Develop Reference

What's the purpose of Symbol in terms of unique object identifiers?

Notes about 'Not a duplicate':
I've been told this is a duplicate of What is the use of Symbol in javascript ECMAScript 6?. Well, it doesn't seem right to me. The code they've given is this:
const door = {};
// library 1
const cake1 = Symbol('cake');
door[cake1] = () => console.log('chocolate');
// library 2
const cake2 = Symbol('cake');
door[cake2] = () => console.log('vanilla');
// your code
door[cake1]();
door[cake2]();
The only thing that makes this work is because cake1 and cake2 are different (unique) names. But the developer has explicitly given these; there is nothing offered by Symbol which helps here.
For example if you change cake1 and cake2 to cake and run it, it will error:
Uncaught SyntaxError: Identifier 'cake' has already been declared
If you're already having to manually come up with unique identifiers then how is Symbol helping?
If you execute this in your console:
Symbol('cake') === Symbol('cake');
It evaluates to false. So they're unique. But in order to actually use them, you're now having to come up with 2 key names (cake1 and cake2) which are unique. This has to be done manually by the developer; there's nothing in Symbol or JavaScript in general which will help with that. You're basically creating a unique identifier using Symbol but then having to assign it manually to...a unique identifier that you've had to come up with as a developer.
With regards to the linked post they cite this as an example which does not use Symbol:
const door = {};
// from library 1
door.cake = () => console.log('chocolate');
// from library 2
door.cake = () => console.log('vanilla');
// your code
door.cake();
They try to claim this is a problem and will only log "vanilla". Well clearly that's because door.cake isn't unique (it's declared twice). The "fix" is as simple as using cake1 and cake2:
door.cake1 = () => console.log('chocolate');
door.cake2 = () => console.log('vanilla');
door.cake1(); // Outputs "chocolate"
door.cake2(); // Outputs "vanilla"
That will now work and log both "chocolate" and "vanilla". In this case Symbol hasn't been used at all, and indeed has no bearing on that working. It's simply a case that the developer has assigned a unique identifier but they have done this manually and without using Symbol.
Original question:
I'm taking a course in JavaScript and the presenter is discussing Symbol.
At the beginning of the video he says:
The thing about Symbol's is that every single one is unique and this makes them very valuable in terms of things like object property identifiers.
However he then goes on to say:
They are not enumerable in for...in loops.
They cannot be used in JSON.stringify. (It results in an empty object).
In the case of point (2) he gives this example:
console.log(JSON.stringify({key: 'prop'})); // object without Symbol
console.log(JSON.stringify({Symbol('sym1'): 'prop'})); // object using Symbol
This logs {"key": "prop"} and {} to the console respectively.
How does any of this make Symbol "valuable" in terms of being unique object keys or identifiers?
In my experience two very common things you'd want to do with an object is enumerate it, or convert the data in them to JSON to send via ajax or some such method.
I can't understand what the purpose of Symbol is at all, but especially why you would want to use them for making object identifiers? Given it will cause things later that you cannot do.
Edit - the following was part of the original question - but is a minor issue in comparison to the actual purpose of Symbol with respect to unique identifiers:
If you needed to send something like {Symbol('sym1'): 'prop'} to a backend via ajax what would you actually need to do in this case?

