I know ECMAScript 6 has constructors but is there such a thing as destructors for ECMAScript 6?
For example if I register some of my object's methods as event listeners in the constructor, I want to remove them when my object is deleted.
One solution is to have a convention of creating a destructor method for every class that needs this kind of behaviour and manually call it. This will remove the references to the event handlers, hence my object will truly be ready for garbage collection. Otherwise it'll stay in memory because of those methods.
But I was hoping if ECMAScript 6 has something native that will be called right before the object is garbage collected.
If there is no such mechanism, what is a pattern/convention for such problems?
Is there such a thing as destructors for ECMAScript 6?
No. EcmaScript 6 does not specify any garbage collection semantics at all[1], so there is nothing like a "destruction" either.
If I register some of my object's methods as event listeners in the constructor, I want to remove them when my object is deleted
A destructor wouldn't even help you here. It's the event listeners themselves that still reference your object, so it would not be able to get garbage-collected before they are unregistered.
What you are actually looking for is a method of registering listeners without marking them as live root objects. (Ask your local eventsource manufacturer for such a feature).
1): Well, there is a beginning with the specification of WeakMap and WeakSet objects. However, true weak references are still in the pipeline [1][2].
I just came across this question in a search about destructors and I thought there was an unanswered part of your question in your comments, so I thought I would address that.
thank you guys. But what would be a good convention if ECMAScript
doesn't have destructors? Should I create a method called destructor
and call it manually when I'm done with the object? Any other idea?
If you want to tell your object that you are now done with it and it should specifically release any event listeners it has, then you can just create an ordinary method for doing that. You can call the method something like release() or deregister() or unhook() or anything of that ilk. The idea is that you're telling the object to disconnect itself from anything else it is hooked up to (deregister event listeners, clear external object references, etc...). You will have to call it manually at the appropriate time.
If, at the same time you also make sure there are no other references to that object, then your object will become eligible for garbage collection at that point.
ES6 does have weakMap and weakSet which are ways of keeping track of a set of objects that are still alive without affecting when they can be garbage collected, but it does not provide any sort of notification when they are garbage collected. They just disappear from the weakMap or weakSet at some point (when they are GCed).
FYI, the issue with this type of destructor you ask for (and probably why there isn't much of a call for it) is that because of garbage collection, an item is not eligible for garbage collection when it has an open event handler against a live object so even if there was such a destructor, it would never get called in your circumstance until you actually removed the event listeners. And, once you've removed the event listeners, there's no need for the destructor for this purpose.
I suppose there's a possible weakListener() that would not prevent garbage collection, but such a thing does not exist either.
FYI, here's another relevant question Why is the object destructor paradigm in garbage collected languages pervasively absent?. This discussion covers finalizer, destructor and disposer design patterns. I found it useful to see the distinction between the three.
Edit in 2020 - proposal for object finalizer
There is a Stage 3 EMCAScript proposal to add a user-defined finalizer function after an object is garbage collected.
A canonical example of something that would benefit from a feature like this is an object that contains a handle to an open file. If the object is garbage collected (because no other code still has a reference to it), then this finalizer scheme allows one to at least put a message to the console that an external resource has just been leaked and code elsewhere should be fixed to prevent this leak.
If you read the proposal thoroughly, you will see that it's nothing like a full-blown destructor in a language like C++. This finalizer is called after the object has already been destroyed and you have to predetermine what part of the instance data needs to be passed to the finalizer for it to do its work. Further, this feature is not meant to be relied upon for normal operation, but rather as a debugging aid and as a backstop against certain types of bugs. You can read the full explanation for these limitations in the proposal.
You have to manually "destruct" objects in JS. Creating a destroy function is common in JS. In other languages this might be called free, release, dispose, close, etc. In my experience though it tends to be destroy which will unhook internal references, events and possibly propagates destroy calls to child objects as well.
WeakMaps are largely useless as they cannot be iterated and this probably wont be available until ECMA 7 if at all. All WeakMaps let you do is have invisible properties detached from the object itself except for lookup by the object reference and GC so that they don't disturb it. This can be useful for caching, extending and dealing with plurality but it doesn't really help with memory management for observables and observers. WeakSet is a subset of WeakMap (like a WeakMap with a default value of boolean true).
