node-weak-ref

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-ref 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 node-weak-ref

Install with yarn:

$ yarn add node-weak-ref

Example

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

import { create } from "weak"

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

// Here's where we set up the weak reference
const ref = create(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 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:

const http = require("http")
const { create } = 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 create(Object obj [, Function callback])

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 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 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 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 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 removeCallback(Weakref ref, Function callback)

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

EventEmitter removeCallbacks(Weakref ref)

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

Array 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.