I replied to your comment in the other question, but since this is open I'll try to elaborate.
You are getting variable names mixed up with Symbols, which are unrelated to one another.
The variable name is just an identifier to reference a value. If I create a variable and then set it to something else, both of those refer to the same value (or in the case of non-primitives in JavaScript, the same reference).
In that case, I can do something like:
const a = Symbol('a');
const b = a;
console.log(a === b); // true
That's because there is only 1 Symbol created and the reference to that Symbol is assigned to both a and b. That isn't what you would use Symbols for.
Symbols are meant to provide unique keys which are not the same as a variable name. Keys are used in objects (or similar). I think the simplicity of the other example may be causing the confusion.
Let us imagine a more complex example. Say I have a program that lets you create an address book of people. I am going to store each person in an object.
const addressBook = {};
const addPerson = ({ name, ...data }) => {
addressBook[name] = data;
};
const listOfPeople = [];
// new user is added in the UI
const newPerson = getPersonFromUserEntry();
listOfPeople.push(newPerson.name);
addPerson(newPerson);
In this case, I would use listOfPeople to display a list and when you click it, it would show the information for that user.
Now, the problem is, since I'm using the person's name, that isn't truly unique. If I have two "Bob Smith"'s added, the second will override the first and clicking the UI from "listOfPeople" will take you to the same one for both.
Now, instead of doing that, lets use a Symbol in the addPerson() and return that and store it in listOfPeople.
const addressBook = {};
const addPerson = ({ name, ...data }) => {
const symbol = Symbol(name);
addressBook[symbol] = data;
return symbol;
};
const listOfPeople = [];
// new user is added in the UI
const newPerson = getPersonFromUserEntry();
listOfPeople.push(addPerson(newPerson));
Now, every entry in listOfPeople is totally unique. If you click the first "Bob Smith" and use that symbol to look him up you'll get the right one. Ditto for the second. They are unique even though the base of the key is the same.
As I mentioned in the other answer, the use-case for Symbol is actually fairly narrow. It is really only when you need to create a key you know will be wholly unique.
Another scenario where you might use it is if you have multiple independent libraries adding code to a common place. For example, the global window object.
If my library exports something to window named "getData" and someone has a library that also exports a "getData" one of us is going to override the other if they are loaded at the same time (whoever is loaded last).
However, if I want to be safer, instead of doing:
window.getData = () => {};
I can instead create a Symbol (whose reference I keep track of) and then call my getData() with the symbol:
window[getDataSymbol]();
I can even export that Symbol to users of my library so they can use that to call it instead.
(Note, all of the above would be fairly poor naming, but again, just an example.)
Also, as someone mentioned in the comments, these Symbols are not for sharing between systems. If I call Symbol('a') that is totally unique to my system. I can't share it with anyone else. If you need to share between systems you have to make sure you are enforcing key uniqueness.

As a very practical example what kind of problem Symbols solve, take angularjs's use of $ and $$:
AngularJS Prefixes $ and $$: To prevent accidental name collisions with your code, AngularJS prefixes names of public objects with $ and names of private objects with $$. Please do not use the $ or $$ prefix in your code.
https://docs.angularjs.org/api
You'll sometimes have to deal with objects that are "yours", but that Angular adds its own $ and $$ prefixed properties to, simply as a necessity for tracking certain states. The $ are meant for public use, but the $$ you're not supposed to touch. If you want to serialise your objects to JSON or such, you need to use Angular's provided functions which strip out the $-prefixed properties, or you need to otherwise be aware of dealing with those properties correctly.
This would be a perfect case for Symbols. Instead of adding public properties to objects which are merely differentiated by a naming convention, Symbols allow you to add truly private properties which only your code can access and which don't interfere with anything else. In practice Angular would define a Symbol once somewhere which it shares across all its modules, e.g.:
export const PRIVATE_PREFIX = Symbol('$$');
Any other module now imports it:
import { PRIVATE_PREFIX } from 'globals';
function foo(userDataObject) {
userDataObject[PRIVATE_PREFIX] = { foo: 'bar' };
}
It can now safely add properties to any and all objects without worrying about name clashes and without having to advise the user about such things, and the user doesn't need to worry about Angular adding any of its own properties since they won't show up anywhere. Only code which has access to the PRIVATE_PREFIX constant can access these properties at all, and if that constant is properly scoped, that's only Angular-related code.
Any other library or code could also add its own Symbol('$$') to the same object, and it would still not clash because they're different symbols. That's the point of Symbols being unique.
(Note that this Angular use is hypothetical, I'm just using its use of $$ as a starting point to illustrate the issue. It doesn't mean Angular actually does this in any way.)