There are various arguments on whether to use various implementations of weak references for this or destructors. Both have potential problems and destructors are more limited.
Destructors are actually potentially useless for observers/listeners as well because typically the listener will hold references to the observer either directly or indirectly. A destructor only really works in a proxy fashion without weak references. If your Observer is really just a proxy taking something else's Listeners and putting them on an observable then it can do something there but this sort of thing is rarely useful. Destructors are more for IO related things or doing things outside of the scope of containment (IE, linking up two instances that it created).
The specific case that I started looking into this for is because I have class A instance that takes class B in the constructor, then creates class C instance which listens to B. I always keep the B instance around somewhere high above. A I sometimes throw away, create new ones, create many, etc. In this situation a Destructor would actually work for me but with a nasty side effect that in the parent if I passed the C instance around but removed all A references then the C and B binding would be broken (C has the ground removed from beneath it).
In JS having no automatic solution is painful but I don't think it's easily solvable. Consider these classes (pseudo):
function Filter(stream) {
stream.on('data', function() {
this.emit('data', data.toString().replace('somenoise', '')); // Pretend chunks/multibyte are not a problem.
});
}
Filter.prototype.__proto__ = EventEmitter.prototype;
function View(df, stream) {
df.on('data', function(data) {
stream.write(data.toUpper()); // Shout.
});
}
On a side note, it's hard to make things work without anonymous/unique functions which will be covered later.
In a normal case instantiation would be as so (pseudo):
var df = new Filter(stdin),
v1 = new View(df, stdout),
v2 = new View(df, stderr);
To GC these normally you would set them to null but it wont work because they've created a tree with stdin at the root. This is basically what event systems do. You give a parent to a child, the child adds itself to the parent and then may or may not maintain a reference to the parent. A tree is a simple example but in reality you may also find yourself with complex graphs albeit rarely.
In this case, Filter adds a reference to itself to stdin in the form of an anonymous function which indirectly references Filter by scope. Scope references are something to be aware of and that can be quite complex. A powerful GC can do some interesting things to carve away at items in scope variables but that's another topic. What is critical to understand is that when you create an anonymous function and add it to something as a listener to ab observable, the observable will maintain a reference to the function and anything the function references in the scopes above it (that it was defined in) will also be maintained. The views do the same but after the execution of their constructors the children do not maintain a reference to their parents.
If I set any or all of the vars declared above to null it isn't going to make a difference to anything (similarly when it finished that "main" scope). They will still be active and pipe data from stdin to stdout and stderr.
If I set them all to null it would be impossible to have them removed or GCed without clearing out the events on stdin or setting stdin to null (assuming it can be freed like this). You basically have a memory leak that way with in effect orphaned objects if the rest of the code needs stdin and has other important events on it prohibiting you from doing the aforementioned.
To get rid of df, v1 and v2 I need to call a destroy method on each of them. In terms of implementation this means that both the Filter and View methods need to keep the reference to the anonymous listener function they create as well as the observable and pass that to removeListener.
On a side note, alternatively you can have an obserable that returns an index to keep track of listeners so that you can add prototyped functions which at least to my understanding should be much better on performance and memory. You still have to keep track of the returned identifier though and pass your object to ensure that the listener is bound to it when called.
A destroy function adds several pains. First is that I would have to call it and free the reference:
df.destroy();
v1.destroy();
v2.destroy();
df = v1 = v2 = null;
This is a minor annoyance as it's a bit more code but that is not the real problem. When I hand these references around to many objects. In this case when exactly do you call destroy? You cannot simply hand these off to other objects. You'll end up with chains of destroys and manual implementation of tracking either through program flow or some other means. You can't fire and forget.
An example of this kind of problem is if I decide that View will also call destroy on df when it is destroyed. If v2 is still around destroying df will break it so destroy cannot simply be relayed to df. Instead when v1 takes df to use it, it would need to then tell df it is used which would raise some counter or similar to df. df's destroy function would decrease than counter and only actually destroy if it is 0. This sort of thing adds a lot of complexity and adds a lot that can go wrong the most obvious of which is destroying something while there is still a reference around somewhere that will be used and circular references (at this point it's no longer a case of managing a counter but a map of referencing objects). When you're thinking of implementing your own reference counters, MM and so on in JS then it's probably deficient.
