Class syntax in JavaScript - Part 2

Published August 2, 2020

In the last blog post, we discussed the basics of class syntax in JavaScript.

Today we will be discussing some of the nitty-gritty details on classes in JavaScript.

We know how to make a class, constructor functions, and to make properties and methods in a class.

But now a big question you have in mind would be how is classes different from the old constructor functions? Or is it just syntactic sugar?

No classes are not just a Syntactic sugar. Because the language checks if you have defined a class or a constructor function in many places.

The first evidence is you need to call the class with the new keyword for making objects out of it otherwise it will throw or show an error. For constructor function, we can call these without new keyword and no objects will be made and no errors are shown.

Type of a class

Class in javascript is of type function.

You can check it like this,

// create a class
class User {
  sayHello() {
    console.log("Hello, guys");
  }
}

// check type of class
console.log(typeof User); // function

Methods are not enumerable in a class by default

Methods in objects will not show up in for...in looping statement. Since they are not enumerable in classes.

// create a class
class User {
  constructor() {
    this.category = "User";
    this.isAdmin = false;
  }
  sayHello() {
    console.log("Hello, guys");
  }
}

// new object from User class
const John = new User();

// loop using for...in statement
for (let properties in John) {
  console.log(properties); // only properties are looped not methods
}

Methods are automatically added to the prototype chain instead of getting added to the object directly.

Class can be defined as an expression

Classes can also be written as an expression

// class an an expression
const User = class {
  constructor() {
    this.category = "User";
    this.isAdmin = false;
  }

  sayHello() {
    console.log("Hello, guys");
  }
};

Or,

// class an an expression
const User = class UserClass {
  constructor() {
    this.category = "User";
    this.isAdmin = false;
  }

  sayHello() {
    console.log("Hello, guys");
  }
};

Getters and setters in Class

We can also define getters and setters in class,

// class an an expression
const User = class UserClass {
  constructor() {
    this.category = "User";
    this.isAdmin = false;
  }

  sayHello() {
    console.log("Hello, guys");
  }

  // getter for virtual property name
  set name(name) {
    this._name = name;
  }

  // setter for virtual property name
  get name() {
    return this._name;
  }
};

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