Static Class Members in ES6

I have been playing with ECMAScript 6 for some time now and I must say that the new features added in this edition are worth taking a look. One of the significant language feature added in this specification is, support for classes. If you are not aware of the features of ES6, checkout the specification on official site for ECMA Script or, the un-official HTML version of the specification on Mozilla’s site.

ES6 classes support most of the features like creating object, constructor, inheritance and overriding that are supported by Object Oriented programming languages like C# or Java. And because it is JavaScript, there is no static type checking.

I found it interesting when I saw the support for static members in the specification. The specification says, we can add a static method to a class using the following syntax:

static methodName(){
}

But, if you attempt to declare a static property as follows:

static propertyName;

It results in an error. So, we can’t declare a static property. But, we can create it inside a method and use it. To access a static member inside a static method, we need to use this keyword. Because, this reference in JavaScript refers to the object using which the method is invoked. Static methods are called using name of the class to which they belong and name of the class in JavaScript refers an object that has a prototype property containing instance members of the class. For example, consider the following class:

class Employee
{
  constructor(){
    this.name = "Ravi";
  }
  setName(name){
    this.name = name;
  }
}

The above code is similar to:

function Employee(){
  this.name="Ravi";
}
Employee.prototype = {
  setName: function(name){
    this.name=name;
  }
};

Here Employee is name of the class and at the same time, it is also an object. All static members of the Employee class are attached to the Employee object.

Let’s add a static method to the Employee class:

class Employee
{
  constructor(){
    this.name = "Ravi";
  }
  setName(name){
    this.name = name;
  }

  static getCounter(){
    if(!this.counter && this.counter!==0){
      this.counter=0;
    }
    else{
      this.counter++;
    }
    return this.counter;
  }
}

The property counter used in the getCounter method of the above snippet becomes a static property, as it is assigned to this reference inside a static method. Similarly, another static method of the class can be called from a static method using this reference.

The following console.log statements will print the results specified in the comments:

console.log(Employee.getCounter());  //0
console.log(Employee.counter);       //0
console.log(Employee.getCounter());  //1
console.log(Employee.counter);       //1
console.log(Employee.getCounter());  //2
console.log(Employee.counter);       //2

Happy Coding!