Understanding JavaScript Promises for Dummies

Understanding JavaScript Promises for Dummies

Javascript Promises are not difficult. However, lots of people find it a little bit hard to understand at the beginning. Therefore, I would like to write down the way I understand promises, in a dummy way.

Javascript Promises are not difficult. However, lots of people find it a little bit hard to understand at the beginning. Therefore, I would like to write down the way I understand promises, in a dummy way.

Table of Contents

  • Understanding Promises
  • Creating a Promise
  • Consuming Promises
  • Chaining Promises
  • Promises are Asynchronous
  • Promises in ES5, ES6/2015, ES7/Next
  • Why Promises and When to Use Them?
  • New Kid On the Block: Observables
  • Summary
Understanding Promises

A Promise in short:

"Imagine you are a kid. Your mom promises you that she'll get you a new phone next week."

You don't know if you will get that phone until next week. Your mom can either really buy you a brand new phone, or stand you up and withhold the phone if she is not happy :(.

That is a promise. A promise has 3 states. They are:

  1. Pending: You don't know if you will get that phone
  2. Fulfilled: Mom is happy, she buys you a brand new phone
  3. Rejected: Your mom is happy, she withholds the phone
Creating a JavaScript Promise

Let's convert this to JavaScript.

/_ ES5 _/
var isMomHappy = false;

// Promise
var willIGetNewPhone = new Promise(
    function (resolve, reject) {
        if (isMomHappy) {
            var phone = {
                brand: 'Samsung',
                color: 'black'
            };
            resolve(phone); // fulfilled
        } else {
            var reason = new Error('mom is not happy');
            reject(reason); // reject
        }

    }
);

The code is quite expressive in itself.

// promise syntax look like this
new Promise(/_ executor_/ function (resolve, reject) { ... } );
Consuming Promises

Now that we have the promise, let's consume it.

/_ ES5 _/
...

// call our promise
var askMom = function () {
    willIGetNewPhone
        .then(function (fulfilled) {
            // yay, you got a new phone
            console.log(fulfilled);
         // output: { brand: 'Samsung', color: 'black' }
        })
        .catch(function (error) {
            // oops, mom don't buy it
            console.log(error.message);
         // output: 'mom is not happy'
        });
};

askMom();

Let's run the example and see the result!

Demo: https://jsbin.com/nifocu/1/edit?js,console

Chaining Promises

Promises are chainable.

Let's say, you, the kid, promises your friend that you will show them the new phone when your mom buy you one.

That is another promise. Let's write it!

...

// 2nd promise
var showOff = function (phone) {
    return new Promise(
        function (resolve, reject) {
            var message = 'Hey friend, I have a new ' +
                phone.color + ' ' + phone.brand + ' phone';

            resolve(message);
        }
    );
};

Notes:

// shorten it
...

// 2nd promise
var showOff = function (phone) {
    var message = 'Hey friend, I have a new ' +
                phone.color + ' ' + phone.brand + ' phone';

    return Promise.resolve(message);
};

Let's chain the promises. You, the kid can only start the showOff promise after the willIGetNewPhone promise.

...

// call our promise
var askMom = function () {
    willIGetNewPhone
    .then(showOff) // chain it here
    .then(function (fulfilled) {
            console.log(fulfilled);
         // output: 'Hey friend, I have a new black Samsung phone.'
        })
        .catch(function (error) {
            // oops, mom don't buy it
            console.log(error.message);
         // output: 'mom is not happy'
        });
};

That's how easy to chain the promise.

Promises are Asynchronous

Promises are asynchronous. Let's log a message before and after we call the promise.

// call our promise
var askMom = function () {
    console.log('before asking Mom'); // log before
    willIGetNewPhone
        .then(showOff)
        .then(function (fulfilled) {
            console.log(fulfilled);
        })
        .catch(function (error) {
            console.log(error.message);
        });
    console.log('after asking mom'); // log after
}

What is the sequence of expected output? Probably you expect:

1. before asking Mom
2. Hey friend, I have a new black Samsung phone.
3. after asking mom

However, the actual output sequence is:

1. before asking Mom
2. after asking mom
3. Hey friend, I have a new black Samsung phone.

Why? Because life (or JS) waits for no man.

