weak-napi

Make weak references to JavaScript Objects.

On certain rarer occasions, you run into the need to be notified when a JavaScript object is going to be garbage collected. This feature is exposed to V8's C++ API, but not to JavaScript.

That's where weak-napi comes in! This module exports the JS engine's GC tracking functionality to JavaScript. This allows you to create weak references, and optionally attach a callback function to any arbitrary JS object. The callback function will be invoked right after the Object is garbage collected (i.e. after there are no more remaining references to the Object in JS-land).

This module can, for example, be used for debugging; to determine whether or not an Object is being garbage collected as it should. Take a look at the example below for commented walkthrough scenario.

Installation

Install with npm:

$ npm install weak-napi

Differences from node-weak

This module exports the full node-weak API. The main differences are:

• This module uses N-API! Yay. That’s a good thing – you don’t need to worry about re-compiling your code anymore when upgrading Node.
• GC callbacks are invoked at a suitable time (after the actual GC run has finished). This is nice because it means your process won’t potentially crash.
• This module works on Node 6+ only, since it uses a Proxy object to give a more complete referral of properties.
• isNearDeath() is not supported (always returns false).

That’s it!

Example

Here's an example of calling a cleanup() function on a Object after it gets garbage collected:

var weak = require('weak-napi')

// we are going to "monitor" this Object and invoke "cleanup"
// before the object is garbage collected
var obj = {
    a: true
  , foo: 'bar'
}

// Here's where we set up the weak reference
var ref = weak(obj, function () {
  // `this` inside the callback is the EventEmitter.
  console.log('"obj" has been garbage collected!')
})

// While `obj` is alive, `ref` proxies everything to it, so:
ref.a   === obj.a
ref.foo === obj.foo

// Clear out any references to the object, so that it will be GC'd at some point...
obj = null

//
//// Time passes, and the garbage collector is run
//

// `callback()` above is called, and `ref` now acts like an empty object.
typeof ref.foo === 'undefined'

Weak Callback Function "Best Practices"

It's important to be careful when using the "callbacks" feature of weak-napi, otherwise you can end up in a situation where the watched object will never be garbage collected.

You should not define the callback function in the same scope as the object that is being watched. It's often best to define the callback function at the highest scope possible (top-level being the best). Named functions work really well for this:

var http = require('http')
  , weak = require('weak-napi')

http.createServer(function (req, res) {
  weak(req, gcReq)
  weak(res, gcRes)
  res.end('Hello World\n')
}).listen(3000)

function gcReq () {
  console.log('GC\'d `req` object')
}

function gcRes () {
  console.log('GC\'d `res` object')
}

API

Weakref weak(Object obj [, Function callback])

The main exports is the function that creates the weak reference. The first argument is the Object that should be monitored. The Object can be a regular Object, an Array, a Function, a RegExp, or any of the primitive types or constructor function created with new.

Optionally, you can set a callback function to be invoked before the object is garbage collected.

Object weak.get(Weakref ref)

get() returns the actual reference to the Object that this weak reference was created with. If this is called with a dead reference, undefined is returned.

Boolean weak.isDead(Weakref ref)

Checks to see if ref is a dead reference. Returns true if the original Object has already been GC'd, false otherwise.

Boolean weak.isNearDeath(Weakref ref)

Note The N-API port of this module does not implement this and always returns false.

Boolean weak.isWeakRef(Object obj)

Checks to see if obj is "weak reference" instance. Returns true if the passed in object is a "weak reference", false otherwise.

EventEmitter weak.addCallback(Weakref ref, Function callback)

Adds callback to the Array of callback functions that will be invoked before the Object gets garbage collected. The callbacks get executed in the order that they are added.

EventEmitter weak.removeCallback(Weakref ref, Function callback)

Removes callback from the Array of callback functions that will be invoked before the Object gets garbage collected.

EventEmitter weak.removeCallbacks(Weakref ref)

Empties the Array of callback functions that will be invoked before the Object gets garbage collected.

Array weak.callbacks(Weakref ref)

Returns an Array that ref iterates through to invoke the GC callbacks. This utilizes node's EventEmitter#listeners() function and therefore returns a copy in node 0.10 and newer.