Develop Reference

Call Class Constructor as Function on Instance

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

Get constructor name of object

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.

Creating a new object by giving a handle to its definition

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()

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.

Javascript: what are some guidelines on how to define new data types?

Suppose you are creating the data type and expose its behavior.
Can you give some examples of when would you use:
a function & new:
// define new data type
var CustomDataType= function(){
this.a='whatever';
this.doX= function(){/*some code*/};
}
// create a new instance of our custom data type
var obj= new customDataType();
an object literal & Object.create:
// define new data type
var customDataType = {
a: 'whatever',
doX: function(){/*some code*/}
}
// create a new instance of our custom data type
var obj= Object.create(customDataType);
a function that builds your object:
function customDataTypeFactory(options){
return {
a: 'whatever',
doX: function(){/*some code*/}
}
};
// create a new instance of our custom data type
var obj= customDataTypeFactory(options);
I feel this could be labeled duplicate for: new vs Object.create but my main interest is not in discussing which one is better but rather to know if there are specific use cases where one should be preferred over the others.
I have read many posts on related questions and the book from Crockford: Javascript: the good parts. So far I have concluded that it is a matter of preference, tough the advice from Crockford resonates a lot with me to me: "try to avoid the features that are dangerous and unnecessary"... I'm talking about new.

I'll start with the way I usually define classes:
function CustomDataType(a)
{
this.a = a;
this.b = 2;
}
CustomDataType.prototype = {
doX : function () {}
};
var obj = new CustomDataType(1);
I assign variables in the constructor because F.prototype = { a : [1,2,3]; } is problematic, the array will be shared between instances unless the property is reinitialized in the constructor (problematic for all non-primitive types). And it's also needed when the property somehow depends on the arguments to the constructor. I declare methods in the prototype so that they are shared between all instances, if you use inheritance it means you can have super calls, it will also lead to less memory usage since the method only has to be allocated once (though that probably isn't a big issue).
I have never used Object.create, I don't see any reason to do to that unless you are into some insane dynamic hackery.
I use object literals whenever it feels unnecessary to create a separate class for the objects, for instance if it's arguments to a methods (so callers don't have to explicitly instantiate something), or if it's some internal values that shouldn't be accessed by anyone else.
Your factory is essentially the same as the object literal. I suppose it can be useful if you need to create some jsonesque structure and set some default values.
I don't see why new would be dangerous or unnecessary, in what context did he say that?
But I think a lot of it comes down to taste. Be consistent and don't complicate things.