Static properties in JavaScript

Published August 5, 2020

In the last post, we have discussed the concept of static methods in JavaScript.

Let's now look at static properties in JavaScript.

Static properties are those properties that are directly attached to the class or a constructor function just like static methods.

To declare a static property you have to declare those propeties with the keyword static like this,

class User {
  constructor(name) {
    // instance property
    this.name = name;
  }

  // static property
  static counter = 0;

  // instance method
  sayMyName() {
    console.log(`Hello, ${this.name}`);
  }
}

Or to make a static property in a constructor function you can add the property directly to the constructor function using the dot . notation like this,

const User = function (name) {
  // instance property
  this.name = name;
};

// instance method
User.prototype.sayMyName = function () {
  console.log(`Hello, ${this.name}`);
};

// static property attached directly
// to constructor function
User.counter = 0;

Now let's use this counter static property to count the number of times our class or constructor function User is used to initialize objects, to achieve that we need to increment the value of counter static property in the constructor method in class User and inside the body of constructor function User.

In class syntax,

class User {
  constructor(name) {
    // instance property
    this.name = name;

    // increment static counter property
    // when the class User is used initialised objects
    User.counter++;
  }

  // static property
  static counter = 0;

  // instance method
  sayMyName() {
    console.log(`Hello, ${this.name}`);
  }
}

// create four objects from User class
const john = new User("John Doe");
const lily = new User("Lily Rose");
const roy = new User("Roy Jose");
const michael = new User("Michael Bob");

// check the counter
console.log(User.counter); // 4

Or when using a constructor function,

const User = function (name) {
  // instance property
  this.name = name;

  // increment static counter property
  // when the class User is initialised
  User.counter++;
};

// instance method
User.prototype.sayMyName = function () {
  console.log(`Hello, ${this.name}`);
};

// static property attached directly
// to constructor function
User.counter = 0;

// create four objects from User class
const john = new User("John Doe");
const lily = new User("Lily Rose");
const roy = new User("Roy Jose");
const michael = new User("Michael Bob");

// check the counter
console.log(User.counter); // 4

Now if you check the counter static property value by using the dot . notation, you see the value is changed to 4 from 0.

Also if you check the contents of john, lily .. objects the static properties will not be shown since they are directly attached to a class and is common to all the objects of the same class.

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