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Is it possible to enable EcmaScript 6 Harmony Proxies in nodejs? If so, what are the pros and cons? And is there any documentation on how to use them? Thanks !

Alexandre Kirszenberg
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5 Answers5

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Invoking node with node --harmony-proxies should do the trick.

Pros: proxies are a very powerful feature when you really need them.

Cons: proxies are a much too powerful feature when you don't need them (which should be most of the time). Also, the implementation should still be regarded experimental.

As for documentation, all there really is atm is the Harmony wiki, in particular this page, which reflects the current implementation of proxies in V8 (and thus node):

http://wiki.ecmascript.org/doku.php?id=harmony:proxies

Andreas Rossberg
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    This indeed was necessary, however I still had trouble getting it to work. The following thread and example was also very useful: https://github.com/joyent/node/issues/4138 – odedfos Aug 13 '13 at 08:41
  • What is "much too powerful" about such a feature? – JSON Mar 10 '15 at 07:22
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    **UPDATE 2015/10/12**: As of now, V8 does not yet support ES6 `new Proxy()` handler, you will have to use `Proxy.create()` instead or you can use [this shim](https://www.npmjs.com/package/harmony-proxy) to alias the function call for you – Blake Regalia Oct 12 '15 at 21:39
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i recommend harmony-reflect, which makes it easy to e.g. set up get/set traps:

UPDATE careful, below is CoffeeScript

require 'harmony-reflect'

handler =

  get: ( target, name ) ->
    console.log 'get' name
    return target[ name ]

  set: ( target, name, value ) ->
    console.log 'set' name
    target[ '%is-clean' ] = no if value isnt target[ name ]
    if value is undefined then delete target[ name ]
    else                       target[ name ] = value
    return value

clean = ( x ) ->
  x[ '%is-clean' ] = yes
  return x

p = Proxy {}, handler
p[ 'a' ] = 1
p[ 'b' ] = undefined
console.log p[ 'a' ], p[ 'b' ]
console.log "c" of p, p[ 'c' ]
console.log p
clean p
p[ 'a' ] = 1
console.log p
p[ 'a' ] = 42
console.log p

the above is the inceptive code to do 'transparent object persistence' in JavaScript. using harmony-reflect, it becomes trivial to make it so that all get and set actions on an object get intercepted—in this demo, we set an %is-clean attribute so we can test whether object members have been changed, and we also delete members that have been set to undefined.

flow
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1

You can use pimped-proxy which a lightweight implementation of proxies, making declaration easier and ES5 compatible. Unlike the native Proxy, it can only proxy properties known at creation time.

https://github.com/Boulangerie/pimped-proxy

user3438710
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1

Proxy is now available natively in Node versions >= 6.

Scott Jungwirth
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Harmony Proxies won't work all that well for nodejs because they're effectively synchronous type function calls. That is, you can't implement a proxy method that's async.

See this GitHub repository for examples: https://github.com/mschwartz/SilkJS-Harmony

mschwartz
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    I don't quite follow what you are saying. Surely, the `get` trap can return an async function as a "method"? – Andreas Rossberg Nov 09 '12 at 06:59
  • Harmony code looks like: `foo = proxy_thing.some_member;` some_member is a function call. I don't know how it could be async. – mschwartz Feb 21 '13 at 00:17
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    SilkJS Fan, I don't think evert function call needs to be async — just don't do IO inside and that's all. – andreypopp Jun 19 '13 at 21:15
  • Not every function call needs to be async, and you can use proxies for primitive kinds of things in a sync manner with nodejs. However, if you're familiar with the tie keyword in Perl, it would be a perfect use case for Harmony Proxies, but you'd have to sync get your value by key from redis (for example) synchronously. – mschwartz Aug 09 '13 at 17:08
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    proxies and (a)synchronicity are as orthogonal concepts as, say, cheese sandwiches and bank holidays. – flow Sep 19 '13 at 15:44
  • To add to the discussion on why this answer is wrong, the upcoming async await will allow you to return values from a getter using a synchronous-like pattern. Until then, there are node-fibers and generators that can also do this. – Levi Roberts Mar 10 '16 at 14:56