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How to use classes in JavaScript

Maham Ghazanfar

Classes are used in many object-oriented programming languages as a way to code efficiently and to bring modularity to the code. Classes are introduced in JavaScript as a part of ES6.

How to declare a class

To declare a class in JavaScript, you can use one of the following methods:

  • Class declaration
  • Class expression

Constructor

A constructor is the first function of a class that is executed and it is called automatically. A constructor is used to create an object of the class upon instantiationThe creation of an object of a class..

It is a function that uses the exact name constructor. constructor is generally an empty function, but it can also be created with parameters and some statements. Unlike other programming languages, there can only be one constructor in JavaScript.

Class declaration

In this method, the name of the class is preceded by the keyword class, and the methods and attributes of the class are contained in curly braces ({}).

class Square {
  constructor {
    this.side = 5;
  }
};

The code above declares a class named Square with a side length of 5 set as the attribute side.

Class expression

To declare a class with an expression, you can use a named expression or an unnamed one.

//unnamed expression
let Shape = class {
  constructor(side_len) {
     this.side = side_len;
  }
};

//named expression
let Shape = class Square {
  constructor(side_len) {
     this.side = side_len;
  }
};

The two methods above show different ways to declare the same class.

In either case, the class can be referenced with the name Shape to which the class has been assigned.

Class methods

A class can have many or few methods, which are basically functions that can use the attributes of a class.

An example of a method with the class Square is provided below.

class Square {
  constructor(side_len) {
    this.side = side_len;
  }
  // Method
  calculatePerimeter() {
    return side*4;
  }
};

Object instantiation

An object is an instance of a class. It is an application of the blueprint that it provides.

const square1 = new Square(10);

The code above shows the syntax to declare an object of a class in JavaScript. Each object has a name, and if the constructor of the class has any parameters, they are passed as arguments to the class at the time of instantiation. In this case, 10 is provided as side_len, which will be set as the attribute side for the Square class.

Code for classes is automatically executed in strict mode, which is semantically different than normal JavaScript in certain ways to make it more secure.

Code

Below is an example of how classes are used in JavaScript.

class Cube { 
  constructor(side_len){ // constructor
    this.side = side_len;
  }

  calculateVolume(){ // method
    return (this.side * this.side * this.side);
  }

  calculateSurfaceArea(){ // method
    return (6 * this.side * this.side);
  }

};

const cube1 = new Cube(3); // object instantiation
console.log("the surface area of the cube is: ", cube1.calculateSurfaceArea());

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