Proxy.handler.ownKeys

var p = new Proxy(target, {
  ownKeys: function(target) {
  }
}

var p = new Proxy{}, {
  ownKeys: function(target) {
    console.log('called'
    return ['a', 'b', 'c'];
  }
}

console.log(Object.getOwnPropertyNames(p) // "called"
                                            // [ 'a', 'b', 'c' ]

var obj = {};
Object.defineProperty(obj, 'a', { 
  configurable: false, 
  enumerable: true, 
  value: 10 }


var p = new Proxy(obj, {
  ownKeys: function(target) {
    return [123, 12.5, true, false, undefined, null, {}, []];
  }
}

console.log(Object.getOwnPropertyNames(p) 

// TypeError: proxy [[OwnPropertyKeys]] must return an array 
// with only string and symbol elements