Introduction to unit testing in JS

If you're new to programming or you're just a hobby programmer and haven't ever done any kind of open source project then you might feel a bit skeptical about the idea of&nbsp;<strong>testing</strong>&nbsp;your code. If you indeed are, then in this article, let me convince you that doing&nbsp;<strong>unit tests</strong>&nbsp;for your code and testing it, in general, is a good practice to follow. Then we'll learn/revise a bit about code testing and finally explore best tools to do unit tests. Enjoy! 😉

If you're new to programming or you're just a hobby programmer and haven't ever done any kind of open source project then you might feel a bit skeptical about the idea of testing your code. If you indeed are, then in this article, let me convince you that doing unit tests for your code and testing it, in general, is a good practice to follow. Then we'll learn/revise a bit about code testing and finally explore best tools to do unit tests. Enjoy! 😉

Why testing?

At the beginning let's talk about why testing is important. When you're developing any kind of app or tool (especially open-source) then tests should be your allies. Consider the quality that they bring. When your tests cover all possible exceptions and cases in your code, you can be sure that it won't fail you in the future. That's yet another reason for testing - assurance. Having tests that cover your entire code-base kept up-to-date allows you to continuously check it for any errors. It makes you sure that everything is fine. If you haven't done any project that needs to be managed by a number of people or needs to serve others (OSS) then you might not really take this whole assurance as a thing. But believe me, it's really important. You can never be sure of wellness of your code without any warranty. 😕 Last but not least we have the advantage of documentation. Believe it or not, properly done tests can sometimes deliver an even better understanding of how to use a particular piece of code than the entire page of text. Just think about it. You have tests that your code passes. This way you have information about how to use the given code and what its outcome is. So, as you can see there are many reasons to start testing your code, thus if you haven't already, it's time to do some testing!

Kinds of tests

If I managed to convince you to do testing, then I'm really happy. 🎉 But the size of the world of code testing can feel a bit overwhelming at the beginning. There are so many terms, concepts, ideologies and tools, and libraries to know about. Let's give it some structure then. First things first, you have to know what kind of tests you're doing, so that you can later choose the right tools for the job. There are 3 main types of tests, divided by the reason they exist.

  • Unit tests - Unit testing allows you to test a really specific aspect of your code, e.g. one function against expected outcome. Here it's really important for your tests to cover all the code that you have, feature by feature. These are the main focal point of this post.
  • Integration tests - Test different parts of your code e.g. components to work as they should. You should also check the way they work together i.e. structural integrity. Side effects are also really important to check. You have to know if there aren't any functions calls and etc. that you didn't plan.
  • Functional tests - I think the name UI tests explains the purpose of these a little better. With functional tests, you check your final product e.g. web app in a specified environment, usually a browser. Here comes the concept of headless browsers, where you execute your test in a browser without visible UI by controlling it with different API calls. It might seem a little awkward at first, but it's a very useful technique, especially for saving some time required by UI and other processes, that are not present in headless mode.
Terminology

Now, that you know a bit more about different kinds of test and what unit tests exactly are, I think it's a good idea to talk a little bit about basic concepts and terms when it comes to testing.

I'd like to start by explaining the TDD and BDD shortcuts that you might have already seen somewhere before but didn't pay much attention to. As these can be seen as basic guidelines when structuring and writing your tests.

Test-Driven-Development (or TDD for short) is a process of developing your software while basing on tests. It's like a cycle, a loop - every time you want to add a feature, you first write your tests (which will obviously fail at this point), then you write the actual code, that fulfills these tests, and then you test again to check that. Your development is based on tests. Pretty interesting idea, won't you agree?

As for Behavior-Driven-Development (BDD), it's yet another ideology, which in fact is based upon TDD. But, the name might not be as self-explanatory as it was with the first one. It can be seen as TDD with some better, additional guidelines. Here, our development is driven not by test specifically, but by behavior, specification, which in fact are tests anyway. 😂 They are just better described, using pure English. This allows your tests to be much better documented and thus more readable. That's why libraries adopt this way of doing tests more often than TDD.

