I am writing an object pool class that holds onto old unused objects, and when a new one is needed can use one of its reserves (Without the expensive creation of objects). However, I want to be able to save any data to this pool (not limited to something such as extends Poolable). The problem is that most object pools require something like this:
class MyData {
constructor(str) {
this.reset(str)
}
reset(str) {
this.str = str;
}
}
So that when a new instance is needed, the pool can call oldInstance.reset(prams). As stated before, I do not want this (Working with lots of third party tools that I am not feeling like writing wrappers for), so my data looks like this:
class MyData {
constructor(str) {
this.str = str;
}
}
When the pool needs to reset an instance, I need to be able to call the constructor as a function and set the this value to the instance that is being wiped.
This is super easy with old classes that used function/prototype syntax when they were made:
const MyClass = function(str) {
this.str = str;
}
// Reset an instance
const instance = new MyClass("foo");
MyClass.apply(instance, ["bar"])
// done
However when I do that for classes, it complains that you can not use a class without the new keyword. How do I go about doing this?
Fiddle: https://jsfiddle.net/wuqek405/
Edit:
As I said, most object pools need a reset function. I am trying to use the constructor as this function, because it is supposed to “set up” the instance. Another solution would be to generate this reset function based on the class. However, I want it to be as fast as possible, so terribly hacky solutions such as stringifying the class and evaling the constructor wouldn’t be optimal.
This is not possible. A class is not callable1, only constructable - which means creating a new object. This was new in ES6, which overhauled the inheritance model of classes and introduced super. Also, ES2022 will introduce class fields, which also get created during construction without being mentioned in the constructor code.
Your only option is to use only ES5 function-based classes, a transpiler, or writing explicit reset methods.
1: technically, it is callable (typeof C == 'function'), but [[Call]] will always throw an exception
Related
Let's say I want to create an array of Person using a random data library.
I could do something like
import {generateRandom} from 'someLib'
let people = []
function getPerson() ({
name: generateRandom.string()
age: generateRandom.number()
})
for (let i = 0; i < 10; i++) {
people.push(getPerson())
}
But I could also do something like
import {generateRandom} from 'someLib'
class Person {
constructor() {
this.name = generateRandom.string(),
this.age = generateRandom.number()
}
}
let people = []
for (let i = 0; i < 10; i++) {
people.push(new Person())
}
On a memory level, is there any difference in the outcome?
(This is just a theoretical question, I am not trying to solve any problem in particular)
I have found this question that is related to this Difference between creating a class in javascript to create an object and creating an class and object in Java
Which states that there are no classes in JS.
Is this just syntactic sugar? 2 ways of doing exactly the same thing?
If you want to examine the memory yourself, you can put this code in the console:
class Test{};
const test = new Test();
class Class {};
test.Class = Class;
test.classObj = new Class();
function func() {return {};};
test.func = func;
test.funcObj = func();
Take a heap snapshot with google chrome dev tools, sort by constructor and find the Test class.
You can then examine the memory of these functions and objects. Here's a screenshot of what I got:
You can see that the object instantiated by the class constructor is slightly larger than the one instantiated by the function. Expanding the prototype, you can see they both use the Object constructor, but the classObj has the additional Class constructor in its prototype chain.
You can also see that the class constructor appears to retain more memory than regular functions, retained size meaning memory that will be cleaned up by garbage collection if the function is no longer in use.
Seems to be an extra 172 bytes for the class constructor, and 24 bytes per object for an empty object.
Following VLAZ's comment, here's the results with 10 methods added, and 10 instances.
class Test{};
const test = new Test();
class Class {
method0(){};
method1(){};
method2(){};
method3(){};
method4(){};
method5(){};
method6(){};
method7(){};
method8(){};
method9(){};
};
test.Class = Class;
for (let i=0; i < 10; i++){
test["classObj" + i] = new Class();
}
function func0(){};
function func1(){};
function func2(){};
function func3(){};
function func4(){};
function func5(){};
function func6(){};
function func7(){};
function func8(){};
function func9(){};
function constructorFunc() {
return {
method0: func0,
method1: func1,
method2: func2,
method3: func3,
method4: func4,
method5: func5,
method6: func6,
method7: func7,
method8: func8,
method9: func9,
};
};
test.constructorFunc = constructorFunc;
for (let i=0; i < 10; i++){
test["funcObj" + i] = constructorFunc();
}
The shallow size of the class objects are now much smaller. This seems to be due to the fact that they can just store a reference to the class prototype rather than reference all of their methods directly.
