Extending JavaScript Maps (or other built-in objects)

Sunday, April 27, 2014
Tags: Instantbird, programming, specifications, Thunderbird, Wat

Finally another technical post, this one is about my adventures in attempting to extend the built-in Map object in JavaScript to extend the functionality. As background, there are two reasons we’d want this:

  1. In the chat backend we currently use JavaScript objects ({}) as hashes/maps to keep track of various things (i.e. there’s a hash of conversations which map from conversation name to prplIConversation objects in the IRC code). Whenever checking to see if something is in this map we have to use hasOwnProperty. This has to be the version from Object.prototype in case the map has a conversation named hasOwnProperty. This is super simple code, but annoying:

    // Similar to Object.hasOwnProperty, but doesn't fail if the object
    // has a hasOwnProperty property set.
    function hasOwnProperty(aObject, aPropertyName)
      Object.prototype.hasOwnProperty.call(aObject, aPropertyName)

    Replacing these custom objects with a Map would alleviate this funky dance.

  2. Frequently in the chat backend we have to “normalize” [1] strings (e.g. #INsTanTBIrd and #instantbird are the same on IRC [2]). This is almost always done for sane storage of data received from the network (or from the user). I figured it’d be great if, instead of having to manually handle this normalization each time we tried to access data, the keys were magically normalized when accessing the data. (Note that although normalization is generally more complicated, just consider to be String.prototype.toLowerCase() for the rest of this post!)

This has been explored before by others, but generally in the context of web sites / cross browser compatibility. Which are concerns that don’t really limit us for backend code.


  1. Replace objects with Maps for safe access. This is pretty easily fixed by switching all obj["foo"] calls to obj.get("foo") (or the appropriate other method: set, delete, etc.)
  2. Automatically “normalize” keys in the some user defined way, e.g. such that obj.get("foo") and obj.get("FoO") return the same value.

First Approach (setting __proto__ to Map.prototype)

My first naive approach was to create an object with __proto__ set to Map.prototype and overwrite anything that uses keys to appropriately call a normalization function.

function NormalizedMap() { }
NormalizedMap.prototype = {
    __proto__: Map.prototype,
    _normalize: function(aStr) aStr.toLowerCase(),

    get: function(aStr) Map.prototype.get.call(this, this._normalize(aStr)),
    set: function(aStr, aVal) Map.prototype.set.call(this, this._normalize(aStr), aVal)

let m = new NormalizedMap();
m.set("foo", 1) // Throws TypeError: set method called on incompatible Object
m instanceof Map; // true . . . wat . . .

This throws an error and does not work. Apparently there are plans to support something like this. The totally fun thing, in my opinion, is that m is an instance of a Map!

Second Approach (modifying __proto__ after instance creation)

My second approach was to generate a real Map and then override the __proto__ to give it the properties I wanted:

function NormalizedMap() {
    let m = new Map();  m.__proto__ = NormalizedMap.prototype;
    return m;
NormalizedMap.prototype = {
    __proto__: Map.prototype,
    _normalize: function(aStr) aStr.toLowerCase(),
    get: function(aStr) Map.prototype.get.call(this, this._normalize(aStr)),
    set: function(aStr, aVal) Map.prototype.set.call(this, this._normalize(aStr), aVal)
let m = new NormalizedMap();
m.set("foo", 1)
m.get("FOO"); // 1
m instanceof Map; // true

This actually works! But will throw a warning each time it is created since changing an objects __proto__ is generally a bad idea. I also thought of overriding individual methods, but this seemed cumbersome and would increase the time in the constructor calls. (Which occur during the start up of each account and is generally a resource constrained time. No, I didn’t profile this, it just seemed like bad design.)

Solution (wrapping a Map)

Finally I settled on the simple solution of just wrapping the Map in a custom object. Initially I thought this would be frustrating to re-declare every function (and prone to breakage in the future if new methods are added), but there’s a nice magic method __noSuchMethod__ that fixes this! (Note that this is a non-standard feature of SpiderMonkey.) __noSuchMethod__ allows an object to intercept a call to a non-existent method (and in this case call that same method on the internal Map object).

Below is the final version that seems to act magically like a Map when necessary (e.g. iterating the map works, all functions and properties exist, the constructor works [3]). I need to thank aleth (another chat developer) who helped out quite a bit with this (and will ultimately be reviewing this code)!

// A Map that automatically normalizes keys before accessing the values.
function NormalizedMap(aNormalizeFunction, aIt = []) {
  if (typeof(aNormalizeFunction) != "function")
    throw "NormalizedMap must have a normalize function!";
  this._normalize = aNormalizeFunction;
  this._map = new Map([[this._normalize(key), val] for ([key, val] of aIt)]);
NormalizedMap.prototype = {
  _map: null,
  // The function to apply to all keys.
  _normalize: null,

  // Anything that accepts a key as an input needs to be manually overridden.
  delete: function(aKey) this._map.delete(this._normalize(aKey)),
  get: function(aKey) this._map.get(this._normalize(aKey)),
  has: function(aKey) this._map.has(this._normalize(aKey)),
  set: function(aKey, aValue) this._map.set(this._normalize(aKey), aValue),

  // Properties must be manually forwarded.
  get size() this._map.size,

  // Here's where the magic happens. If a method is called that isn't defined
  // here, just pass it to the internal _map object.
  __noSuchMethod__: function(aId, aArgs) this._map[aId].apply(this._map, aArgs)

The one downside of see of this is that properties must be declared manually to forward to the internal _map object. Maybe there is a matching __noSuchProperty__ method I’m missing? Overall, I’m happy with this solution, but please leave a comment if you can think of an easier / better way to do this! (Or see a glaring way this will break!)

[1]This is always a little bit of a sore subject in #instantbird since we’ve had a variety of issues with this over the years. I think we’ve fixed most of them at this point though!
[2]As I’ve written before, IRC tends to have crazy specifications. In IRC, the characters of A-Z[]\~ are considered the upper case of a-z{}|^ (“because of IRC’s Scandinavian origin”). Oh, also this can change based on an ISUPPORT response from the server to pure ASCII or RFC 1459 casemapping (A-Z[]\ map to a-z{}|). It seems like this could theoretically change at any point on a live server too, although that would be INSANE and I hope no one ever does that.
[3]I wrote some xpcshell tests to ensure these properties work as expected, but they’re uhh…not up anywhere yet though. Oops.