With the knowledge of these 2 trends that are so important when it comes to testing, it's time to explore some terms, explained just as BDD suggests, in English. 😉

  • Assertion functions - functions that we use to test our code against expected output. In Jest and many other unit testing libraries they look like this:
expect(value).toBeTruthy()
  • Code coverage - indicate how big part of our code the tests cover. These are some incredibly useful stats, which can be an additional feature to consider when choosing your next testing library of choice. Of course, there are standalone tools for this, but having everything in one bundle is much more comfortable.
  • Environment - generally a (headlessbrowser or something similar for your functional tests. We're not going to dig into this one, but there are some nice options available here as well. 😄
  • Mocks - also called fakes, are used to fake certain behaviors to later use them in tests, to provide reliable input/output checks even without different features implemented in a realistic manner.
  • Spies - provide information about functions. By using them you can know e.g. how many times, when and with what arguments a function was called. They are possibly the best way to check for side effects.
  • Stubs - alternatively called dubs, give you the ability to replace chosen function with something different, to test expected functionality and behavior.
  • Snapshot testing - a way of testing, based on comparing output data with an already saved copy (snapshot).

With this, you should have a basic understanding of ways of testing and functionalities that you would expect from your library-of-choice. As here I'm focusing only on unit tests, be sure to check out some links in above-listed terms for tools that provide given functionality in a standalone package.

Unit testing

With theory out of the way, it's time to do some testing - unit testing! But first, let's choose the best tool for the job. Here comes the list of some of the best libraries and tools for unit testing & more. 🔥

Jest
test('adds 1 + 2 to equal 3', () => {
  expect(1 + 2).toBe(3);
});

Jest is my personal go-to when it comes to unit testing. Started by guys from Facebook, it has been well battle-tested with a number of popular libraries, such as React itself. It provides almost all features required for high-quality unit testing. Readable assertion functions, great coverage reports, mocking APIparallel test runner and general ease-of-use makes this library one of the best choices available on the market right now, especially for BDD. Aside from that, a great community and support and well-written documentation are very much noticeable.

Mocha
var assert = require('assert');
describe('Array', function() {
  describe('#indexOf()', function() {
    it('should return -1 when the value is not present', function() {
      assert.equal([1,2,3].indexOf(4), -1);
    });
  });
});

Mocha is yet another library, with the goal of making testing fun and simple. Following BDD ideology, it has well-design descriptive API. Also, Mocha is different when it comes to its architecture. It provides developers with minimalflexible setup, allowing them to extend it with other custom libraries, responsible for different tasks. With Mocha, you can use any assertion library you want (it doesn't have its own), including the NodeJS built-in one or Chai. Its API might feel a little similar to Jest's with some small differences. Because of its architecture, Mocha lacks features that Jest has built-in. We're talking about code coverage and more importantly parallel test runner (Mocha runs tests on one process only). Well, where it lacks in functionality, Mocha makes up in visuals with a great choice of tests progress reporters (a feature supported by Jest too). But, as I said, Mocha is for those you like to have their own configurable setups. Besides that, its docs might feel less polished that Jest's but they defiantly exhaust the topic.

Jasmine
describe("A suite is just a function", function() {
  var a;
  it("and so is a spec", function() {
    a = true;
    expect(a).toBe(true);
  });
});

Jasmine might be a bit older than some of its competitors on this list. It's advertised as batteries-included, trying to provide every feature that developers would possibly need. Thus, Jasmine has assertion functionality built-in with expect style implementation. With that comes other built-in features, such as spies, mocks, reporters etc. Also, if you're doing some development with Ruby or Python, you might find it comfortable to use the same library, as Jasmine has official support for those two. As for the docs, they cover all the topics well, but their structure didn't really impress me.

