Javascript Question posted in
A very good W3school tutorial can be found here.

Classes in PYTHON vs JAVASCRIPT

Am studying the difference between those two languages and i was wondering why i can’t access the variables in javascript classes without initiating an instance but i can do that in python

here is an example of what am talking about:

PYTHON CLASS

    class Car: 
       all =[]
       def __init__(self, name):
          self.name = name

          Car.all.append(self)

       def get_car_name(self):
          return self.name
 
        
        
    bmw = Car("BMW")
    mercedez = Car("MERCEDEZ")

    print(Car.all)

Running this code returns a list of all cars (which are the instance that i have created)
[<main.Car object at 0x0000022D667664E0>, <main.Car object at 0x0000022D66766510>]

JAVASCRIPT Class

    class Car {
      all = [];
      constructor(name, miles) {
      this.name = name;
      this.miles = miles;

      this.all.push(this);
      }
     }

    let ford = new Car("ford", 324);
    let tesla = new Car("tesla", 3433);

    console.log(Car.all);

if i used this code the console.log will return
undefined

in javascript if i want to get the value of all i have to use an instance like this

console.log(ford.all);

this will return only the instance of ford that was created
[ Car { all: [Circular *1], name: 'ford', miles: 324 } ]

but otherwise in python this if i printed out an instance all it will return this

print(bmw.all)

[<__main__.Car object at 0x00000199CAF764E0>, <__main__.Car object at 0x00000199CAF76510>]

it returns the two instance that is created even if i called the all of one instance

Tags:

2

Answers


  1. 0

    You need to declare it static, otherwise it’s an instance property:

    class Car {
  static all = [];
  constructor(name, miles) {
  this.name = name;
  this.miles = miles;

  Car.all.push(this);
  }
 }

let ford = new Car("ford", 324);
let tesla = new Car("tesla", 3433);

console.log(Car.all);

    In Python variables declared outside methods are static (no static keyword is needed).

    Login or Signup to reply.
  2. 0

    The all cars array even can be, or rather should be, implemented as a private static field with a controlled static all getter as its counterpart.

    One does not want to have a directly public accessible Car.all since everyone is capable of manipulating this array. The static all getter allows for instance a shallow copy of the all cars array to be the return value, thus no other process than the constructor pushes into this array, and just the static all getter is the 2nd process which does control how the all data is exposed into public.

    class Car {
  // - note the hash prefix ... private static .
  static #all = [];

  constructor(name, miles) {

    Object.assign(this, { name, miles });

    // - push into the private static array.
    Car.#all.push(this);
  }
  // - implement a controlled static getter.
  static get all() {

    // - never ever allow direct access to
    //   a private/protected instance.

    // - return e.g. a shallow copy of the
    //   private static `all` (cars) array.
    return [...Car.#all];
  }
 }

let ford = new Car("ford", 324);
let tesla = new Car("tesla", 3433);

console.log(Car.all);
    .as-console-wrapper { min-height: 100%!important; top: 0; }
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search