At first glance, the retained size of Class seems to be smaller than constructorFunc, but expanding Class you can see a property named prototype which is an object retaining an extra 1.38 KB. Adding that to the 520 B of the class itself pushes it above the retained memory of constructorFunc. But the memory saved by creating instances of the class instead of an object will outweigh that pretty quick.
So seems like classes are the way to go.
Your first getPerson() is invalid syntax and the function does not even attempt to return anything (if it was valid syntax). So, people.push(getPerson()) would generate an array of undefined(if the syntaxError was fixed) which will be entirely different than your second code block which generates an array of objects.
If, what you meant to ask about was something like this:
let people = []
function getPerson() {
return {
name: generateRandom.string()
age: generateRandom.number()
}
}
for (let i = 0; i < 10; i++) {
people.push(getPerson())
}
Then, this and your class will each create an array of objects and those objects will each have your two properties on them. The Class will contain some additional setup on the prototype that allows for subclassing, but if you're just asking about using the object with two properties, then these two would fundamentally do the same thing.
On a memory level, is there any difference in the outcome?
Without methods, your two properties will occupy the same amount of memory. The two properties are assigned directly to the object so those two properties use the same memory.
The class will actually set up a few other things on the object like a .constructor property and the object will get its own separate prototype. The prototype is shared among all instances. So, there could be slightly more memory usage for the class instance, but it's unlikely this makes a material difference.
If you defined methods directly on the object in the first implementation and methods in the class in the second implementation, then the class would definitely be more efficient because there would be one copy of the methods on the prototype vs. many separate copies of the methods on each instance of the object. But, you didn't show that in your question.
Which states that there are no classes in JS. Is this just syntactic sugar? 2 ways of doing exactly the same thing?
Classes are syntactic sugar, but they make it easy to define things in an efficient way without having to do a bunch of manual things. And, because the language sets things up for you, then everyone using a class definition creates code that works the same way.
You can manually build the same object that instantiating an instance of a class does, but it's a lot more code to do write all the bookkeeping that a class does for you. Your simple object with just two instance properties doesn't show any of that or need any of that, but when you start having methods and sub-classing things and overriding base methods and then calling base methods in your derived implementation, the class definition takes care of a some bookkeeping for you and simply lets you write, good extensible code faster and with everyone doing it the same way. Again, your simple object with just two properties does not show or need what a class does, but other uses of objects do.
For example, the class definition and new Person() creates an object that automatically sets yourObject.constructor to be the constructor that created the object. This allows other code to know what type of object it is or allows other code to abstractly create new instances of the same object. Methods are put on the prototype of the object and made non-enumerable. Constructors can call super(...) to execute the base class constructor (whatever it happens to be). Without the class definition, you have to do that very manually by calling the exact right function in the base class.
TLDR: Are there any pitfalls in using this.constructor(props) in a class method to duplicate that class?
I have a use case where defining class models for entities proves quite useful for management and processing throughout a complex system.
These entity model classes all require a set of common methods.
I do not wish to duplicate code across multiple entity model classes. I am exploring the use of a BaseEntityModel class which can be extended by all other model classes requiring the common functionality.
One of these methods is a clone() method, which provides a deep clone of the object as the ExtendedEntityModel type.
clone() will be declared on the BaseEntityModel class. I want to preserve the class name and properties when calling clone from a ExtendedEntityModel object which has extended BaseEntityModel class.
I need a way of creating a new instance of a ExtendedEntityModel through the BaseEntityModel without having to explicitly know what it is when calling clone(). In otherwords, I wanted to avoid using new ParentClassEntity() because the BaseEntityModel methods need to work for all extensions of it.
I did some searching but could not find much on this pattern. After some tinkering, I found that calling this.constructor(props) does exactly what I want. See below class, and clone() function
BaseEntityModel.js
class BaseEntityModel {
constructor({ entityType, dataKeys }) {
this.entityType = entityType;
this.dataKeys = dataKeys;
}
/**
* Provide deep clone of self, Preserving extended class properties
*/
clone() {
return new this.constructor(JSON.parse(JSON.stringify(this))); //<-- here, this.constructor
};
}
ExtendedModel.js
class ExtendedModel extends BaseEntityModel {
constructor({param1, param2} : ExtendedModel) {
super({ entityType, dataKeys });
this.param1 = param1;
this.param2 = param2;
}
}
and then usage
var extendedModel1 = new ExtendedModel({param1, param2};
var clonedExtendedModel = extendedModel1.clone(); //returns a cloned version and of type ExtendedModel
I cannot find much on calling this.constructor. Can anyone provide some insight into why this might be a bad choice, or confirm that this is not a javascript bomb waiting to go off?