Tape
var test = require('tape');

test('timing test', function (t) {
t.plan(1);
var start = Date.now();

setTimeout(function () {
    t.equal(Date.now() - start, 100);
}, 100);

});

Tape is, again, minimal and flexible library for doing tests for NodeJS and browser. Its API is a bit different than the others' but still readable, with the same ideology in mind. Everything you need to know about it has its place in a single README file. And... it has pretty support for a great number of TAP reporters which is always an advantage.

AVA
import test from 'ava';

test('foo', t => {
t.pass();
});

test('bar', async t => {
const bar = Promise.resolve('bar');
t.is(await bar, 'bar');
});

AVA is a testing library with built-in assertion functions and great focus on async tests. It has simple API (just like other BDD tools) and the ability to run tests in parallel. Just like Tape (which it's inspired by), it has no implicit globals. In addition to that, it has Babel v7 built-in, so that you can write your tests in ES-Next without any additional configuration. All its documentation can be found on its GitHub repo.

Intern

Intern is a TDD/BDD testing framework and tests runner for JavaScript, as well as TypeScript. It allows you to perform both unit and functional tests. It uses Chai as the assertion library and Istanbul to generate your code coverage stats. It can also run your tests concurrently. Generally speaking, it's an extendable framework for doing tests. Did I mention that it has very good, comprehensive documentation?

Cucumber.js
Feature: Simple maths
In order to do maths
As a developer
I want to increment variables

Scenario: easy maths
Given a variable set to 1
When I increment the variable by 1
Then the variable should contain 2

Scenario Outline: much more complex stuff
Given a variable set to <var>
When I increment the variable by <increment>
Then the variable should contain <result>

Examples:
  | var | increment | result |
  | 100 |         5 |    105 |
  |  99 |      1234 |   1333 |
  |  12 |         5 |     18 |

Cucumber.js is another unit testing tool, but this time a little different... It allows you to write your tests in plain language. Basically what you do is you write your test in a structured text format using some keywords like ScenarioGivenWhen etc. and then in your code you define what each step written in plain language should do to run expected tests. I haven't ever used such an approach, so I won't tell you how comfortable it is in real use. But, at least it seems interesting. 😅

Testing, testing...

So, that's it for my list of best tools for doing unit tests. Of course, if I missed an entry worth including in this list, let me know about that down in the comments. Keep in mind that I've covered unit testing only in this list, so no tools for integration, functional or end-to-end (covering every type of tests) testing here. I think it would be too much for one article. It's better to explore smaller topics one-by-one IMHO. In fact, by doing that article I learned a lot too. I'm definitely not a testing expert, but at least now I've got some new knowledge. I hope this post also helped you in your journey with code tests. That's all for now.


By : Areknawo



What is JavaScript – All You Need To Know About JavaScript

What is JavaScript – All You Need To Know About JavaScript

In this article on what is JavaScript, we will learn the basic concepts of JavaScript.

After decades of improvement, JavaScript has become one of the most popular programming languages of all time. It all started in the year 1995 when Brendan Eich created JavaScript in a span of 10 days. Since then, it has seen multiple versions, updates and has grown to the next level.

Here’s a list of topics that I’ll be covering in this blog:

  1. What is JavaScript
  2. What can JavaScript do?
  3. JavaScript Frameworks
  4. The Big Picture: HTML, CSS & JavaScript
  5. Benefits of JavaScript
  6. Fundamentals of JavaScript
    VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case
What is JavaScript?

JavaScript is a high level, interpreted, programming language used to make web pages more interactive.

Have you ever thought that your website is missing something? Maybe it’s not engaging enough or it’s not as creative as you want it to be. JavaScript is that missing piece which can be used to enhance web pages, applications, etc to provide a more user-friendly experience.

What is JavaScript?

JavaScript is the language of the web, it is used to make the web look alive by adding motion to it. To be more precise, it’s a programming language that let’s you implement complex and beautiful things/design on web pages. When you notice a web page doing more than just sit there and gawk at you, you can bet that the web page is using JavaScript.