If WeakSets were iterable, this could be used:
function Observable() {
this.events = {open: new WeakSet(), close: new WeakSet()};
}
Observable.prototype.on = function(type, f) {
this.events[type].add(f);
};
Observable.prototype.emit = function(type, ...args) {
this.events[type].forEach(f => f(...args));
};
Observable.prototype.off = function(type, f) {
this.events[type].delete(f);
};
In this case the owning class must also keep a token reference to f otherwise it will go poof.
If Observable were used instead of EventListener then memory management would be automatic in regards to the event listeners.
Instead of calling destroy on each object this would be enough to fully remove them:
df = v1 = v2 = null;
If you didn't set df to null it would still exist but v1 and v2 would automatically be unhooked.
There are two problems with this approach however.
Problem one is that it adds a new complexity. Sometimes people do not actually want this behaviour. I could create a very large chain of objects linked to each other by events rather than containment (references in constructor scopes or object properties). Eventually a tree and I would only have to pass around the root and worry about that. Freeing the root would conveniently free the entire thing. Both behaviours depending on coding style, etc are useful and when creating reusable objects it's going to be hard to either know what people want, what they have done, what you have done and a pain to work around what has been done. If I use Observable instead of EventListener then either df will need to reference v1 and v2 or I'll have to pass them all if I want to transfer ownership of the reference to something else out of scope. A weak reference like thing would mitigate the problem a little by transferring control from Observable to an observer but would not solve it entirely (and needs check on every emit or event on itself). This problem can be fixed I suppose if the behaviour only applies to isolated graphs which would complicate the GC severely and would not apply to cases where there are references outside the graph that are in practice noops (only consume CPU cycles, no changes made).
Problem two is that either it is unpredictable in certain cases or forces the JS engine to traverse the GC graph for those objects on demand which can have a horrific performance impact (although if it is clever it can avoid doing it per member by doing it per WeakMap loop instead). The GC may never run if memory usage does not reach a certain threshold and the object with its events wont be removed. If I set v1 to null it may still relay to stdout forever. Even if it does get GCed this will be arbitrary, it may continue to relay to stdout for any amount of time (1 lines, 10 lines, 2.5 lines, etc).
The reason WeakMap gets away with not caring about the GC when non-iterable is that to access an object you have to have a reference to it anyway so either it hasn't been GCed or hasn't been added to the map.
I am not sure what I think about this kind of thing. You're sort of breaking memory management to fix it with the iterable WeakMap approach. Problem two can also exist for destructors as well.
All of this invokes several levels of hell so I would suggest to try to work around it with good program design, good practices, avoiding certain things, etc. It can be frustrating in JS however because of how flexible it is in certain aspects and because it is more naturally asynchronous and event based with heavy inversion of control.
There is one other solution that is fairly elegant but again still has some potentially serious hangups. If you have a class that extends an observable class you can override the event functions. Add your events to other observables only when events are added to yourself. When all events are removed from you then remove your events from children. You can also make a class to extend your observable class to do this for you. Such a class could provide hooks for empty and non-empty so in a since you would be Observing yourself. This approach isn't bad but also has hangups. There is a complexity increase as well as performance decrease. You'll have to keep a reference to object you observe. Critically, it also will not work for leaves but at least the intermediates will self destruct if you destroy the leaf. It's like chaining destroy but hidden behind calls that you already have to chain. A large performance problem is with this however is that you may have to reinitialise internal data from the Observable everytime your class becomes active. If this process takes a very long time then you might be in trouble.
If you could iterate WeakMap then you could perhaps combine things (switch to Weak when no events, Strong when events) but all that is really doing is putting the performance problem on someone else.