You, the kid, wouldn't stop playing while waiting for your mom promise (the new phone). Don't you? That's something we call asynchronous, the code will run without blocking or waiting for the result. Anything that need to wait for promise to proceed, you put that in .then.

Promises in ES5, ES6/2015, ES7/Next

ES5 - Majority browsers

The demo code is workable in ES5 environments (all major browsers + NodeJs) if you include Bluebird promise library. It's because ES5 doesn't support promises out of the box. Another famous promise library is Q by Kris Kowal.

ES6 / ES2015 - Modern browsers, NodeJs v6

The demo code works out of the box because ES6 supports promises natively. In addition, with ES6 functions, we can further simplify the code with fat arrow => and use const and let.

Here is an example of ES6 code:

/_ ES6 _/
const isMomHappy = true;

// Promise
const willIGetNewPhone = new Promise(
    (resolve, reject) => { // fat arrow
        if (isMomHappy) {
            const phone = {
                brand: 'Samsung',
                color: 'black'
            };
            resolve(phone);
        } else {
            const reason = new Error('mom is not happy');
            reject(reason);
        }

    }
);

const showOff = function (phone) {
    const message = 'Hey friend, I have a new ' +
                phone.color + ' ' + phone.brand + ' phone';
    return Promise.resolve(message);
};

// call our promise
const askMom = function () {
    willIGetNewPhone
        .then(showOff)
        .then(fulfilled => console.log(fulfilled)) // fat arrow
        .catch(error => console.log(error.message)); // fat arrow
};

askMom();

Notes that all the var are replaced with const. All the function(resolve, reject) has been simplified to (resolve, reject) =>. There are a few benefits come with these changes. Read more on:-

ES7 - Async Await make the syntax look prettier

ES7 introduce async and await syntax. It makes the asynchronous syntax look prettier and easier to understand, without the .then and .catch.

Rewrite our example with ES7 syntax.

/_ ES7 _/
const isMomHappy = true;

// Promise
const willIGetNewPhone = new Promise(
    (resolve, reject) => {
        if (isMomHappy) {
            const phone = {
                brand: 'Samsung',
                color: 'black'
            };
            resolve(phone);
        } else {
            const reason = new Error('mom is not happy');
            reject(reason);
        }

    }
);

// 2nd promise
async function showOff(phone) {
    return new Promise(
        (resolve, reject) => {
            var message = 'Hey friend, I have a new ' +
                phone.color + ' ' + phone.brand + ' phone';

            resolve(message);
        }
    );
};

// call our promise
async function askMom() {
    try {
        console.log('before asking Mom');

        let phone = await willIGetNewPhone;
        let message = await showOff(phone);

        console.log(message);
        console.log('after asking mom');
    }
    catch (error) {
        console.log(error.message);
    }
}

(async () => {
    await askMom();
})();
Why Promises and When to Use Them?

Why do we need promises? How's the world look like before promise? Before answering these questions, let's go back to the fundamental.

Normal Function vs Async Function

Let's take a look at these two example, both example perform addition of two number, one add using normal function, the other add remotely.

Normal Function to Add Two Numbers

// add two numbers normally

function add (num1, num2) {
    return num1 + num2;
}

const result = add(1, 2); // you get result = 3 immediately
Async Function to Add Two numbers
// add two numbers remotely

// get the result by calling an API
const result = getAddResultFromServer('http://www.example.com?num1=1&num2=2');
// you get result  = "undefined"

If you add the numbers with normal function, you get the result immediately. However when you issue a remote call to get result, you need to wait, you can't get the result immediately.

Or put it this way, you don't know if you will get the result because the server might be down, slow in response, etc. You don't want your entire process to be blocked while waiting for the result.

Calling APIs, downloading files, reading files are among some of the usual async operations that you'll perform.

World Before Promises: Callback

Must we use promise for asynchronous call? Nope. Prior to Promise, we use callback. Callback is just a function you call when you get the return result. Let's modify the previous example to accept a callback.

// add two numbers remotely
// get the result by calling an API

function addAsync (num1, num2, callback) {
    // use the famous jQuery getJSON callback API
    return $.getJSON('http://www.example.com', {
        num1: num1,
        num2: num2
    }, callback);
}

addAsync(1, 2, success => {
    // callback
    const result = success; // you get result = 3 here
});

The syntax looks ok, why do we need promises then?

