Proxy() constructor

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.

The Proxy() constructor creates Proxy objects.

Syntax

new Proxy(target, handler)

Note: Proxy() can only be constructed with new. Attempting to call it without new throws a TypeError.

Parameters

target

A target object to wrap with Proxy. It can be any sort of object, including a native array, a function, or even another proxy.

handler

An object whose properties are functions that define the behavior of the proxy when an operation is performed on it.

Description

Use the Proxy() constructor to create a new Proxy object. This constructor takes two mandatory arguments:

target is the object for which you want to create the proxy
handler is the object that defines the custom behavior of the proxy.

An empty handler will create a proxy that behaves, in almost all respects, exactly like the target. By defining any of a set group of functions on the handler object, you can customize specific aspects of the proxy's behavior. For example, by defining get() you can provide a customized version of the target's property accessor.

Handler functions

This section lists all the handler functions you can define. Handler functions are sometimes called traps, because they trap calls to the underlying target object.

handler.apply()

A trap for a function call.

handler.construct()

A trap for the new operator.

handler.defineProperty()

A trap for Object.defineProperty.

handler.deleteProperty()

A trap for the delete operator.

handler.get()

A trap for getting property values.

handler.getOwnPropertyDescriptor()

A trap for Object.getOwnPropertyDescriptor.

handler.getPrototypeOf()

A trap for Object.getPrototypeOf.

handler.has()

A trap for the in operator.

handler.isExtensible()

A trap for Object.isExtensible.

handler.ownKeys()

A trap for Object.getOwnPropertyNames and Object.getOwnPropertySymbols.

handler.preventExtensions()

A trap for Object.preventExtensions.

handler.set()

A trap for setting property values.

handler.setPrototypeOf()

A trap for Object.setPrototypeOf.

Examples

Selectively proxy property accessors

In this example the target has two properties, notProxied and proxied. We define a handler that returns a different value for proxied, and lets any other accesses through to the target.

js
const target = {
  notProxied: "original value",
  proxied: "original value",
};

const handler = {
  get(target, prop, receiver) {
    if (prop === "proxied") {
      return "replaced value";
    }
    return Reflect.get(...arguments);
  },
};

const proxy = new Proxy(target, handler);

console.log(proxy.notProxied); // "original value"
console.log(proxy.proxied); // "replaced value"