There are also immediate annoyances with iterable WeakMap when it comes to behaviour. I mentioned briefly before about functions having scope references and carving. If I instantiate a child that in the constructor that hooks the listener 'console.log(param)' to parent and fails to persist the parent then when I remove all references to the child it could be freed entirely as the anonymous function added to the parent references nothing from within the child. This leaves the question of what to do about parent.weakmap.add(child, (param) => console.log(param)). To my knowledge the key is weak but not the value so weakmap.add(object, object) is persistent. This is something I need to reevaluate though. To me that looks like a memory leak if I dispose all other object references but I suspect in reality it manages that basically by seeing it as a circular reference. Either the anonymous function maintains an implicit reference to objects resulting from parent scopes for consistency wasting a lot of memory or you have behaviour varying based on circumstances which is hard to predict or manage. I think the former is actually impossible. In the latter case if I have a method on a class that simply takes an object and adds console.log it would be freed when I clear the references to the class even if I returned the function and maintained a reference. To be fair this particular scenario is rarely needed legitimately but eventually someone will find an angle and will be asking for a HalfWeakMap which is iterable (free on key and value refs released) but that is unpredictable as well (obj = null magically ending IO, f = null magically ending IO, both doable at incredible distances).
If there is no such mechanism, what is a pattern/convention for such problems?
The term 'cleanup' might be more appropriate, but will use 'destructor' to match OP
Suppose you write some javascript entirely with 'function's and 'var's.
Then you can use the pattern of writing all the functions code within the framework of a try/catch/finally lattice. Within finally perform the destruction code.
Instead of the C++ style of writing object classes with unspecified lifetimes, and then specifying the lifetime by arbitrary scopes and the implicit call to ~() at scope end (~() is destructor in C++), in this javascript pattern the object is the function, the scope is exactly the function scope, and the destructor is the finally block.
If you are now thinking this pattern is inherently flawed because try/catch/finally doesn't encompass asynchronous execution which is essential to javascript, then you are correct. Fortunately, since 2018 the asynchronous programming helper object Promise has had a prototype function finally added to the already existing resolve and catch prototype functions. That means that that asynchronous scopes requiring destructors can be written with a Promise object, using finally as the destructor. Furthermore you can use try/catch/finally in an async function calling Promises with or without await, but must be aware that Promises called without await will be execute asynchronously outside the scope and so handle the desctructor code in a final then.
In the following code PromiseA and PromiseB are some legacy API level promises which don't have finally function arguments specified. PromiseC DOES have a finally argument defined.
async function afunc(a,b){
try {
function resolveB(r){ ... }
function catchB(e){ ... }
function cleanupB(){ ... }
function resolveC(r){ ... }
function catchC(e){ ... }
function cleanupC(){ ... }
...
// PromiseA preced by await sp will finish before finally block.
// If no rush then safe to handle PromiseA cleanup in finally block
var x = await PromiseA(a);
// PromiseB,PromiseC not preceded by await - will execute asynchronously
// so might finish after finally block so we must provide
// explicit cleanup (if necessary)
PromiseB(b).then(resolveB,catchB).then(cleanupB,cleanupB);
PromiseC(c).then(resolveC,catchC,cleanupC);
}
catch(e) { ... }
finally { /* scope destructor/cleanup code here */ }
}
I am not advocating that every object in javascript be written as a function. Instead, consider the case where you have a scope identified which really 'wants' a destructor to be called at its end of life. Formulate that scope as a function object, using the pattern's finally block (or finally function in the case of an asynchronous scope) as the destructor. It is quite like likely that formulating that functional object obviated the need for a non-function class which would otherwise have been written - no extra code was required, aligning scope and class might even be cleaner.
Note: As others have written, we should not confuse destructors and garbage collection. As it happens C++ destructors are often or mainly concerned with manual garbage collection, but not exclusively so. Javascript has no need for manual garbage collection, but asynchronous scope end-of-life is often a place for (de)registering event listeners, etc..
Here you go. The Subscribe/Publish object will unsubscribe a callback function automatically if it goes out of scope and gets garbage collected.
const createWeakPublisher = () => {
const weakSet = new WeakSet();
const subscriptions = new Set();
return {
subscribe(callback) {
if (!weakSet.has(callback)) {
weakSet.add(callback);
subscriptions.add(new WeakRef(callback));
}
return callback;
},
publish() {
for (const weakRef of subscriptions) {
const callback = weakRef.deref();
console.log(callback?.toString());
if (callback) callback();
else subscriptions.delete(weakRef);
}
},
};
};
Although it might not happen immediately after the callback function goes out of scope, or it might not happen at all. See weakRef documentation for more details. But it works like a charm for my use case.
