

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

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))

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

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