node-weak

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 node-weak comes in! This module exports V8's Persistent<Object> 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 before 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

Example

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

var weak = require('weak')

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

// The function to call before Garbage Collection.
// Note that by the time this is called, 'obj' has been set to `null`.
function cleanup (o) {
  delete o.a
  delete o.foo
}

// Here's where we set up the weak reference
var ref = weak(obj, function (obj) {
  // `this` inside the callback is the EventEmitter.
  // The first argument to the callback is the `obj`.
  // DO NOT store any new references to the object, and
  // DO NOT use the object in any async functions.
  cleanup(obj)
})

// 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 node-weak, 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')

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. The callback will be invoked with obj as its first argument.

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)

Checks to see if ref is "near death". This will be true exactly during the weak reference callback function, and false any other time.

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.

undefined 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.

Array weak.callbacks(Weakref ref)

Returns the internal Array that ref iterates through to invoke the GC callbacks. The array can be push()ed or unshift()ed onto, to have more control over the execution order of the callbacks. This is similar in concept to node's EventEmitter#listeners() function.