harmony-reflect

What is harmony-reflect?

The harmony-reflect package is a library for ECMAScript 2015 (also known as ES6) that provides a full implementation of the ES6 Reflect API and Proxy handlers. It allows developers to intercept and define custom behavior for fundamental language operations (e.g., property lookup, assignment, enumeration, function invocation, etc.) using the Proxy API. It also provides utility functions that mirror the standard Reflect API, which provides methods for interceptable JavaScript operations.

What are harmony-reflect's main functionalities?

Proxying an object

This code demonstrates how to create a proxy for an object that returns the property value if it exists, or 42 if it does not.

{"handler": {"get": function(target, name) { return name in target ? target[name] : 42; }}, "proxy": new Proxy({}, handler)}

Reflecting property access

This code uses the Reflect API to access the property 'foo' on the target object.

{"target": {"foo": 1}, "status": Reflect.get(target, 'foo')}

Defining a property on an object

This code snippet uses Reflect.defineProperty to define a new property 'foo' on the target object with the value 123.

{"target": {}, "status": Reflect.defineProperty(target, 'foo', {value: 123})}

Observing function invocations

This code shows how to create a proxy for a function that intercepts the call and returns the result multiplied by 2.

{"handler": {"apply": function(target, thisArg, argumentsList) { return target.apply(thisArg, argumentsList) * 2; }}, "proxy": new Proxy(function(x) { return x; }, handler)}

Other packages similar to harmony-reflect

proxy-polyfill

The proxy-polyfill package provides a very limited polyfill for the Proxy object, which allows you to use a subset of the Proxy API features in environments that do not support it natively. It does not support the full range of metaprogramming capabilities that harmony-reflect offers.

reflect-metadata

The reflect-metadata package is a polyfill for the experimental metadata reflection API. It allows developers to add and read metadata on classes and properties. While it deals with reflection, it is more focused on metadata than the general proxy and reflection capabilities provided by harmony-reflect.

README

This is a shim for the ECMAScript 6 Reflect and Proxy objects.

This library does two things:

• It defines an ES6-compliant Reflect global object that exports the ECMAScript 6 reflection API.
• It patches the harmony-era (pre-ES6) Proxy object to be up-to-date with the ES6 spec.

Read Why should I use this library?

Feb 2016 update: the recently released V8 v4.9 includes native support for ES2015 Proxies and Reflect, making this library obsolete for environments that embed V8 4.9 or newer (like Chrome 49).

Installation

For node.js, install via npm:

npm install harmony-reflect

Then:

node --harmony-proxies
> var Reflect = require('harmony-reflect');

See release notes for changes to the npm releases.

To use in a browser, just download the single reflect.js file. After loading

<script src="reflect.js"></script>

a global object Reflect is defined that contains reflection methods as defined in the ES6 spec.

This library also updates the "harmony-era" Proxy object in the V8 engine (also used in node.js) to follow the latest ECMAScript 2015 spec. To create such a proxy, call:

var proxy = new Proxy(target, handler);

See below for a list of spec incompatibilities and other gotcha's.

API Docs

This module exports an object named Reflect and updates the global Proxy object (if it exists) to be compatible with the latest ECMAScript 6 spec.

The ECMAScript 6 Proxy API allows one to intercept various operations on Javascript objects.

• Overview of all supported traps on proxies
• The Reflect API
• The Proxy Handler API

Compatibility

The Reflect API, with support for proxies, was tested on:

• Firefox (>= v4.0)
• node --harmony_proxies (>= v0.7.8)
• iojs --harmony_proxies (>= 2.3.0)
• v8 --harmony_proxies (>= v3.6)
• Any recent js spidermonkey shell

If you need only Reflect and not an up-to-date Proxy object, this library should work on any modern ES5 engine (including all browsers).

Compatibility notes:

• Chrome (>= v19 && <= v37) used to support proxies behind a flag (chrome://flags/#enable-javascript-harmony) but Chrome v38 removed the Proxy constructor. As a result, this library cannot patch the harmony-era Proxy object on Chrome v38 or above. If you're working with Chromium directly, it's still possible to enable proxies using chromium-browser --js-flags="--harmony_proxies".
• In older versions of V8, the Proxy constructor was enabled by default when starting V8 with --harmony. For recent versions of V8, Proxy must be explicitly enabled with --harmony_proxies.

Dependencies

• ECMAScript 5/strict
• To emulate direct proxies: * old Harmony Proxies * Harmony WeakMaps

After loading reflect.js into your page or other JS environment, be aware that the following globals are patched to be able to recognize emulated direct proxies:

Object.getOwnPropertyDescriptor
Object.defineProperty
Object.defineProperties
Object.getOwnPropertyNames
Object.getOwnPropertySymbols
Object.keys
Object.{get,set}PrototypeOf
Object.{freeze,seal,preventExtensions}
Object.{isFrozen,isSealed,isExtensible}
Object.prototype.valueOf
Object.prototype.isPrototypeOf
Object.prototype.toString
Object.prototype.hasOwnProperty
Function.prototype.toString
Date.prototype.toString
Array.isArray
Array.prototype.concat
Proxy
Reflect

:warning: In node.js, when you require('harmony-reflect'), only the current module's globals are patched. If you pass an emulated direct proxy to an external module, and that module uses the unpatched globals, the module may not interact with the proxy according to the latest ES6 Proxy API, instead falling back on the old pre-ES6 Proxy API. This can cause bugs, e.g. the built-in Array.isArray will return false when passed a proxy-for-array, while the patched Array.isArray will return true. I know of no good fix to reliably patch the globals for all node modules. If you do, let me know.

Examples

The examples directory contains a number of examples demonstrating the use of proxies:

• membranes: wrappers that transitively isolate two object-graphs.
• observer: a self-hosted implementation of the ES7 Object.observe notification mechanism.
• profiler: a simple profiler to collect usage statistics of an object.

Other example uses of proxies (not done by me, but using this library):

• supporting negative array indices a la Python
• tpyo: using proxies to correct typo's in JS property names
• persistent objects: shows how one might go about using proxies to save updates to objects in a database incrementally
• defaultdict: default values for new keys in objects (as known from Python)

For more examples of proxies, and a good overview of their design rationale, I recommend reading Axel Rauschmayer's blog post on proxies.

Proxy Handler API

The sister project proxy-handlers defines a number of predefined Proxy handlers as "abstract classes" that your code can "subclass" The goal is to minimize the number of traps that your proxy handlers must implement.

Spec Incompatibilities and other gotcha's

This library differs from the ECMAScript 2015 spec as follows:

• In ES6, Proxy is a constructor function that requires the use of new. That is, you must write new Proxy(target, handler). This library exports Proxy as an ordinary function which may be called with or without using the new operator.

• In ES6, Function.prototype.toString and Date.prototype.toString do not operate transparently on Proxies. This shim patches those functions so that stringifying a Proxy-for-a-function or a Proxy-for-a-date "unwraps" the proxy and instead stringifies the target of the Proxy. This behavior may change in the future to be more spec-compatible.

• This library does not shim Symbol objects. On modern V8 or io.js which supports Symbol objects natively, due to a bug in V8, Symbols and Proxies don't play well together. Read more.

• Proxies-for-arrays are serialized as JSON objects rather than as JSON arrays. That is, JSON.stringify(new Proxy([], {})) returns "{}" rather than "[]". Read more.