To expand on #samanime's excellent answer, I'd just like to really put emphasis on how Symbols are most commonly used by real developers.
Symbols prevent key name collision on objects.
Let's inspect the following page from MDN on Symbols. Under "Properties", you can see some built-in Symbols. We'll look at the first one, Symbol.iterator.
Imagine for a second that you're designing a language like JavaScript. You've added special syntax like for..of and would like to allow developers to define their own behavior when their special object or class is iterated over using this syntax. Perhaps for..of could check for a special function defined on the object/class, named iterator:
const myObject = {
iterator: function() {
console.log("I'm being iterated over!");
}
};
However, this presents a problem. What if some developer, for whatever reason, happens to name their own function property iterator:
const myObject = {
iterator: function() {
//Iterate over and modify a bunch of data
}
};
Clearly this iterator function is only meant to be called to perform some data manipulation, probably very infrequently. And yet if some consumer of this library were to think myObject is iterable and use for..of on it, JavaScript will go right ahead and call that function, thinking it's supposed to return an iterator.
This is called a name collision and even if you tell every developer very firmly "don't name your object properties iterator unless it returns a proper iterator!", someone is bound to not listen and cause problems.
Even if you don't think just that one example is worthy of this whole Symbol thing, just look at the rest of the list of well-known symbols. replace, match, search, hasInstance, toPrimitive... So many possible collisions! Even if every developer is made to never use these as keys on their objects, you're really restricting the set of usable key names and therefore developer freedom to implement things how they want.
Symbols are the perfect solution for this. Take the above example, but now JavaScript doesn't check for a property named "iterator", but instead for a property with a key exactly equal to the unique Symbol Symbol.iterator. A developer wishing to implement their own iterator function writes it like this:
const myObject = {
[Symbol.iterator]: function() {
console.log("I'm being iterated over!");
}
};
...and a developer wishing to simply not be bothered and use their own property named iterator can do so completely freely without any possible hiccups.
This is a pattern developers of libraries may implement for any unique key they'd like to check for on an object, the same way the JavaScript developers have done it. This way, the problem of name collisions and needing to restrict the valid namespace for properties is completely solved.
Comment from the asker:
The bit which confused me on the linked OP is they've created 2 variables with the names cake1 and cake2. These names are unique and the developer has had to determine them so I didn't understand why they couldn't assign the variable to the same name, as a string (const cake1 = 'cake1'; const cake2 = 'cake2'). This could be used to make 2 unique key names since the strings 'cake1' !== 'cake2'. Also the answer says for Symbol you "can't share it" (e.g. between libraries) so what use is that in terms of avoiding conflict with other libraries or other developers code?
The linked OP I think is misleading - it seems the point was supposed to be that both symbols have the value "cake" and thus you technically have two duplicate property keys with the name "cake" on the object which normally isn't possible. However, in practice the capability for symbols to contain values is not really useful. I understand your confusion there, again, I think it was just another example of avoiding key name collision.
About the libraries, when a library is published, it doesn't publish the value generated for the symbol at runtime, it publishes code which, when added to your project, generates a completely unique symbol different than what the developers of the library had. However, this means nothing to users of the library. The point is that you can't save the value of a symbol, transfer it to another machine, and expect that symbol reference to work when running the same code. To reiterate, a library has code to create a symbol, it doesn't export the generated value of any symbols.

What's the purpose of Symbol in terms of unique object identifiers?
Well,
Symbol( 'description' ) !== Symbol( 'description' )
How does any of this make Symbol "valuable" in terms of being unique object keys or identifiers?
In a visitor pattern or chain of responsibility, some logic may add additional metadata to any object and that's it (imagine some validation OR ORM metadata) attached to objects but that does not persist *.
If you needed to send something like {Symbol('sym1'): 'prop'} to a backend via ajax what would you actually need to do in this case?
If I may assure you, you won't need to do that. you would consider { sym1: 'prop' } instead.
Now, this page even has a note about it
Note: If you are familiar with Ruby's (or another language) that also has a feature called "symbols", please don’t be misguided. JavaScript symbols are different.
As I said, there are useful for runtime metadata and not effective data.