Feature of JavaScript

Scripting language and not Java: In fact, JavaScript has nothing to do with Java. Then why is it called “Java” Script? When JavaScript was first released it was called Mocha, it was later renamed to LiveScript and then to JavaScript when Netscape (founded JavaScript) and Sun did a license agreement. Object-based scripting language which supports polymorphism, encapsulation and to some extent inheritance as well.**Interpreted language: **It doesn’t have to be compiled like Java and C which require a compiler.JavaScript runs in a browser: You can run it on Google Chrome, Internet Explorer, Safari, etc. JavaScript can execute not only in the browser but also on the server and any device which has a JavaScript Engine.

What is JavaScript – Stackoverflow stats

Currently, we have 100s of programming languages and every day new languages are being created. Among these are few powerful languages that bring about big changes in the market and JavaScript is one of them.

JavaScript has always been on the list of popular programming languages. According to StackOverflow, for the 6th year in a row, JavaScript has remained the most popular and commonly used programming language.

What can JavaScript do?

JavaScript is mainly known for creating beautiful web pages & applications. An example of this is Google Maps. If you want to explore a specific map, all you have to do is click and drag with the mouse. And what sort of language could do that? You guessed it! It’s JavaScript.JavaScript can also be used in smart watches. An example of this is the popular smartwatch maker called Pebble. Pebble has created Pebble.js which is a small JavaScript Framework that allows a developer to create an application for the Pebble line of watches in JavaScript.

What is JavaScript – Applications of JavaScript
Most popular websites like Google, Facebook, Netflix, Amazon, etc make use of JavaScript to build their websites.Among things like mobile applications, digital art, web servers and server applications, JavaScript is also used to make Games. A lot of developers are building small-scale games and apps using JavaScript.## JavaScript Frameworks

One major reason for the popularity of JavaScript is the JavaScript Frameworks. Here’s a brief introduction of the most trending JavaScript frameworks :

  1. AngularJS is Google’s web development framework which provides a set of modern development and design features for rapid application development.

  2. ReactJS is another top JavaScript framework mainly maintained by Facebook and it’s behind the User Interface of Facebook and Instagram, showing off its efficiency in maintaining such high traffic applications.

What is JavaScript – JavaScript Frameworks

  1. MeteorJS is mainly used for providing back-end development. Using JavaScript on the back-end to save time and build expertise is one of the major ideas behind Meteor.

  2. jQuery can be used when you want to extend your website and make it more interactive. Companies like Google, WordPress and IBM rely on jQuery.

The Big Picture: HTML, CSS & JavaScript

Anyone familiar with JavaScript knows that it has something to do with HTML and CSS. But what is the relationship between these three? Let me explain this with an analogy.

What is JavaScript – HTML, CSS and JavaScript

Think of HTML (HyperText Markup Language) as the skeleton of the web. It is used for displaying the web.

On the other hand, CSS is like our clothes. We put on fashionable clothes to look better. Similarly, the web is quite stylish as well. It uses CSS which stands for Cascading Style Sheets for styling purpose.

Then there is JavaScript which puts life into a web page. Just like how kids move around using the skateboard, the web also motions with the help of JavaScript.

Benefits of JavaScript

There has to be a reason why so many developers love working on JavaScript. Well, there are several benefits of using JavaScript for developing web applications, here’s a few benefits:

It’s easy to learn and simple to implement. It is a weak-type programming language unlike the strong-type programming languages like Java and C++, which have strict rules for coding.

It’s all about being fast in today’s world and since JavaScript is mainly a client-side programming language, it is very fast because any code can run immediately instead of having to contact the server and wait for an answer.

Rich set of frameworks like AngularJS, ReactJS are used to build web applications and perform different tasks.

**Builds interactive websites: **We all get attracted to beautifully designed websites and JavaScript is the reason behind such attractive websites and applications.

JavaScript is an interpreted language that does not require a compiler because the web interprets JavaScript. All you need is a browser like Google Chrome or Internet Explorer and you can do all sorts of stuff in the browser.