See CodeSandbox of working implementation
I am learning about compositions in Javascript. So I want to ask if this is correct way of doing things.
I made some exercises that look like this:
class Animal {
// constructor() {
// }
eat = () => {
console.log("this creature is eating");
}
}
const AnimalsWithWings = superclass => class extends superclass {
constructor(Feathers, ...args) {
super(...args);
Object.assign(this, { Feathers });
}
}
const CanDive = superclass => class extends superclass {
// constructor( ...args) {
// super(...args);
// }
dive = () => {
console.log("Can Dive");
}
}
class Duck extends AnimalsWithWings(CanDive(Animal)) {
constructor(eats, ...args) {
super(...args);
Object.assign(this, { eats });
}
}
const duffy = new Duck("watermelon", true);
console.log(duffy);
duffy.dive();
duffy.eat()
I am still in learning process so I just need some pointers.
Did it do more or less what you expected? Then, sure, it's a correct way to do it, whatever "correct" means here.
It looks to me like it does what it was meant to do when I pop it into the console. I can't really say much more on your code specifically because I'm not sure what concrete domain it's trying to model, aside from maybe breaking down Ducks into atomic pieces.
If you're going to do it this way, though, I'd personally prefer to use a params object instead of just changing the constructor signature like that with AnimalsWithWings. That way, the order of extra parametrizations doesn't depend on the order in which the mixins were applied, which I would consider a Surprise. Surprises are bad.
const AnimalsWithWings = superclass => class extends superclass {
// Everyone receives the same `params` object.
// They only take what they know about, and ignore the rest.
constructor(params) {
super(params);
Object.assign(this, { Feathers: params.Feathers });
}
}
Even more personal opiniony, I'd name them WithDiving and WithWings instead, just to keep a somewhat consistent naming scheme, and to better imply that these are modifiers, not "real" base classes.
Your code does saddle every Duck with a prototype chain 4 prototypes long, but eh, whatever. If it somehow becomes a performance problem then you can create a utility function to optimize the mixin process or something. Flatten the prototypes, maybe.
Your code does also let you call super.method() in methods, though it's debatable whether you should ever use that in a mixin at all. I'd say you shouldn't, unless you want your mixins to implicitly depend on each other, which is a Surprise.
There are plenty of other ways of doing mixins, too.
You could create a utility function to flatten all the prototypes into a single new one and return a base class from that which you extend. (Just be sure to iterate property descriptors rather than just using Object.assign() when doing that flattening, if you want to properly handle things like get/set accessors, etc.)
You could eschew Classes and just directly create prototype objects and use Object.create() to create instances. (same thing about iterating property descriptors.)
You could create a Duck prototype using a bunch of iterative calls to Object.create() instead of iteratively extending base classes.
You could control the additional behaviors with helper Controller Classes instead of composing behavior directly into the base.
You could deal just in plain objects with data, and pass the objects to functions that expect the object to have certain properties on it in order to do things. (Amusingly, called "duck typing") I'll grant that's not really mixins, just calling functions, but if it accomplishes the same thing in effect...
Probably a bunch others I can't really think about at the moment. It's all sticking sets of behaviors onto some base thing.
How can I get name of object's class? I mean "Process" in this example
I see two ways to get it. First one is to write a getter in this class like
getClassName(){return "Process"}
But I suppose it will be an error if I try to call this method in object which doesn't belong to this class and hasn't got method like this.
And second one is using object instanceof Process. But maybe there is some way to make it better and more correctly?
You can get it from name on constructor:
console.log(object.constructor.name);
When you do ex = new Example, for instance, in the normal course of things that makes Example.prototype the prototype of the object that was created (ex), and the object inherits a constructor property from that object that refers back to the constructor (Example).