Best (most performant) way to declare (class) properties with unknown values in v8

So I learned a bit about the hidden class concept in v8. It is said that you should declare all properties in the constructor (if using prototype based "pseudo classes") and that you should not delete them or add new ones outside of the constructor. So far, so good.
1) But what about properties where you know the type (that you also shouldn't change) but not the (initial) value?
For example, is it sufficient to do something like this:
var Foo = function () {
this.myString;
this.myNumber;
}
... and assign concrete values later on, or would it be better to assign a "bogus" value upfront, like this:
var Foo = function () {
this.myString = "";
this.myNumber = 0;
}
2) Another thing is with objects. Sometimes I just know that an object wont have a fixed structure, but I want to use it as a hash map. Is there any (non verbose) way to tell the compiler I want to use it this way, so that it isn't optimized (and deopted later on)?
Update
Thanks for your input! So after reading your comments (and more on the internet) I consider these points as "best practices":
Do define all properties of a class in the constructor (also applies for defining simple objects)
You have to assign something to these properties, even if thats just null or undefined - just stating this.myString; is apparently not enough
Because you have to assign something anyways I think assigning a "bogus" value in case you can't assign the final value immediatly cannot hurt, so that the compiler does "know" ASAP what type you want to use. So, for example this.myString = "";
In case of objects, do assign the whole structure if you know it beforehand, and again assign dummy values to it's properties if you don't know them immediatly. Otherwise, for example when intending to use the Object as a hashmap, just do: this.myObject = {};. Think its not worth indicating to the compiler that this should be a hashmap. If you really want to do this, I found a trick that assigns a dummy property to this object and deletes it immediatly afterwards. But I won't do this.
As for smaller Arrays it's apparently recommended (reference: https://www.youtube.com/watch?v=UJPdhx5zTaw&feature=youtu.be&t=25m40s) to preallocate them especially if you know the final size, so for example: this.myArray = new Array(4);
Don't delete properties later on! Just null them if needed
Don't change types after assigning! This will add another hidden class and hurt performance. I think thats best practice anyways. The only case where I have different types is for certain function arguments anyways. In that case I usually convert them to the same target type.
Same applies if you keep adding additional properties later on.
That being said, I also think doing this will lean to cleaner and more organized code, and also helps with documenting.
Yeah, so one little thing I am unsure remains: What if I define properties in a function (for example a kind of configure() method) called within the constructor?

Re 1): Just reading properties, like in your first snippet, does not do anything to the object. You need to assign them to create the properties.
But for object properties it doesn't actually matter much what values you initialise them with, as long as you do initialise them. Even undefined should be fine.
The concrete values are much more relevant for arrays, where you want to make sure to create them with the right elements (and without any holes!) because the VM tries to keep them homogeneous. In particular, never use the Array constructor, because that creates just holes.
Re 2): There are ways to trick the VM into using a dictionary representation, but they depend on VM and version and aren't really reliable. In general, it is best to avoid using objects as maps altogether. Since ES6, there is a proper Map class.

How to obtain true de-referenced object instances in javascript

I'm trying to get a better understanding of object oriented patterns in JavaScript. I particulary like the way EmberJS implements their classes with .extend and .create from Parent class Objects.
I've tried to implement a basic version of this on my own, but to no success, my newly instantiated Objects reference the same Object. I.e If I increment a private counter var in instance a via a public method, then separately do the same to instance b, b will reflect both increments.
I was able to achieve a de-referenced object via Object.create(myClass), however this is undesirable as I'd like to achieve this internally and also not rely on client support for that native method.
Here's a jsbin of what I've got: http://jsbin.com/zepaju/6/edit?js,console
Thanks for any help!

