How to code in JavaScript

JavaScript is a programming language used to make websites dynamic & interactive.

For example, it allows you to:

JavaScript can also be used on the server side to write the backend or API of your website. In that context it's called Node or NodeJs.

On the frontend side, JavaScript is often used in combination with HTML & CSS.

Basics of JavaScript

Let's start with the basics of JavaScript by looking at a simple example:

function sum(a, b) {
  return a + b;
This code defines a function (which is a reusable piece of code) called sum. This means that you can now call sum(1, 3) which returns 4. You can run it again with different values, such as sum(2, 5) and it will return the result of 2 + 5 which is 7.

A note on the return keyword

The keyword return instructs the function that this is the result of that function. Think of it as an operation and then when it completes, it will give you the result of the sum.


You can create a string in JavaScript by simply using the double quotes (") or single quotes (') and later you will see the backtick character.
Here's an example:

"This is a string";
'this is another string!'

There is no difference between using a double quote or a single quote. They are exactly the same. Both of these strings do not support interpolation (which means having a variable inside of them).

Basic String properties

The .length property is used to return the length of the string. Here's an example of getting the length of "Nice!":

// returns 5

Basic String methods

Here are some common methods that you can call on strings:


This method returns true when the searchString is included inside the parent string. For example:

"Hello World".includes("Hello"); // true
"Hello World".includes("Potato"); // false


This will return a new string that has all of its characters in upper case:

  "hello".toUpperCase(); // "HELLO";


This will return a new string that has all of its characters in lower case:

"NICe".toLowerCase(); // "nice";


1, 2, 0, -5, 3.5, 2000, 2019, -23.51.
All of these are examples of Numbers in JavaScript. It doesn't matter if it's negative or positive, if it has a decimal (values after the .) or not. We call them numbers.

Converting from number to string

You can convert a number to string by simply calling the .toString() method. Let's take an example where we have a variable called answer with a value 42:

answer.toString(); //"42"

Converting from string to number

In some scenarios, you'd like to convert from a string to a number. For that, you'd have to use the Number.parseInt method. Here's an example:

Number.parseInt("42", 10); //42

Note that the Number in Number.parseInt("42") is an Object in JavaScript that contains methods related to numbers, and .parseInt() is one of them. It's a method that you call on Number to convert a string into a number.

The 2nd argument is called radix. A radix of 10 means that we're using the decimal system. In short, humans use the decimal system (because we have 10 fingers, from 0 to 9), and computers use the binary system (on and off, 0s and 1s).

Unfortunately parseInt does not default to a radix of 10, thus it is recommended that you always specify it. If you omit the radix, it may or may not work as you expect based on the string you provided as a first argument. Thus to be safe, always specify the radix.

How to learn JavaScript interactively?

You can read short lessons and then practise them immediately.
And in order to persist your new knowledge, you can recap those topics with a flashcards app.

Learn JavaScript