Proxies

• create a proxy

var proxy = new Proxy({}, {
    get: function(target, property) {
        return 35;
    }
});
var handler = {
  // target.prop
  get: ...,
  // target.prop = value
  set: ...,
  // 'prop' in target
  has: ...,
  // delete target.prop
  deleteProperty: ...,
  // target(...args)
  apply: ...,
  // new target(...args)
  construct: ...,
  // Object.getOwnPropertyDescriptor(target, 'prop')
  getOwnPropertyDescriptor: ...,
  // Object.defineProperty(target, 'prop', descriptor)
  defineProperty: ...,
  // Object.getPrototypeOf(target), Reflect.getPrototypeOf(target),
  // target.__proto__, object.isPrototypeOf(target), object instanceof target
  getPrototypeOf: ...,
  // Object.setPrototypeOf(target), Reflect.setPrototypeOf(target)
  setPrototypeOf: ...,
  // for (let i in target) {}
  enumerate: ...,
  // Object.keys(target)
  ownKeys: ...,
  // Object.preventExtensions(target)
  preventExtensions: ...,
  // Object.isExtensible(target)
  isExtensible :...
}
console.log(proxy.time);        // 35
console.log(proxy.name);        // 35
console.log(proxy.title);       // 35
Array.isArray(new Proxy([], {})); // true

Uses

Defensive Objects

function createDefensiveObject(target) {
    return new Proxy(target, {
        get: function(target, property) {
            if (property in target) {
                return target[property];
            } else {
                throw new ReferenceError(
                "Property \"" + property + "\" does not exist.");
            }
        }
    });
}
let person = {
    name: "Nicholas"
};
let defensivePerson = createDefensiveObject(person);
console.log(defensivePerson.name);      // "Nicholas"
console.log(defensivePerson.age);       // ReferenceError!

Type Safety

function createTypeSafeObject(object) {
    return new Proxy(object, {
          set: function(target, property, value) {
              var currentType = typeof target[property],
                  newType = typeof value;
              if (property in target && currentType !== newType) {
                  throw new Error(
                  "Property " 
                  + property 
                  + " must be a " 
                  + currentType + ".");
              } else {
                  target[property] = value;
              }
          }
    });
}

Ref

• Proxy - MDN