This is a pretty big subject, because there isn't a perfect way to make JavaScript work like Java-- you'll always have to invent some new coding idiom, and different people have different preferences.
Looking at your linked code, it's hard to be sure what you're gunning for but it looks like the problem is that you're thinking of an object's prototype as a "class", which is copied into each "instance" (like in Java)-- this isn't the case.
Your create() function is creating each "instance" by doing Object.create(Poll), which makes a new object with the Poll object as its prototype. When you refer to properties of the resulting objects, and those properties are not directly defined on the object, what you get is a reference to a property of the single Poll object.
The fact that you've sealed the Poll object's internal variables within a closure doesn't make any difference to this; the closure variables are hidden from the outside world, but they are accessible to the methods of the Poll object, and those methods are shared between all "instances".
If you want a function that spits out objects with a particular set of methods, and which hide their internal data in a closure, that might look like:
function Poll(challenger,incumbent) {
var challengerVotes=0;
var incumbentVotes=0;
return {
voteForChallenger: function() {challengerVotes++},
voteForIncumbent: function() {incumbentVotes++},
winner: function() {return challengerVotes>incumbentVotes ? challenger : incumbent}
}
}
var poll1 = Poll("Edward","Jacob");
var poll2 = Poll("Vanilla","Stilton");
poll1 and poll2 would not affect one another, and there would be no way to access the vote counts of either except through the supplied methods. I appreciate you're looking for a more generic approach but this is an example of how you might start.

Benefits/drawbacks of additional abstraction layer within Angular's $scope

I have slightly modified the example from the following URL (http://docs.angularjs.org/cookbook/helloworld) as follows, placing the name value within an attrs object property:
<!doctype html>
<html ng-app>
<head>
<script src="http://code.angularjs.org/1.2.9/angular.min.js"></script>
<script>
function HelloCntl($scope) {
$scope.attrs = {
name : 'World'
}
}
</script>
</head>
<body>
<div ng-controller="HelloCntl">
Your name: <input type="text" ng-model="attrs.name"/>
<hr/>
Hello {{attrs.name || "World"}}!
</div>
</body>
</html>
One benefit I can see is that the HTML source code can be searched for /attrs\.\w+/ (e.g.) if there is ever a need to easily find all such attributes within the view rather than the controller (e.g. a search for name could collide with form element names). Also within the controller I can only imagine that partitioning attributes necessary for the front end might lend itself to better organization.
Is anybody else using such a level of abstraction. Are there any possible specific further benefits to it's usage? And most importantly, might there be any specific drawbacks to it.

It's recommended that you always use a dot in your ngModels in order to avoid potential issues with prototypal inheritance that are discussed in Angular's Guide to Understanding Scopes:
This issue with primitives can be easily avoided by following the
"best practice" of always have a '.' in your ng-models – watch 3
minutes worth. Misko demonstrates the primitive binding issue with
ng-switch.
Prototypal inheritance and primitives
In javascripts' approach to inheritance reading and writing to a primitive act differently. When reading, if the primitive doesn't exist on the current scope it tries to find it on any parent scope. However, if you write to a primitive that doesn't exist on the current scope it immediately creates one on that scope.
You can see the problem this can cause in this fiddle that has 3 scopes- one parent and two children that are siblings. First type something in the "parent" and you'll see that both children are updated. Then type something different in one of the children. Now. only that child is updated, because the write caused the child to creates it's own copy of the variable. If you now update the parent again, only the other child will track it. And if you type something into the sibling child all three scopes will now have their own copies.
This can obviously cause lots of issues.
Prototypal inheritance and objects
Try the same experiment with this fiddle in which each ngModel uses a property of an object instead of a primitive. Now both reading and writing act consistently.
When you write to a property of an object it acts just like reading does (and the opposite of how writing to a primitive does). If the object you're writing to does not exist on the current scope it looks up it's parent chain trying to find that object. If it finds one with that name then it writes to the property on that found object.
So, while in the primitive example we started with 1 variable and then after writing to the children ended up with 3 copies of the variable- when we use an object we only ever have the one property on the one object.
Since we almost always, perhaps just always, want this consistent behavior the recommendation is to only use objects properties, not primitives in an ngModel or, said more commonly, "always use a dot in your ngModel"

