Is there a way to count instances from a class?

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I need to keep track of how many instances of a class have been created, from within the said class. I need every instance natively to know 'which one' it is.

The only way I could think of was to use global variables but... there may be a better way.

Here is what I want to avoid:

class MyClass {
    this.instanceId = (window.instanceCount == undefined) ? 0 : window.instanceCount + 1;
    window.instanceCount = this.instanceId;
    ...
}

You can statically add a counter to your "class". Here I use a symbol to prevent names collision. Note that I check whether you use Burrito as a function or a class prior to incrementing the counter.

function Burrito() {
  if (this instanceof Burrito) {
    Burrito[Burrito.counterName]++;
  }
}

Burrito.counterName = Symbol();
Burrito[Burrito.counterName] = 0;
Burrito.count = () => Burrito[Burrito.counterName];

new Burrito(); // 1
new Burrito(); // 2
Burrito();     // skipped
new Burrito(); // 3

console.log(Burrito.count());

How to Count the Number of Objects in a Class in Python, Therefore, we put a simple incrementor in the __init__ method that counts up each time an object is created. In our example code below, we create a class named� The following code will help to find the number of instances created in a class, using C#. In this example, we created 4 objects in a test class. Thus, before the execution of a constructor, we need to add static int count = 0. In each object initialization time of a class, the constructor will execute once.

It's not necessary for the property to be in the global scope:

class MyClass {
    static count = 0;
    instanceId = MyClass.count++;
}

How to count total number of occurrences of an object in a Python list?, You can use the list class count function to count the occurrences of an object in a Python list. Use this only if you want the count of one object only. It finds the total number of the object you pass it in the list it is called on. I am looking for a way to programatically get the number of instances of a certain class type in .NET and Java. Say for example I have class Foo. I want to be able to, in the same process, get a current count of all instance of Foo. However I cannot modify Foo, so a static int with counting is out.

You can use static property and increase it in the constructor()

class MyClass{
  static count = 0;
  constructor(){
    this.instanceId = ++MyClass.count;
  }
}
let x = new MyClass();
let y = new MyClass();

console.log(MyClass.count) //2

console.log(x.instanceId); //1
console.log(y.instanceId); //2

Python program to count number of objects created, Counting objects in python program: Here, we are going to learn how to 2 instance variables or object attributes (name and age), the methods are: The constructor method is used to initialize the attributes of the class at� assuming i have the Class Animals and i have created 3 instances of animals: There's a way to count the instances of a class? mutexe. use a static int member in

Count number of a class objects created in Java, The idea is to use static member in the class to count objects. instance block is preferred to make this program generic. Java | Class and Object | Question 1 � How to create a Class in JShell of Java 9 � Byte Class Fields in� If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created. A static field, sometimes called a class variable, is incarnated when the class is initialized (§12.4). Note that counter is implicitly set to zero

Instances of a Class — Python Like You Mean It, Each of these instances share the common attributes append , count , reverse , and so on Now that we know the basics of how to create an instance of a class, � The idea is to use static member in the class to count objects. A static member is shared by all objects of the class, all static data is initialized to zero when the first object is created if no other initialization is present, And constructor and static member function can only access static data member, other static member functions and any other functions from outside the class.

Python does not keep track of all instances of a class (or of a built-in type), so if you need this functionality, you have to implement it yourself. One way to do it is to store a weak reference to each instance in class attribute.

Comments
  • Use a class variable rather than a global variable, and increment it in the constructor.
  • Just exchange window for a better suited object… And no need to make it a global variable, any variable outside the constructor scope will suffice.
  • Thanks, it's actually the only cross-browser valid answer. I'm having a hard time understanding the "Prototype[Prototype.property]" syntax though. How does that make a static property?
  • Please note that I don't actually use the prototype in this answer. The property is set on the function itself and not on its prototype.
  • Right :-) What does Function[Function.property] mean, though? It does works but I feel like I'm missing something.
  • This is the same as accessing a property. If you know the name of the property, you'd simply do obj.foo. However if the property name is given to you, you need to evaluate it e.g. var key = 'some-random-string'; obj[key] which is different than obj['key'] or obj.key. In the last two examples you access a property which name is literally 'key'. In my answer I have used a Symbol() to prevent name collision, since I don't know what the symbol would be I need to save it and evaluate it when accessing the counter property. (The symbol is the name given to the counter property).
  • That doesn't look like valid JavaScript
  • @Bergi it is valid.
  • I'm getting a SyntaxError: bad method definition in Firefox. Classes (in the current standard) can only contain method (and getter/setter) definitions.
  • @Bergi Because your browser hasn't implemented this syntax. Download chrome - or transpile.
  • Chrome implementing non-standard features and enabling them by default doesn't make this valid JavaScript. Don't take them for granted, and mention in your answer when you're using any.
  • Except that's not valid ES8.
  • @Bergi Why it's not valid ES8
  • Class fields are still a proposal only. You should mention that when posting answers with code that is using them.