Proxy

Description

The Proxy object allows you to create an object that can be used in place of the original object, but which may redefine fundamental Object operations like getting, setting, and defining properties. Proxy objects are commonly used to log property accesses, validate, format, or sanitize inputs, and so on.

You create a Proxy with two parameters:

• target: the original object which you want to proxy
• handler: an object that defines which operations will be intercepted and how to redefine intercepted operations.

For example, this code creates a proxy for the target object.

const target = {
  message1: "hello",
  message2: "everyone",
};

const handler1 = {};

const proxy1 = new Proxy(target, handler1);

Because the handler is empty, this proxy behaves just like the original target:

console.log(proxy1.message1); // hello
console.log(proxy1.message2); // everyone

To customize the proxy, we define functions on the handler object:

const target = {
  message1: "hello",
  message2: "everyone",
};

const handler2 = {
  get(target, prop, receiver) {
    return "world";
  },
};

const proxy2 = new Proxy(target, handler2);

Here we've provided an implementation of the get() handler, which intercepts attempts to access properties in the target.

Handler functions are sometimes called traps, presumably because they trap calls to the target object. The trap in handler2 above redefines all property accessors:

console.log(proxy2.message1); // world
console.log(proxy2.message2); // world

Proxies are often used with the Reflect object, which provides some methods with the same names as the Proxy traps. The Reflect methods provide the reflective semantics for invoking the corresponding object internal methods. For example, we can call Reflect.get if we don't wish to redefine the object's behavior:

const target = {
  message1: "hello",
  message2: "everyone",
};

const handler3 = {
  get(target, prop, receiver) {
    if (prop === "message2") {
      return "world";
    }
    return Reflect.get(...arguments);
  },
};

const proxy3 = new Proxy(target, handler3);

console.log(proxy3.message1); // hello
console.log(proxy3.message2); // world

The Reflect method still interacts with the object through object internal methods — it doesn't "de-proxify" the proxy if it's invoked on a proxy. If you use Reflect methods within a proxy trap, and the Reflect method call gets intercepted by the trap again, there may be infinite recursion.

Terminology

The following terms are used when talking about the functionality of proxies.

handler

The object passed as the second argument to the Proxy constructor. It contains the traps which define the behavior of the proxy.

trap

The function that defines the behavior for the corresponding object internal method. (This is analogous to the concept of traps in operating systems.)

target

Object which the proxy virtualizes. It is often used as storage backend for the proxy. Invariants (semantics that remain unchanged) regarding object non-extensibility or non-configurable properties are verified against the target.

invariants

Semantics that remain unchanged when implementing custom operations. If your trap implementation violates the invariants of a handler, a TypeError will be thrown.

Object internal methods

Objects are collections of properties. However, the language doesn't provide any machinery to directly manipulate data stored in the object — rather, the object defines some internal methods specifying how it can be interacted with. For example, when you read obj.x, you may expect the following to happen:

• The x property is searched up the prototype chain until it is found.
• If x is a data property, the property descriptor's value attribute is returned.
• If x is an accessor property, the getter is invoked, and the return value of the getter is returned.

There isn't anything special about this process in the language — it's just because ordinary objects, by default, have a [[Get]] internal method that is defined with this behavior. The obj.x property access syntax simply invokes the [[Get]] method on the object, and the object uses its own internal method implementation to determine what to return.

As another example, arrays differ from normal objects, because they have a magic length property that, when modified, automatically allocates empty slots or removes elements from the array. Similarly, adding array elements automatically changes the length property. This is because arrays have a [[DefineOwnProperty]] internal method that knows to update length when an integer index is written to, or update the array contents when length is written to. Such objects whose internal methods have different implementations from ordinary objects are called exotic objects. Proxy enable developers to define their own exotic objects with full capacity.

All objects have the following internal methods: