Conventions for ES6 iterables? - javascript

I'm the author of a graph datastructure library for JavaScript. I'm currently ES6-ifying the library.
I also want to make it more usable for ES6 programmers, which means implementing the iterable protocol. But a graph has multiple things to iterate over, and multiple ways to iterate over them (vertices, edges, successors, predecessors, vertices in topological order, etc.)
I have an idea of how to design this interface, but I'd like to follow existing conventions if they exist. Here's an example of how I might do the 'vertices' part:
class JsGraph {
// ...
get vertices() {
return {
_graph: this,
get length() { return this._graph._vertexCount },
*[Symbol.iterator]() {
var keys = Object.keys(this._graph._vertices);
for (let i = 0; i < keys.length; ++i) {
yield [keys[i], this._graph._vertices[keys[i]]];
}
}
};
}
// ...
}
If there are any existng convention I should probably be following (or any problems with this code), your feedback would be much appreciated.

You might get some inspiration from the Map and Set ES6 data structures. They do provide multiple methods to get different iterators: .values(), .keys(), and .entries(). Their ##iterator method defaults to entries or values respectively.
I don't know of any other existing conventions, I guess these will have to form yet.
any problems with this code
First, I'm not sure whether the getter vertices that returns such an object is a good idea. I'd rather do
get vertexCount() { … }
vertices*() { … }
but that'll basically come down to preference. Your current code is not as efficient because it creates two functions at every .vertices access, you could improve that using prototypes if you see the need (class VertexIterator {…}?).
Second, iterators on mutable data are a hazzle to implement, as it may change while being looped over. You should be aware of that and choose a strategy (making your structures immutable is probably out of question). For example, the MapIterator's next method is specified to "redetermine the number of elements each time it is evaluated.". Similarly, for object enumerations (for in) there is the rule that deleted properties must never appear (if not already enumerated before they were deleted), and of course no property may be enumerated twice. However, they explicitly are allowed to be inconsistent about keys that are added during the execution, no guarantees are given on whether those appear in the enumeration or not.

Related

How object basically work when get value by key

I have a question about how object key-value work when we get value by key? Does it need to lookup where the key first and return the corresponding value?
For example: If we have object like
var obj = { keyX: valueX, keyY: valueY, keyZ: valueZ }
Then we retrieve the value obj.keyY, so does it lookup one by one on those object keys to find for keyY?
So for large object (e.g object with 1 million keys), is it slow when we get value by key?
I appreciate any helps. Thank you.
Then we retrieve the value obj.keyY, so does it lookup one by one on those object keys to find for keyY?
It's up to the implementation, but even in old JavaScript engines, the answer is no, it's much, much more efficient than that. Object property access is a hugely common operation, so JavaScript engines aggressively optimize it, and are quite sophisticated at doing so.
In modern JavaScript engines objects are often optimized to just-in-time generated class machine code, so property lookup is blindingly fast. If they aren't optimized for some reason (perhaps only used infrequently), typically a structure like a hash table is used, so lookup remains much better than linear access (looking at each property).
Objects with large numbers of properties, or properties that vary over time, may not be as well optimized as objects with more reasonable numbers of properties. But they'll at least be optimized (in anything vaguely modern) to a hash table level of access time. (FWIW: For objects that have varying properties over time, you may be better off with a Map in the first place.)
Although interesting academically, don't worry about this in terms of writing your code until/unless you run into a performance problem that you trace to slow property access. (Which I literally never have in ~20 years of JavaScript coding. :-) )
Little benchmark:
//setup..
for(var obj = {}, i =0; i<1000000; obj['key'+i] = i++);
var a;
//find 1
console.time(1);
a = obj.key1;
console.timeEnd(1);
//find last
console.time(2);
a = obj.key999999;
console.timeEnd(2);
As you see, it is not lookup one by one

Is it memory inefficient to instantiate classes in a getter method? What OOP design prevents this?

I have a getter method that instantiates classes from deserialized data. I suspect this is a terribly inefficient way of doing things
class Container {
get content() {
let contentList = []
for (let c of this._content) { // this._content is an array of objects
contentList.push(new ContentTypes[c.type](c)) //ContentTypes is a mapping between type names and classes
}
return contentList
}
}
What's the appropriate design pattern in OOP land?
Whether the code is in a getter or a method, the efficiency doesn't change. But what changes is the expectation of the person writing code using your class.
I don't expect accessing a property on an object to incur much overhead. So when using an instance of Container, if I see a contents property, I'm not going to expect it to do a lot of work. Yours does (a bit).
I would either:
Keep a copy of what you return (making what you return immutable) and only discard it when the things it relies on change, or
Make it a method (explicitly).
(Or I might do both.)
I expect method calls to incur more overhead than property accesses. I don't know if there's a specific OOP principle one can cite about that, but that's my view from experience (for what it may or may not be worth).
But, note that there are counterexamples out in the wild. A famous and directly applicable one is the innerHTML property of DOM elements, which is an accessor property that does a fair bit of work when you use it.
Here's an example of returning an immutable thing you keep and re-return, but invalidate when its underlying data changes:
class Container {
constructor() {
this._content = [];
this._exposedContent = null;
}
addContent(content) {
this._content.push(content);
this._exposedContent = null; // Invalidating the cached copy if any
}
get content() {
// Do we have a cached copy?
if (!this._exposedContent) {
// No, create it
this._exposedContent = this._content.map(
c => Object.freeze(new ContentTypes[c.type](c))
);
}
return this._exposedContent;
}
}
(You might also expire the copy after a period of time.)
But it's a judgement call whether it's worth the memory overhead and complexity vs. what your code actually in your content accessor. My default position would be to use a method instead (getContent or similar) and not cache, but it's a judgement call.

Extending JavaScript arrays in V8 and maintaining PACKED kind

I've read about packed arrays in the V8 engine and would like use them in some performance-critical parts of my game (for example as a container for particles).
I would like to make a custom subclass of Array that will always stay PACKED but it seems it's not possible to extend self with another array.
class PackedArray extends Array {
constructor() {
super();
}
extend(values) {
// `values` is another array.
// This obviously won't work, is there another way?
this = this.concat(values);
}
}
Is there a way to extend sub-classed array instance and maintain the PACKED kind in V8?
I know I can push values one by one or even call this.push(...values), but it's slower than .concat() and fails for large arrays.
V8 developer here. TL;DR: the best thing you can do with "packed elements" is not to worry about them. The difference is almost never measurable.
In certain microbenchmarks, where there are only a handful of machine instructions in the optimized code for the hot core loop, every single instruction matters, and if the "hole"-check for the current element is one of them (actually two: cmp + je on x86), then tracking which arrays don't need it can affect the benchmark score. But in real-world applications where you do non-trivial operations on the array element, the impact of two machine instructions is not measurable. Any contortions that you go through with your custom wrapper class are most likely more expensive than the minuscule bit of overhead you might be able to save.
The specific question you ask can be solved by preferring "has-a" over "is-a" composition:
class PackedArray {
extend(values) {
this.#data = this.#data.concat(values);
}
get(i) { return this.#data[i]; }
#data = [];
}
Which would also address the issue that with a subclass, code could still use my_packed_array[10000] = "now you have holes" to side-step the .extend() method. However, keep in mind what I wrote above: the impact of a hole-check is tiny, and any of these extra wrappings probably cost way more than they save.
EDIT: what #MathiasBynens writes is also a very good point: Don't optimize for V8, let V8 optimize for you! :-)
You can use this.splice() to modify the current array.
extend(values) {
this.splice(this.length, 0, ...values);
}

When should I prefer a clone over an reference in javascript?

at the moment I'm writing a small app and came to the point, where I thought it would be clever to clone an object, instead of using a reference.
The reason I'm doing this is, because I'm collecting objects in a list. Later I will only work with this list, because it's part of a model. The reference isn't something I need and I want to avoid having references to outside objects in the list, because I don't want someone to build a construct, where the model can be changed from an inconsiderate place in their code. (The integrity of the information in the model is very important.)
Additional I thought I will get a better performance out of it, when I don't use references.
So my overall question still is: When should I prefer a clone over an reference in javascript?
Thanks!
If stability is important, then clone it. If testing shows that this is a bottleneck, consider changing it to a reference. I'd be very surprised if it is a bottleneck though, unless you have a very complicated object which is passed back and forth very frequently (and if you're doing that it's probably an indication of a bad design).
Also remember that you can only do so much to save other developers from their own stupidity. If they really want to break your API, they could just replace your functions with their own by copying the source or modifying it at runtime. If you document that the object must not be changed, a good developer (yes, there are some) will follow that rule.
For what it's worth, I've used both approaches in my own projects. For small structs which don't get passed around much, I've made copies for stability, and for larger data (e.g. 3D vertex data which may be passed around every frame), I don't copy.
Why not just make the objects stored in the list immutable? Instead of storing simple JSON-like objects you would store closures.
Say you have an object with two properties A and B. It looks like that:
myObj = {
"A" : "someValue",
"B" : "someOtherValue"
}
But then, as you said, anyone could alter the state of this object by simply overriding it's properties A or B. Instead of passing such objects in a list to the client, you could pass read-only data created from your actual objects.
First define a function that takes an ordinary object and returns a set of accessors to it:
var readOnlyObj = function(builder) {
return {
getA : function() { return builder.A; },
getB : function() { return builder.B; }
}
}
Then instead of your object myObj give the user readOnlyObj(myObj) so that they can access the properties by methods getA and getB.
This way you avoid the costs of cloning and provide a clear set of valid actions that a user can perform on your objects.

Why would I need to freeze an object in JavaScript?

It is not clear to me when anyone would need to use Object.freeze in JavaScript. MDN and MSDN don't give real life examples when it is useful.
I get it that an attempt to change such an object at runtime means a crash. The question is rather, when would I appreciate this crash?
To me the immutability is a design time constraint which is supposed to be guaranteed by the type checker.
So is there any point in having a runtime crash in a dynamically typed language, besides detecting a violation better later than never?
The Object.freeze function does the following:
Makes the object non-extensible, so that new properties cannot be added to it.
Sets the configurable attribute to false for all properties of the object. When - configurable is false, the property attributes cannot be changed and the property cannot be deleted.
Sets the writable attribute to false for all data properties of the object. When writable is false, the data property value cannot be changed.
That's the what part, but why would anyone do this?
Well, in the object-oriented paradigm, the notion exists that an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. The final keyword in various languages is the most suitable analogy of this. Even in languages that are not compiled and therefore easily modified, it still exists, i.e. PHP, and in this case, JavaScript.
You can use this when you have an object representing a logically immutable data structure, especially if:
Changing the properties of the object or altering its "duck type" could lead to bad behavior elsewhere in your application
The object is similar to a mutable type or otherwise looks mutable, and you want programmers to be warned on attempting to change it rather than obtain undefined behavior.
As an API author, this may be exactly the behavior you want. For example, you may have an internally cached structure that represents a canonical server response that you provide to the user of your API by reference but still use internally for a variety of purposes. Your users can reference this structure, but altering it may result in your API having undefined behavior. In this case, you want an exception to be thrown if your users attempt to modify it.
In my nodejs server environment, I use freeze for the same reason I use 'use strict'. If I have an object that I do not want being extended or modified, I will freeze it. If something attempts to extend or modify my frozen object, I WANT my app to throw an error.
To me this relates to consistent, quality, more secure code.
Also,
Chrome is showing significant performance increases working with frozen objects.
Edit:
In my most recent project, I'm sending/receiving encrypted data between a government entity. There are a lot of configuration values. I'm using frozen object(s) for these values. Modification of these values could have serious, adverse side effects. Additionally, as I linked previously, Chrome is showing performance advantages with frozen objects, I assume nodejs does as well.
For simplicity, an example would be:
var US_COIN_VALUE = {
QUARTER: 25,
DIME: 10,
NICKEL: 5,
PENNY: 1
};
return Object.freeze( US_COIN_VALUE );
There is no reason to modify the values in this example. And enjoy the benefits of speed optimizations.
Object.freeze() mainly using in Functional Programming (Immutability)
Immutability is a central concept of functional programming because without it, the data flow in your program is lossy. State history is abandoned, and strange bugs can creep into your software.
In JavaScript, it’s important not to confuse const, with immutability. const creates a variable name binding which can’t be reassigned after creation. const does not create immutable objects. You can’t change the object that the binding refers to, but you can still change the properties of the object, which means that bindings created with const are mutable, not immutable.
Immutable objects can’t be changed at all. You can make a value truly immutable by deep freezing the object. JavaScript has a method that freezes an object one-level deep.
const a = Object.freeze({
foo: 'Hello',
bar: 'world',
baz: '!'
});
When you're writing a library/framework in JS and you don't want some developer to break your dynamic language creation by re-assigning "internal" or public properties.
This is the most obvious use case for immutability.
With the V8 release v7.6 the performance of frozen/sealed arrays is greatly improved. Therefore, one reason you would like to freeze an object is when your code is performance-critical.
What is a practical situation when you might want to freeze an object?
One example, on application startup you create an object containing app settings. You may pass that configuration object around to various modules of the application. But once that settings object is created you want to know that it won't be changed.
This is an old question, but I think I have a good case where freeze might help. I had this problem today.
The problem
class Node {
constructor() {
this._children = [];
this._parent = undefined;
}
get children() { return this._children; }
get parent() { return this._parent; }
set parent(newParent) {
// 1. if _parent is not undefined, remove this node from _parent's children
// 2. set _parent to newParent
// 3. if newParent is not undefined, add this node to newParent's children
}
addChild(node) { node.parent = this; }
removeChild(node) { node.parent === this && (node.parent = undefined); }
...
}
As you can see, when you change the parent, it automatically handles the connection between these nodes, keeping children and parent in sync. However, there is one problem here:
let newNode = new Node();
myNode.children.push(newNode);
Now, myNode has newNode in its children, but newNode does not have myNode as its parent. So you've just broken it.
(OFF-TOPIC) Why are you exposing the children anyway?
Yes, I could just create lots of methods: countChildren(), getChild(index), getChildrenIterator() (which returns a generator), findChildIndex(node), and so on... but is it really a better approach than just returning an array, which provides an interface all javascript programmers already know?
You can access its length to see how many children it has;
You can access the children by their index (i.e. children[i]);
You can iterate over it using for .. of;
And you can use some other nice methods provided by an Array.
Note: returning a copy of the array is out of question! It costs linear time, and any updates to the original array do not propagate to the copy!
The solution
get children() { return Object.freeze(Object.create(this._children)); }
// OR, if you deeply care about performance:
get children() {
return this._PUBLIC_children === undefined
? (this._PUBLIC_children = Object.freeze(Object.create(this._children)))
: this._PUBLIC_children;
}
Done!
Object.create: we create an object that inherits from this._children (i.e. has this._children as its __proto__). This alone solves almost the entire problem:
It's simple and fast (constant time)
You can use anything provided by the Array interface
If you modify the returned object, it does not change the original!
Object.freeze: however, the fact that you can modify the returned object BUT the changes do not affect the original array is extremely confusing for the user of the class! So, we just freeze it. If he tries to modify it, an exception is thrown (assuming strict mode) and he knows he can't (and why). It's sad no exception is thrown for myFrozenObject[x] = y if you are not in strict mode, but myFrozenObject is not modified anyway, so it's still not-so-weird.
Of course the programmer could bypass it by accessing __proto__, e.g:
someNode.children.__proto__.push(new Node());
But I like to think that in this case they actually know what they are doing and have a good reason to do so.
IMPORTANT: notice that this doesn't work so well for objects: using hasOwnProperty in the for .. in will always return false.
UPDATE: using Proxy to solve the same problem for objects
Just for completion: if you have an object instead of an Array you can still solve this problem by using Proxy. Actually, this is a generic solution that should work with any kind of element, but I recommend against (if you can avoid it) due to performance issues:
get myObject() { return Object.freeze(new Proxy(this._myObject, {})); }
This still returns an object that can't be changed, but keeps all the read-only functionality of it. If you really need, you can drop the Object.freeze and implement the required traps (set, deleteProperty, ...) in the Proxy, but that takes extra effort, and that's why the Object.freeze comes in handy with proxies.
I can think of several places that Object.freeze would come in very handy.
The first real world implementation that could use freeze is when developing an application that requires 'state' on the server to match what's in the browser. For instance, imagine you need to add in a level of permissions to your function calls. If you are working in an application there may be places where a Developer could easily change or overwrite the permission settings without even realizing it (especially if the object were being passed through by reference!). But permissions by and large can never change and error'ing when they are changed is preferred. So in this case, the permissions object could be frozen, thereby limiting developer from mistakenly 'setting' permissions erroneously. The same could be said for user-like data like a login name or email address. These things can be mistakenly or maliciously broken with bad code.
Another typical solution would be in a game loop code. There are many settings of game state that you would want to freeze to retain that the state of the game is kept in sync with the server.
Think of Object.freeze as a way to make an object as a Constant. Anytime you would want to have variable constant, you could have an object constant with freeze for similar reasons.
There are also times where you want to pass immutable objects through functions and data passing, and only allow updating the original object with setters. This can be done by cloning and freezing the object for 'getters' and only updating the original with 'setters'.
Are any of these not valid things? It can also be said that frozen objects could be more performant due to the lack of dynamic variables, but I haven't seen any proof of that yet.
The only practical use for Object.freeze is during development. For production code, there is absolutely no benefit for freezing/sealing objects.
Silly Typos
It could help you catch this very common problem during development:
if (myObject.someProp = 5) {
doSomething();
}
In strict mode, this would throw an error if myObject was frozen.
Enforce Coding Protocol / Restriction
It would also help in enforcing a certain protocol in a team, especially with new members who may not have the same coding style as everyone else.
A lot of Java guys like to add a lot of methods to objects to make JS feel more familiar. Freezing objects would prevent them from doing that.
I could see this being useful when you're working with an interactive tool. Rather than:
if ( ! obj.isFrozen() ) {
obj.x = mouse[0];
obj.y = mouse[1];
}
You could simply do:
obj.x = mouse[0];
obj.y = mouse[1];
Properties will only update if the object isn't frozen.
Don't know if this helps, but I use it to create simple enumerations. It allows me to hopefully not get duff data in a database, by knowing the source of the data has been attempted to be unchangeable without purposefully trying to break the code. From a statically typed perspective, it allows for reasoning over code construction.
All the other answers pretty much answer the question.
I just wanted to summarise everything here along with an example.
Use Object.freeze when you need utmost surety regarding its state in the future. You need to make sure that other developers or users of your code do not change internal/public properties. Alexander Mills's answer
Object.freeze has better performance since 19th June, 2019, ever since V8 v7.6 released. Philippe's answer. Also take a look at the V8 docs.
Here is what Object.freeze does, and it should clear out doubts for people who only have surface level understanding of Object.freeze.
const obj = {
name: "Fanoflix"
};
const mutateObject = (testObj) => {
testObj.name = 'Arthas' // NOT Allowed if parameter is frozen
}
obj.name = "Lich King" // Allowed
obj.age = 29; // Allowed
mutateObject(obj) // Allowed
Object.freeze(obj) // ========== Freezing obj ==========
mutateObject(obj) // passed by reference NOT Allowed
obj.name = "Illidan" // mutation NOT Allowed
obj.age = 25; // addition NOT Allowed
delete obj.name // deletion NOT Allowed

Categories