JavaScript is platform independent and it is supported by all major browsers like Internet Explorer, Google Chrome, Mozilla Firefox, Safari, etc.

JavaScript Fundamentals

In this What is JavaScript blog, we’ll cover the following basic fundamentals of JavaScript
VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case## Variables

Variable is a name given to a memory location which acts as a container for storing data temporarily. They are nothing but reserved memory locations to store values.

What is JavaScript – Variables

To declare a variable in JavaScript use the ‘let’ keyword. For example:

let age;
age=22;

In the above example, I’ve declared a variable ‘age’ by using the ‘let’ keyword and then I’ve stored a value (22) in it. So here a memory location is assigned to the ‘age’ variable and it contains a value i.e. ’22’.

Constants

Constants are fixed values that don’t change during execution time.

To declare a constant in JavaScript use the ‘const’ keyword. For example:

const mybirthday;
mybirthday='3rd August'; 

Data types

You can assign different types of values to a variable such as a number or a string. In JavaScript, there are two categories of data types :

What is JavaScript – Data Types

Objects

An object is a standalone entity with properties and types and it is a lot like an object in real life. For example, consider a girl, whose name is Emily, age is 22 and eye-color is brown. In this example the object is the girl and her name, age and eye-color are her properties.

What is JavaScript – Objects example

Objects are variables too, but they contain many values, so instead of declaring different variables for each property, you can declare an object which stores all these properties.

To declare an object in JavaScript use the ‘let’ keyword and make sure to use curly brackets in such a way that all property-value pairs are defined within the curly brackets. For example:

let girl= {
name: 'Emily',
age: 22,
eyeColour: 'Brown'
};

In the above example, I’ve declared an object called ‘girl’ and it has 3 properties (name, age, eye colour) with values (Emily, 22, Brown).

Arrays

An array is a data structure that contains a list of elements which store multiple values in a single variable.

For example, let’s consider a scenario where you went shopping to buy art supplies. The list of items you bought can be put into an array.

What is JavaScript – Arrays example

To declare an array in JavaScript use the ‘let’ keyword with square brackets and all the array elements must be enclosed within them. For example:

let shopping=[];
shopping=['paintBrush','sprayPaint','waterColours','canvas'];

In the above example I’ve declared an array called ‘shopping’ and I’ve added four elements in it.

Also, array elements are numbered from zero. For example this is how you access the first array element:

shopping[0];		

Functions

A function is a block of organised, reusable code that is used to perform single, related action.

Let’s create a function that calculates the product of two numbers.

To declare a function in JavaScript use the ‘function’ keyword. For example:

function product(a, b) {
return a*b;
}

In the above example, I’ve declared a function called ‘product’ and I’ve passed 2 parameters to this function, ‘a’ and ‘b’ which are variables whose product is returned by this function. Now, in order to call a function and pass a value to these parameters you’ll have to follow the below syntax:

product(8,2);

In the above code snippet I’m calling the product function with a set of values (8 & 2). These are values of the variables ‘a’ and ‘b’ and they’re called as arguments to the function.

Conditional statements – if

Conditional statement is a set of rules performed if a certain condition is met. The ‘if’ statement is used to execute a block of code, only if the condition specified holds true.

What is JavaScript – if flowchart

To declare an if statement in JavaScript use the ‘if’ keyword. The syntax is:

if(condition) {
statement;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[2]) {
console.log('Correct!');
}

In the above example I’ve defined an array of numbers and then I’ve defined an if block. Within this block is a condition and a statement. The condition is ‘(numbers[0]==numbers[2])’ and the statement is ‘console.log(‘Correct!’)’. If the condition is met, only then the statement will be executed.

Conditional statements- Else if

Else statement is used to execute a block of code if the same condition is false.

What is JavaScript – Else-if flowchart

The syntax is:

if(condition) {
statement a;
}
else (condition) {
statement b;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[4] {
console.log("Correct!");
}
else {
console.log("Wrong, please try again");
}

In the above example, I’ve defined an if block as well as an else block. So if the conditions within the if block holds false then the else block gets executed. Try this for yourself and see what you get!