You might also want to check out the FinalizationRegistry API for a different approach.
"A destructor wouldn't even help you here. It's the event listeners
themselves that still reference your object, so it would not be able
to get garbage-collected before they are unregistered."
Not so. The purpose of a destructor is to allow the item that registered the listeners to unregister them. Once an object has no other references to it, it will be garbage collected.
For instance, in AngularJS, when a controller is destroyed, it can listen for a destroy event and respond to it. This isn't the same as having a destructor automatically called, but it's close, and gives us the opportunity to remove listeners that were set when the controller was initialized.
// Set event listeners, hanging onto the returned listener removal functions
function initialize() {
$scope.listenerCleanup = [];
$scope.listenerCleanup.push( $scope.$on( EVENTS.DESTROY, instance.onDestroy) );
$scope.listenerCleanup.push( $scope.$on( AUTH_SERVICE_RESPONSES.CREATE_USER.SUCCESS, instance.onCreateUserResponse ) );
$scope.listenerCleanup.push( $scope.$on( AUTH_SERVICE_RESPONSES.CREATE_USER.FAILURE, instance.onCreateUserResponse ) );
}
// Remove event listeners when the controller is destroyed
function onDestroy(){
$scope.listenerCleanup.forEach( remove => remove() );
}
Javascript does not have destructures the same way C++ does. Instead, alternative design patterns should be used to manage resources. Here are a couple of examples:
You can restrict users to using the instance for the duration of a callback, after which it'll automatically be cleaned up. (This pattern is similar to the beloved "with" statement in Python)
connectToDatabase(async db => {
const resource = await db.doSomeRequest()
await useResource(resource)
}) // The db connection is closed once the callback ends
When the above example is too restrictive, another alternative is to just create explicit cleanup functions.
const db = makeDatabaseConnection()
const resource = await db.doSomeRequest()
updatePageWithResource(resource)
pageChangeEvent.addListener(() => {
db.destroy()
})
The other answers already explained in detail that there is no destructor. But your actual goal seems to be event related. You have an object which is connected to some event and you want this connection to go away automatically when the object is garbage collected. But this won't happen because the event subscription itself references the listener function. Well, UNLESS you use this nifty new WeakRef stuff.
Here is an example:
<!DOCTYPE html>
<html>
<body>
<button onclick="subscribe()">Subscribe</button>
<button id="emitter">Emit</button>
<button onclick="free()">Free</button>
<script>
const emitter = document.getElementById("emitter");
let listener = null;
function addWeakEventListener(element, event, callback) {
// Weakrefs only can store objects, so we put the callback into an object
const weakRef = new WeakRef({ callback });
const listener = () => {
const obj = weakRef.deref();
if (obj == null) {
console.log("Removing garbage collected event listener");
element.removeEventListener(event, listener);
} else {
obj.callback();
}
};
element.addEventListener(event, listener);
}
function subscribe() {
listener = () => console.log("Event fired!");
addWeakEventListener(emitter, "click", listener);
console.log("Listener created and subscribed to emitter");
}
function free() {
listener = null;
console.log("Reference cleared. Now force garbage collection in dev console or wait some time before clicking Emit again.");
}
</script>
</body>
</html>
(JSFiddle)
Clicking the Subscribe button creates a new listener function and registers it at the click event of the Emit button. So clicking the Emit button after that prints a message to the console. Now click the Free button which simply sets the listener variable to null so the garbage collector can remove the listener. Wait some time or force gargabe collection in the developer console and then click the Emit button again. The wrapper listener function now sees that the actual listener (wrapped in a WeakRef) is no longer there and then unsubscribes itself from the button.
WeakRefs are quite powerful but note that there is no guarantee if and when your stuff is garbage collected.