I say "in the normal course of things" because there are various ways those normal relationships can be changed. For instance, code could have overridden the constructor property with an own property on the object (ex.constructor = somethingElse;). To rule out that specific scenario, you could use:
console.log(Object.getPrototypeOf(object).constructor.name);
Live example:
class Example1 {
}
const e1 = new Example1();
console.log(e1.constructor.name); // "Example1"
class Example2 {
constructor() {
this.constructor = "I'm special";
}
}
const e2 = new Example2();
console.log(Object.getPrototypeOf(e2).constructor.name); // "Example2"
The TC39 committee members that specify JavaScript were happy enough to use the instance's constructor property in Promises when building the new promise that then and catch return (see Step 3 here which goes here and reads constructor from the instance) (and in some other places), so you wouldn't be out on your own if you also used it. They don't even go to the prototype of the instance.
But yes, just for completeness, even if you go to the prototype for it, it's still possible for that to lead you astray, since the prototype's constructor property can also be mucked with:
class Example {
}
Example.prototype.constructor = Object; // Why would anyone do this? People are weird.
const e = new Example();
console.log(Object.getPrototypeOf(e).constructor.name); // "Object"
It's also possible to redefine the name on a function:
class Example {
}
// Why would someone do this? People are weird.
Object.defineProperty(Example, "name", {
value: "flibberdeegibbit"
});
const e = new Example();
console.log(Object.getPrototypeOf(e).constructor.name); // "flibberdeegibbit"
So...caveat user.
Note that the function name property is new as of ES2015 (as is class syntax). If you're using class syntax via a transpiler, it may or may not set name correctly.
Generally object instanceof Process is desirable if it's known for sure that object originates from this class/function. There may be situations where this won't be so. The appearance of several Process can be caused by iframes, multiple package versions, etc.
There is name property that already exists in regular functions class constructors. A known pitfall is that it will be mangled in minified code, so it is generally useless in browser JS, and its use can be considered an antipattern. name cannot be reassigned (in some browsers), so a separate property is needed to identify the class.
The proper way is to avoid this problem
But I suppose it will be an error if I try to call this method in object which doesn't belong to this class and hasn't got method like this.
is to use a getter:
class Process {
get className() { return 'Process'; }
...
}
Or a property:
class Process {
...
}
Process.prototype.className = 'Process';
As a result, there may be several Process classes that have Process className identifier. This may be desirable or not. While instanceof associates class instance with one particular class.
Use .constructor.name on the object. Each object's constructor by default refers to his creation function, which has a name property. It returns the name of the function.
class SomeClass {
}
const obj = new SomeClass();
console.log(obj.constructor.name);
Use the name property as follows:
class Process {}
console.log(Process.name);
const process = new Process;
console.log(process.constructor.name);
This is the same way it works for normal prototypal inheritance using functions.
In this Udacity video on game development, the instructor mentions that Javascript allows us to create an object by giving a handle to its definition. Then, it says that to allow this "an overloaded object definition will update a hash table with a pointer to its class definition".
I quite know what a hash table, a pointer, an overloaded method, and the factory pattern are, but I cannot make sense of this mysterious statement, or of the rest of the explanation.
"Hash table" is just a fancier way of saying "ordinary Javascript object". What the instructor means by "handle to its definition" is just another way of saying "the function that acts as a constructor for the class".
Ultimately, what he means by the statement you mentioned:
each overloaded entity definition will update a hash table with a pointer to its class definition
is the following:
All "subclasses" of Entity will register their constructor function in a single shared hashmap/object using the key which is the type value.
This allows you to get the constructor function (in other words, the function to call new on, which will return an instance of that entity) for any type by accessing gGameEngine.factory[type].
This is nice, because whenever a programmer adds a new type of entity, so long as they remember to add a new entry to that gGameEngine.factory object with the correct key, then that object will contain everything you need to construct any type of supported object.
So the code that iterates over the JSON structure generated by the level editor can create an instance of any type the same way, using something like:
var typeConstructor = gGameEngine.factory(tileSpec.type),
instance;
if (typeConstructor) {
instance = new(typeConstructor)(tileSpec /* or whatever params */);
}
This is similar to the code visible at around the 1 minute mark of the video you linked to.
Make sense now?
I think all he's saying is that you can map references to functions / objects / variables inside another object. He's using one type of property accessor syntax, but I think he's overcomplicating things by using language like 'handle' and 'hash table'.
var someClass = function () {
// stuff
}
var containingObject = {};
containingObject["someClass"] = someClass;
// same thing as
containingObject.someClass = someClass;
Then you can instantiate the class by calling the containingObject property.
var classInstance = new containingObject["someClass"]()
// or
var classInstance = new containingObject.someClass()