**Loops **

Loops are used to repeat a specific block until some end condition is met. There are three categories of loops in JavaScript :

  1. while loop
  2. do while loop
  3. for loop
While loop

While the condition is true, the code within the loop is executed.

What is JavaScript – while loop flowchart

The syntax is:

while(condition) {
loop code;
}

Now let’s look at an example:

let i=0;
while(i < 5) {
console.log("The number is " +i);
i++;
}

In the above example, I’ve defined a while loop wherein I’ve set a condition. As long as the condition holds true, the while loop is executed. Try this for yourself and see what you get!

Do while loop

This loop will first execute the code, then check the condition and while the condition holds true, execute repeatedly.

What is JavaScript – Do while loop flowchart

Refer the syntax to better understand it:

do {
loop code;
} while(condition);

This loop executes the code block once before checking if the condition is true, then it will repeat the loop as long as the condition holds true.

Now let’s look at an example:

do {
console.log("The number is " +i);
i++;
}
while(i > 5);

The above code is similar to the while loop code except, the code block within the do loop is first executed and only then the condition within the while loop is checked. If the condition holds true then the do loop is executed again.

For loop

The for loop repeatedly executes the loop code while a given condition is TRUE. It tests the condition before executing the loop body.

What is JavaScript – for loop flowchart

The syntax is:

for(begin; condition; step) {
loop code;
}

In the above syntax:

  • begin statement is executed one time before the execution of the loop code
  • condition defines the condition for executing the loop code
  • step statement is executed every time after the code block has been executed

For example:

for (i=0;i<5;i++) {
console.log("The number is " +i);
}

In the above example, I’ve defined a for loop within which I’ve defined the begin, condition and step statements. The begin statement is that ‘i=0’. After executing the begin statement the code within the for loop is executed one time. Next, the condition is checked, if ‘i<5’ then, the code within the loop is executed. After this, the last step statement (i++) is executed. Try this and see what you get!

Switch Case

The switch statement is used to perform different actions based on different conditions.

What is JavaScript – Switch case flowchart

Let’s look at the syntax for switch case:

switch(expression) {
case 1:
code block 1
break;
case 2:
code block 2
break;
default:
code block 3
break;
}

How does it work?

  • Switch expression gets evaluated once
  • Value of the expression is compared with the values of each case
  • If there is a match, the associated block of code is executed

Let’s try this with an example:

let games='football';
switch(games) {
case "throwball":
console.log("I dislike throwball!");
break;
case "football":
console.log("I love football!");
break;
case "cricket":
console.log("I'm a huge cricket fan!");
break;
default:
console.log("I like other games");
break;
}

In the above example the switch expression is ‘games’ and the value of games is ‘football’. The value of ‘games’ is compared with the value of each case. In this example it is compared to ‘throwball’, ‘cricket’ and ‘football’. The value of ‘games’ matches with the case ‘football’, therefore the code within the ‘football’ case is executed. Try this for yourself and see what you get!

With this, we come to the end of this blog. I hope you found this blog informative and I hope you have a basic understanding of JavaScript. In my next blog on JavaScript I’ll be covering in-depth concepts, so stay tuned.

Also, check out our video on JavaScript Fundamentals if you want to get started as soon as possible and don’t forget to leave a comment if you have any doubt and also, let us know whether you’d want us to create more content on JavaScript. We are listening!

Learn JavaScript in 60 Minutes | JavaScript Crash Course | JavaScript Tutorial |

This video on "JavaScript" will help you learn JavaScript basics and fundamental concepts in 60 minutes. This will provide you in-depth knowledge about the JavaScript fundamentals that will help you write your own code in JavaScript and build a website. This JavaScript tutorial covers following topics..