The answer to the question as-stated in the title is FinalizationRegistry, available since Firefox 79 (June 2020), Chrome 84 and derivatives (July 2020), Safari 14.1 (April 2021), and Node 14.6.0 (July 2020)… however, a native JS destructor is probably not the right solution for your use-case.
function create_eval_worker(f) {
let src_worker_blob = new Blob([f.toString()], {type: 'application/javascript'});
let src_worker_url = URL.createObjectURL(src_worker_blob);
async function g() {
let w = new Worker(src_worker_url);
…
}
// Run URL.revokeObjectURL(src_worker_url) as a destructor of g
let registry = new FinalizationRegistry(u => URL.revokeObjectURL(u));
registry.register(g, src_worker_url);
return g;
}
}
Caveat:
Avoid where possible
Correct use of FinalizationRegistry takes careful thought, and it's best avoided if possible. When, how, and whether garbage collection occurs is down to the implementation of any given JavaScript engine. Any behavior you observe in one engine may be different in another engine, in another version of the same engine, or even in a slightly different situation with the same version of the same engine.
…
Developers shouldn't rely on cleanup callbacks for essential program logic. Cleanup callbacks may be useful for reducing memory usage across the course of a program, but are unlikely to be useful otherwise.
A conforming JavaScript implementation, even one that does garbage collection, is not required to call cleanup callbacks. When and whether it does so is entirely down to the implementation of the JavaScript engine. When a registered object is reclaimed, any cleanup callbacks for it may be called then, or some time later, or not at all.
–Mozilla Developer Network
I'm currently working with sockets.io in Node.js, I have a class called Rooms, their functions are self explanatory, its basic model looks like:
Room (owner)
this.owner = owner
occupants = []
Room.prototype = {
sendMessage()
getUsers()
leaveParty()
}
But I want to make one for destroying its self, I tried doing,
Room.prototype.destroy = function() {
this = undefined
}
and then doing
var roomVariable = new Room('blah');
roomVariable.destroy.call(roomVariable);
But that does not work, I'm running out of ideas on how to make this destroy its self, basically after there are no more users left in occupants, I want it erased from memory and all. Thanks!
Quick answer is: you can't.
Long answer is: destruction of a JS object and subsequent garbage collection is only possible outside of scope. You will need to hunt down and delete/unset all references of an item. This sucks, I know, but that's how it is.
Much like in PHP, this in a prototype method is actually not the object itself, merely an interface to it. You can't unset it, you can't re-define it (it'd lead to chaos and confusion otherwise).
The proper form for this is to let something else, an object manager, know that you intend to delete this. At which point, this object does the actual GC.
Your Room's destroy()/dispose() method should release any resources that must be explicitly released (like an open transaction that doesn't auto-commit) and signal any known "watchers" or "subscribers" that it's closing (so they can remove their references to it). this can include your RoomContainer, or you can make your RoomContainer responsible for removing references to expired rooms. In either case, once there are no more references to the room, the garbage collector is free to remove it from memory.
I am creating an object inside of an enclosure. Also in the enclosure are private properties that the object's functions can access - and this works as expected.
My issue: I want others to be able to extend my object with functions of their own (functions from a different context), but those functions will also need access to the same private properties - and I have not been able to find a way to make this work.
I've tried various configurations of .call, and also wrapping their function in a new function, amongst other things. I feel like I've gotten close to a solution, but have just fallen short.
Here's a bit of simplified example code that accurately reflects my situation:
//extension object
//fn2 can be any function, with any number of arguments, etc.
var obj1 = {};
obj1.fn2 = function (s1, s2){ console.log(priv); };
//actual object
var obj2 = (function (){
//private property
var priv = "hello world";
//return object
var obj3 = {};
//return object's native fn (works)
obj3.fn = function (s){ console.log(priv); };
//extension happens here - but is obviously not correct
obj3.fn2 = obj1.fn2;
//return object
return obj3;
})();
//try output
obj2.fn("goodbye world"); //works
obj2.fn2("goodbye world", "thx 4 teh phish"); //fails
Any insight would be appreciated. And I totally understand if what I want just isn't possible - but it sure seems like it should be :P
EDIT: Thank you all for the responses. I fully understand that the properties are more easily accessed as public, and that normally inherited objects won't have access to them otherwise. However, since the new function is being attached to the original object I have to believe there's a way to use the original context and not the context the new function was created in.