I do this as well. I also put all action functions (button clicks etc) into a $scope.actions object. And since i use socket.io i put those callbacks into a $scope.events object it usually keeps my controllers nice and organized and easily able to find the function i need to if i need to do any editing.
app.controller('Ctrl',['$scope', function ($scope) {
$scope.data = {
//contains data like arrays,strings,numbers etc
};
$scope.actions = {
//contains callback functions for actions like button clicks, select boxes changed etc
};
$scope.events = {
//contains callback functions for socket.io events
}
]);
Then in like my templates I can do like
<input ng-click="actions.doSomething()">
I also do a partial of this for services. I use a private and public data object
app.factory('$sysMsgService',['$rootScope',function($rootScope){
//data that the outside scope does not need to see.
var privateData = {};
var service = {
data:{
//contains the public data the service needs to keep track of
},
//service functions defined after this
};
return service;
}]);

Javascript object encapsulation that tracks changes

Is it possible to create an object container where changes can be tracked
Said object is a complex nested object of data. (compliant with JSON).
The wrapper allows you to get the object, and save changes, without specifically stating what the changes are
Does there exist a design pattern for this kind of encapsulation
Deep cloning is not an option since I'm trying to write a wrapper like this to avoid doing just that.
The solution of serialization should only be considered if there are no other solutions.
An example of use would be
var foo = state.get();
// change state
state.update(); // or state.save();
client.tell(state.recentChange());
A jsfiddle snippet might help : http://jsfiddle.net/Raynos/kzKEp/
It seems like implementing an internal hash to keep track of changes is the best option.
[Edit]
To clarify this is actaully done on node.js on the server. The only thing that changes is that the solution can be specific to the V8 implementation.

Stripping away the javascript aspect of this problem, there are only three ways to know if something has changed:
Keep a copy or representation to compare with.
Observe the change itself happening in-transit.
Be notified of the change.
Now take these concepts back to javascript, and you have the following patterns:
Copy: either a deep clone, full serialization, or a hash.
Observe: force the use of a setter, or tap into the javascript engine (not very applicable)
Notify: modifying the code that makes the changes to publish events (again, not very applicable).
Seeing as you've ruled out a deep clone and the use of setters, I think your only option is some form of serialisation... see a hash implementation here.

You'll have to wrap all your nested objects with a class that reports you when something changes. The thing is, if you put an observer only in the first level object, you'll only receive notifications for the properties contained in this object.
For example, imagine you have this object:
var obj = new WrappedObject({
property1: {
property1a: "foo",
property1b: 20,
}
})
If you don't wrap the object contained in porperty1, you'll only receive a "get" event for property1, and just that, because when someone runs obj.property1.property1a = "bar" the only interaction that you'll have with obj, will be when it asks for the reference of the object contained in property1, and the modification will happen in an unobserved object.
The best approach I can imagine, is iterating over all the properties when you wrap the first object, and constructing recursively a wrapper object for every typeOf(property) == "Object".
I hope my understanding of your question was right. Sorry if not! It's my first answer here :$.

There's something called reactive programming that kind of resembles what you ask about, but its more involved and would probably be overkill.
It seems like you would like to keep a history of values, correct? This shouldn't be too hard as long as you restrit changes to a setter function. Of course, this is more difficult in javascript than it is in some other languages. Real private fields demand some clever use of closures.
Assuming you can do all of that, just write something like this into the setter.
function setVal(x)
{
history.push(value);
value = x;
}

You can use the solution that processing.js uses.
Write the script that accesses the wrapped object normally...
var foo = state.get();
foo.bar = "baz";
state.update();
client.tell(state.recentChange());
...but in the browser (or on the server if loading speed is important) before it runs, parse the code and convert it to this,
var foo = state.get();
state.set(foo, "bar", "baz");
state.update();
client.tell(state.recentChange());
This could also be used to do other useful things, like operator overloading:
// Before conversion
var a=new Vector(), b=new Vector();
return a + b * 3;
// After conversion
var a=new Vector(), b=new Vector();
return Vector.add(a,Vector.multiply(b,3));

It would appear that node-proxy implements a way of doing this by wrapping a proxy around the entire object. I'll look into more detail as to how it works.
https://github.com/samshull/node-proxy