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JavaScript | How to use classes in JavaScript

JavaScript | How to use classes in JavaScript

Classes in JavaScript are a special syntax for its prototypical inheritance model that resembles class based inheritance in other object oriented languages. Classes are just special functions that can be declared to resembles classes in other languages. In JavaScript, we can have class declarations and class expressions, because they are just functions. So like all other functions, there are function declarations and function expressions. Classes serve a templates to create new objects.

Defining Classes

To declare a class, or make a class declaration, we use the class keyword to do so. For example, to declare a simple class, we can write:

class Person{
  constructor(firstName, lastName) {
    this.firstName= firstName;
    this.lastName = lastName;
  }
}

Class declarations aren’t hoisted so they can used before they are defined in the code, as the JavaScript interpreter will not automatically pull them up to the top. So the class above won’t work before it’s defined in the code like the following:

const person = new Person('John', 'Smith');
class Person{
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

We will get a ReferenceError if we run the code above.

We can also define a class by a class expression, which is an alternative syntax for defining a class. They can be named or unnamed. We can also assign a class to a variable like we do with functions. If we do that, we can reference the class by its name. For example, we can define:

let Person = class {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

To get the name of the unnamed classes above, we can get the name with the name property, like so:

console.log(Person.name);


We can also undefined a named class like the following:


let Person = class Person2{
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

Then to get the name of the class, we can use the name property again. So we if we write:

console.log(Person.name)


we get Person2 logged.

The class body is defined with curly brackets. We define the class members inside the brackets. The body of the class is executed in strict mode, so everything defined in strict mode applies to the definition of a class, so we can’t define variables with out some keyword before it like var , let or const , and many other rules apply when you define a class. Classes in JavaScript also have a constructor method that lets us set fields when the object is instantiated with a class . Each class can only have one constructor method in it. If there’s more than one, then SyntaxError will be thrown. A constructor have to also call the super method to call the constructor of the super class inside if it the class extends a parent class.

Methods that aren’t declared static constitutes of the prototypical methods of the class. They are called after an object has been created by using the new keyword. For example, the following class have only prototypical methods:


class Person{
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  get fullName(){
    return `${this.firstName} ${this.lastName}`  
  }
  sayHi(){
    return `Hi, ${this.firstName} ${this.lastName}`
  }
}

In the Person class above, fullName and sayHi are prototypical methods. They are called like this:

const person = new Person('Jane', 'Smith');
person.fullName() // 'Jane Smith'

Static methods are methods that can be called without creating an object from the class using the new keyword. For instance, we can have something like the following:


class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  get fullName() {
    return `${this.firstName} ${this.lastName}`
  }
  sayHi() {
    return `Hi, ${this.firstName} ${this.lastName}`
  }
  static personCount() {
    return 3;
  }
}

We can call the personCount function without using the new keyword to create an instance of the class. So if we write:

Person.personCount

We get 3 returned.

The this value inside prototypical methods will be the value of the object. For static methods the value of this has the class that the static method is in as the value.

Getters and Setters

JavaScript classes can have getters and setter functions. Getters, as the name suggests, is a method that lets us get some data from a class. Setters are methods that gives us the ability to set some fields of the class. We denote getter functions with the get keyword and setters with the set keyword. For example, we can write a class that has getters and setters like the following:

class Person {
  constructor(firstName, lastName) {
    this._firstName = firstName;
    this._lastName = lastName;
  }
  get fullName() {
    return `${this.firstName} ${this.lastName}`
  }
  get firstName() {
    return this._firstName
  }
  get lastName() {
    return this._lastName
  }
  sayHi() {
    return `Hi, ${this.firstName} ${this.lastName}`
  }
  set firstName(firstName) {
    this._firstName = firstName;
  }
  set lastName(lastName) {
    this._lastName = lastName;
  }
}

Then when we use the new keyword to construct a Person object, we can use them in the following way:


const person = new Person('Jane', 'Smith');
person.firstName = 'John';
person.lastName = 'Doe';
console.log(person.firstName, person.lastName)

Since we have the getter and setter functions, we can use them to set the data directly to set the data for firstName and lastName of the Person class. In the setter functions, which start with the keyword set , what we assign to them get passed into the parameters and set in the member of the class. In the getter functions, which are denote by get we return the member values so that we can use them.