Now, I'm the first to say that eval is evil - and, in fact, I've never used it, or even considered using it, before. However, I'm trying everything I can think of to make this work - and I stumbled across this (seemingly) working solution:
obj3.fn2 = eval(obj1.fn2.toString());
So, if I check to make sure that obj1.fn2 is a typeof function, is there any way this could be harmful to my code? It doesn't execute the function, so I can't see how - but maybe I'm missing something?
Javascript doesn't have a "protected" analog. You either get super private or completely public. From here you can choose to:
Reconsider your class design, and have the subclasses depend only on the public interface of the parent class.
Add getter and setter functions to the public interface. Not necessarily the best thing though as you might just as well make the properties public (besides best practice issues and whatnot)
Just use public properties instead. This is the "natural" way to do OO inheritance in Javascript and is usually not a problem if you use a donvention like adding an underscore to the beggining of the name. As a bonus you can use the prototypal inheritance feature (it is nice knowing how to use this instead of only closure-based classes)
function Base(){
this._priv = "Hello world"
};
Base.prototype = {
fn: function(){
console.log(this._priv);
}
}
var obj2 = new Base();
obj2.fn = function(){ ... }
I hate to answer my own question - seems like a bit of a faux pas - but c'est la vie. (because I woke up French today?)
So, while I found that the eval() solution I presented last night in the edit to my original question does seem to be a valid solution, and a proper use of eval for retaining the object's context within the new function, it is far from perfect.
Firstly, it works in FF, but both IE and Chrome seem to hate it (those were the next ones I tried, and I quit trying others after they both failed). Though I'm sure it could probably be made to work across browsers, it seems like a hassle.
Secondly, it does give quite a bit of power to the new function, and as I look at my code more I do like the idea of controlling exactly what these new functions being added to my object get access to.
Thirdly, .eval() is typically pretty slow - and it turns out that .apply() (which is typically faster) just may work well enough.
This is because I realized at some point last night that no new functions on this object will need to set any of the private variables (at least, I'm fairly certain they won't) - and .apply() works fine to pass the values through for them to read.
I'm sure there's more to it than just those 3 things, but for now I think I'm going to go with more of a 'wrapper' solution - something like this:
var f = function (){
var fauxThis = {};
fauxThis.priv = priv;
obj1.fn2.apply(fauxThis, arguments);
};
obj3.fn2 = f;
//(To be placed where I had "obj3.fn2 = obj1.fn2;")
I am certainly willing now to consider the use of eval() in very specific cases - and may even revisit this specific use of it before I make my final decision of which direction to take. (especially if I can think of a case where the private value would need to be set)
Thanks all for your input!
The quickest and easiest solution is to prefix any supposedly private properties with the underscore (_).
Personally I like to bottle my private properties into a single object which would be placed on the object, like so:
obj.publicProp = 20;
obj._.privateProp = true;
I wouldn't worry so much about it though, the underscore is basically a universal symbol for private so those using the script will know that it's private and shouldn't be touched. Or, better yet, just leave it out of the public documentation ;)
There are other methods and you can use which do emulate "true" protected variables, but they're not the best as they avoid garbage collection, and can be clunky to use.
In terms of memory consumption, are these equivalent or do we get a new function instance for every object in the latter?
var f=function(){alert(this.animal);}
var items=[];
for(var i=0;i<10;++i)
{
var item={"animal":"monkey"};
item.alertAnimal=f;
items.push(item);
}
and
var items=[];
for(var i=0;i<10;++i)
{
var item={"animal":"monkey"};
item.alertAnimal=function(){alert(this.animal);};
items.push(item);
}
EDIT
I'm thinking that in order for closure to work correctly, the second instance would indeed create a new function each pass. Is this correct?
You should pefer the first method, since the second one creates a function every time the interpreter passes that line.
Regarding your edit: We are in the same scope all the time, since JavaScript has function scope instead of block scope, so this might be optimizable, but i did not encounter an implementation that doesn't create it every time. I would recommend not to rely on this (probably possible) optimization, since implementations that lack support could likely exceed memory limits if you use this technique extensively (which is bad, since you do not know what implementation will run it, right?).
