JavaScript Class in ES6 , write less, do more - AkuCode

May 02, 2020 October 12, 2022

Traditionally JavaScript is the only mainstream language with prototype-based inheritance. Programmers switching to JS from class-based language found it puzzling, but ES2015 introduced classes, which are just syntactic sugar over the inner working, but changed a lot how we built JavaScript programs.
Now inheritance is very easy and resembles other object-oriented programming language:

   class Person {
     contructor(name){
        this.name = name
                    }
            hello(){
             return ‘hello, Iam’ + this.name+ ‘.’
                    }
                }
   class Actor extend Person{
            hello(){
             return super.hello()+ ‘I am an actor.’
                }
        }
   var tomCruise = new Actor(‘Tom Cruise’)
   tomCruise.hello()

(the above program prints “Hello, I am Tom Cruise. I am an actor.”)

Classes do not have explicit class variable declarations, but you must initialize any variable in the constructor.

Constructor

Classes have a special method called constructor which is called when a class is initialized via new.

Super

The parent class can be referenced using super().

Getters and Setters

A getter for a property can be declared as

class Person{
 get fullName(){
 return `${this.firtsName} ${this.lastName}`
        }
}

Setters are written in the same way:

class Person{
        set age(years){
 this.theAge = years
        }
}

Modules

Before ES2015, there were at least 3 major modules competing standards, which fragmented the community:

AMD
RequireJS
CommonJS

ES2015 standardized these into a common format.

Importing Modules

Importing is done via the import … from … construct:

 import + from ‘mymodule;
 import React from ‘react’
 import { react, Componenent} from ‘react’
 import React as MyLibrary from ‘react’

Exporting modules

You can write modules and export anything to other modules using the export keyword:

 export var foo = 2
 export function bar(){ /*…*/}

Template Literals

Template literals are a new syntax to create string:

const aString = `A string`

They provide a way to embed expressions into string, effectively interpolating the values, by using the ${a_variable} syntax:

 const var = ‘test’
 const string = `something ${var}` //something test

You can perform more complex expressions as well:

 const string = `something ${1 + 2 + 3}`
 const string2 = `something ${foo() ? ‘x’ : ‘y’}`

And strings can span over multiple lines:

 const string3 = `Hey
        this
        string
        is awesome!`

Compare how we used to do multiline string pre-ES2015:

 var str = ‘One\n’ + 
 ‘Two\n’ + 
        ‘Three’

Default Parameters

Functions now support default parameters:

 const foo =  function(index = 0, testing = true){/* … */}
        foo()

The spread operator

You can expand an array, an object or a string using the spread operator.

Let’s start with an array example. Given

const a = [1,2,3]

You can create a new array using

const b = […a,4,5,6]

You can also create a copy of an array using

const c = […a]

This work for object as well. Clone an object with:

const newObj = {…oldObj}

Using strings, the spread operator creates an array with each char in the string:

 const hey = ‘hey’
        const arrayized = […hey] // [ ‘h’, ’e’, ’y’]

This operator has some pretty useful applications. The most important one is the ability to use an array as function argument in a very simple way:

 const f = (foo, bar) => {}
 const a = [1, 2]
        f(…a)

(in the past you could do this using f.apply(null, a) but that's not a niche and readable).

Desctructuring assignments

Given an object, you can extract just some values and put them into name variables:

 const Person = {
  firstName: ‘Tom’,
  lastName: ‘Cruise’,
  actor: true,
  age: 54,
 const (firstName: name,age) = person
        
}

Name and age contain the desired values.

The syntax also works on arrays:

 const a = [1,2,3,4,5]
        [first, second, , ,fifth] = a

Enhanced Object Literals

In ES2015 Object Literals gained superpowers.

Simpler syntax to include variabels

Instead of doing

 const something = ’y’
     const x = {
     something : something
        }

You can do

 const something = ‘y’
     const x = {
     something
        }

Prototype

A prototype can be specified with

 const anObject = { y: ‘y’}
     const x = {
     __proto__: anObject
        }

Super()

 const anObject = { y: ‘y’, test: () => ‘zoo’ }
 const x = {
         __proto__: anObject,
         test(){
                return super.test()+ ‘x’
            }
        }
        x.test() //zoox

Dynamic properties

 const x = {
         [‘a’ + ‘_’ + ‘b’]: ‘z’
            }
        x.a_b //z

For-of loop

ES5 back in 2009 introduced forEach() loops. While niche, they offered no way to break, like for loops always did. ES5 introduced the for-of loop, which combines the consiness of forEach with the ability to break:

for (const v of [‘a’, ‘b’, ‘c’]){
     console.log(v);
        }
        for (const [I, v] of [‘a’, ‘b’, ‘c’].entries()){
     console.log(I, v)
        
}

Map and Set

Map and Set (and their respective garbage collected Weakmap and WeakSet) are the official implementations of two very popular data structures.

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