JavaScript Inheritance

In JavaScript, we can create classes where the properties can be included in the properties of a child class.

So, we can have a high-level class that contains the properties that are common to all the child classes, and the child class can have its own special properties that are not in any other classes.

For example, if we have an Animal class with the common properties and methods, like name and the eat method, then the Bird class can just inherit the common properties in the Animal class. They don’t have to be defined in the Bird class again.

We can write the following to do inheritance in JavaScript:

class Animal {
  constructor(name) {
    this.name = name;
  }
  eat() {
    console.log('eat');
  }
}
class Bird extends Animal {
  constructor(name, numWings) {
    super(name);
    this.numWings = numWings;
  }
}
const bird = new Bird('Joe', 2);
console.log(bird.name)
bird.eat();

In the example above, we have the parent class, Animal, that has the eat method, which all classes that extends from Animal will have, so they don’t have to define eat again.

We have the Bird class which extends the Animal class. Note that in the constructor of the Bird class, we have the super() function call to call the parent’s class constructor to populate the properties of the parent class in addition to the properties of the child class.

Classes cannot extend regular objects, which cannot be constructed with the new keyword. If we want to inherit from a regular object, we have to use the Object.setPrototypeOf function to set a class to inherit from a regular object. For example:

const Animal = {
  eat() {
    console.log(`${this.name} eats`);
  }
};
class Cat{
  constructor(name) {
    this.name = name;
  }
}
class Chicken{
  constructor(name) {
    this.name = name;
  }
}
Object.setPrototypeOf(Cat.prototype, Animal);
Object.setPrototypeOf(Chicken.prototype, Animal);
let cat = new Cat('Bob');
let chicken = new Chicken('Joe');
cat.eat();
chicken.eat();

If we run the example code above, we have see Bob eats and Joe eats logged because we have inherited the eat function from the Animal object.

this Keyword

The this keyword allows us to access the current object’s properties inside an object, unless you’re using arrow functions.

As we can see from the above example, we can get the properties of the instance of the child and the parent class in the object.

Mixins

We can use mixins to do multiple inheritance in JavaScript. Mixins are templates for creating classes. We need mixins to do multiple inheritance because JavaScript classes can only inherit from one super class, so multiple inheritance isn’t possible.

For example, if we have a base class, we can define mixins to incorporate the members from multiple classes into one by composing the mixins by calling one and then pass the returned result into the next one as the argument, an so on, like so:

class Base {
  baseFn() {
    console.log('baseFn called');
  }
}
let classAMixin = Base => class extends Base {
  a() {
    console.log('classAMixin called');
  }
};
let classBMixin = Base => class extends Base {
  b() {
    console.log('classBMixin called');
  }
};
class Bar extends classAMixin(classBMixin(Base)) {}
const bar = new Bar();
bar.baseFn()
bar.a()
bar.b()

In the code above, we have the Base class which we pass into the classBMixin to get the b function into the Base class, then we call the classAMixin by passing in the result of classBMixin(Base) into the argument of the classAMixin to return the a function from classAMixin into the Base class and then return the whole class with all the functions from all the classes incorporated into one.

If we call all the functions above like we did by creating an instance of the Bar object and then call the baseFn , a and b functions, then we get:

baseFn called
classAMixin called
classBMixin called

This means that we have all the functions from the mixins incorporated into the new Bar class.

In JavaScript, classes are just syntactic sugar to make the prototypical inheritance of JavaScript clearer by letting us structure the code in a way that’s more like typical inheritance class based object oriented inheritance pattern. This means that we write classes to and use the new keyword to create objects from the classes, but underneath the syntactic sugar, we are still using prototypical inheritance to extend objects. We can extend classes from objects and we can also use mixins to do multiple inheritance in of JavaScript classes.