I am not an expert, but it seems to me that different javascript engines could be handling this in different ways.
For example, V8 has something called hidden classes, which could affect memory consumption when accessing the same property. Maybe somebody can confirm or deny this.
Consider the following Javascript function (1):
function setData(domElement) {
domElement.myDataProperty = {
'suppose': 'this',
'object': 'is',
'static': 'and',
'pretty': 'big'
};
};
Now what I don't like about this function is that the exact same object is created every time the function is called. Since the object does not change I would rather create it just once. So we could make the following adjustments (2):
var dataObject = {
'suppose': 'this',
'object': 'is',
'static': 'and',
'pretty': 'big'
};
function setData(domElement) {
domElement.myDataProperty = dataObject;
};
Now the object is created once when the script is loaded and stored in dataObject. But let's assume that setData is called only occasionally -- most of the times that the script is loaded the function is not used. What I don't like about this function in that case is that the object is always created and held in memory, including many occasions in which it will never be used. I figured you could do something like this to strike the ideal balance (3):
var dataObject;
function setData(domElement) {
if (!dataObject) {
dataObject = {
'suppose': 'this',
'object': 'is',
'static': 'and',
'pretty': 'big'
};
}
domElement.myDataProperty = dataObject;
};
Would that make sense? I figure it depends on when the interpreter decides to create an object. Does it really wait until it passes the !dataObject condition, or does it enter the function, tries to be smart and decides to construct it in advance? Perhaps different Javascript engines have different policies with regard to this?
Then of course there is the question of whether these optimizations will ever matter in practice. Obviously this depends on factors like the size of the object, the speed of the engine, the amount of resources available, etc.. But in general, which one would you say is the more significant optimization: from (1) to (2) or from (2) to (3)?
The answer is, you're not supposed to know. The examples you showed have very little difference between them. The only way you'd ever reasonably worry about this is if you had actual evidence that one way or another was noticably harming performance or memory usage on a particular interpreter. Until then, it's the interpreter's job to worry about that stuff for you.
That said, if you really want to know... Try it and find out. call the different versions 1,000,000 times and see what difference it makes.
Make a giant version of the object and see if that makes a dent. Watch task manager. Try different browsers. Report back your results. It's a much better way to find out than just asking a bunch of jerks on the internet what they guess might be the case.
just keep in mind that object has to be in memory anyway, regardless ... as source text
A new object must be created -- it cannot not be, partially because the spec requires it, but mostly because alternative behaviour would be counter intuitive, take:
function f() {
return {a : "b", c: "d"};
}
o=f();
alert([o.c, o.e]); // Alerts "b,"
delete o.c;
o.e="f";
o=f();
alert([o.c, o.e]); // If the object was only created once this would produce ",f"
Do you really expect a new object expression to not actually produce the object you're asking for? Because that's what you seem to want.
Conceivably you just want to do:
var myFunction = (function(){
var object = {a: "b", c: "d"};
return function() { return object; }
})();
Which would get the effect you want, although you would have to realise that the object you're returning is a completely mutable object that can be changed, and everyone would be sharing that same mutating instance.
First, I'd implement it in situation #2 and load it once immediately after the page is loaded.
If there was a problem with page speed, I would measure the time taken for specific tasks within the page.
If it was very expensive to create the object (relatively speaking), then I would move to situation #3.
There's no point in adding the 'if' statement if it really doesn't buy you anything... and in this case, creating a simple/big object is no sweat off your CPU's back. Without measurements, you're not optimizing - you're just shooting blind.
It's actually a fairly common method of initializing things that I've personally used in C++ and Java.
First, this optimization will never matter in practice.
Second, the last function is exactly as good as the first function. Well, almost. In the first I suppose you're at the mercy of the garbage collector, which should destroy the old object when you reassign domElement.myDataProperty. Still, without knowing exactly how the garbage collector works on your target platform (and it can be very different across browsers), you can't be sure you're saving any work at all really.
Try all three of them in a couple of browsers and find out which is faster.