Safe object/dictionary in JavaScript

safe-map.js

/* jshint node:true, esnext:true */
'use strict';

// Run: node --harmony_proxies --harmony-collections safe-map.js

function createSafeObject(props) {
  if (typeof props !== 'object' || props === null) {
    props = {};
  }

  var m = new Map();
  var keys = [];

  for (var k in props) {
    if (props.hasOwnProperty(k)) {
      m.set(k, props[k]);
    }
  }

  return Proxy.create({
    has: function(name) {
      return m.has(name);
    },
    hasOwn: function(name) {
      return m.has(name);
    },
    get: function(receiver, name) {
      if (!m.has(name)) {
        throw new Error('Unknown property: ' + name);
      }
      return m.get(name);
    },
    set: function(receiver, name, val) {
      if (keys.indexOf(name) === -1) {
        keys.push(name);
      }
      return m.set(name, val);
    },
    delete: function(name) {
      if (keys.indexOf(name) === -1) {
        keys = keys.filter(function(key) {
          return key !== name;
        });
      }
      return m.delete(name);
    },
    enumerate: function() {
      return keys;
    },
    keys: function() {
      return keys;
    }
  }, m);
}

var obj = createSafeObject({ x: 10, y: 20 });
console.log('Position: %s/%s', obj.x, obj.y);

try {
  console.log(obj.z);
} catch (err) {
  console.log('Expected error:\n  %s', err.message);
}

obj.z = 40;
console.log('Position: %s/%s/%s', obj.x, obj.y, obj.z);