What if You Want to Perform Subsequent Async Action?

Let's say, instead of just add the numbers one time, we want to add 3 times. In a normal function, we do this:-

// add two numbers normally

let resultA, resultB, resultC;

 function add (num1, num2) {
    return num1 + num2;
}

resultA = add(1, 2); // you get resultA = 3 immediately
resultB = add(resultA, 3); // you get resultB = 6 immediately
resultC = add(resultB, 4); // you get resultC = 10 immediately

console.log('total' + resultC);
console.log(resultA, resultB, resultC);

How it looks like with callbacks?

// add two numbers remotely
// get the result by calling an API

let resultA, resultB, resultC;

function addAsync (num1, num2, callback) {
    // use the famous jQuery getJSON callback API
    return $.getJSON('http://www.example.com', {
        num1: num1,
        num2: num2
    }, callback);
}

addAsync(1, 2, success => {
    // callback 1
    resultA = success; // you get result = 3 here

    addAsync(resultA, 3, success => {
        // callback 2
        resultB = success; // you get result = 6 here

        addAsync(resultB, 4, success => {
            // callback 3
            resultC = success; // you get result = 10 here

            console.log('total' + resultC);
            console.log(resultA, resultB, resultC);
        });
    });
});

Demo: https://jsbin.com/barimo/edit?html,js,console

The syntax is less user friendly. In a nicer term, It looks like a pyramid, but people usually refer this as "callback hell", because the callback nested into another callback. Imagine you have 10 callbacks, your code will nested 10 times!

Escape From Callback Hell

Promises come in to rescue. Let's look at the promise version of the same example.

// add two numbers remotely using observable

let resultA, resultB, resultC;

function addAsync(num1, num2) {
    // use ES6 fetch API, which return a promise
    return fetch(`http://www.example.com?num1=${num1}&num2=${num2}`)
        .then(x => x.json());
}

addAsync(1, 2)
    .then(success => {
        resultA = success;
        return resultA;
    })
    .then(success => addAsync(success, 3))
    .then(success => {
        resultB = success;
        return resultB;
    })
    .then(success => addAsync(success, 4))
    .then(success => {
        resultC = success;
        return resultC;
    })
    .then(success => {
        console.log('total: ' + success)
        console.log(resultA, resultB, resultC)
    });

Demo: https://jsbin.com/qafane/edit?js,console

With promises, we flatten the callback with .then. In a way, it looks cleaner because of no callback nesting. Of course, with ES7 async syntax, we can even further enhance this example, but I leave that to you. :)

New Kid On the Block: Observables

Before you settle down with promises, there is something that has come about to make it even easier to deal with async data called Observables.

Observables are lazy event streams which can emit zero or more events, and may or may not finish.source

Some key differences between promises and observable are:

Fear not, let look at the same demo written with Observables. In this example, I am using RxJS for the observables.

let Observable = Rx.Observable;
let resultA, resultB, resultC;

function addAsync(num1, num2) {
    // use ES6 fetch API, which return a promise
    const promise = fetch(`http://www.example.com?num1=${num1}&num2=${num2}`)
        .then(x => x.json());

    return Observable.fromPromise(promise);
}

addAsync(1,2)
  .do(x => resultA = x)
  .flatMap(x => addAsync(x, 3))
  .do(x => resultB = x)
  .flatMap(x => addAsync(x, 4))
  .do(x => resultC = x)
  .subscribe(x => {
    console.log('total: ' + x)
    console.log(resultA, resultB, resultC)
  });

Demo: https://jsbin.com/dosaviwalu/edit?js,console

Notes:

Observables can do more funky stuff easily. For example, delay add function by 3 seconds with just one line of code or retry so you can retry a call a certain number of times.

...

addAsync(1,2)
  .delay(3000) // delay 3 seconds
  .do(x => resultA = x)
  ...

Well, let's talk about Observables in future post!

Summary

Get yourself familiar with callbacks and promises. Understand them and use them. Don't worry about Observables, just yet. All three can factor into your development depending on the situation.

Here are the demo code for all mom promise to buy phone examples:

That's it. Hopefully this article smoothen your path to tame the JavaScript promises.

Happy coding!

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!

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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.