Variables  are the first building block you will be introduced to when learning most languages. Variables are values in your code that can represent different values each time the code runs. Here is an example of two variables in a script:

firstname = "maaike";
x = 2;

And they can be assigned a new value while the code is running:

firstname = "Edward";
x=7;

Without variables, a piece of code would do the exact same thing every single time it was run. Even though that could still be helpful in some cases, it can be made much more powerful by working with variables to allow our code to do something different every time we run it.

Declaring variables

The first time you create a variable, you declare it. And you need a special word for that: let, var,  or const. We’ll discuss the use of these three arguments shortly. The second time you call a variable, you only use the name of the existing variable to assign it a new value:

let firstname = "Maria";
firstname = "Jacky";

In our examples, we will be assigning a value to our variables in the code. This is called “hardcoded” since the value of your variable is defined in your script instead of coming dynamically from some external input. This is something you won’t be doing that often in actual code, as more commonly the value comes from an external source, such as an input box on a website that a user filled out, a database, or some other code that calls your code. The use of variables coming from external sources instead of being hardcoded into a script is actually the reason that scripts are adaptable to new information, without having to rewrite the code.

We have just established how powerful the variable building block is in code. Right now, we are going to hardcode variables into our scripts, and they therefore will not vary until a coder changes the program. However, we will soon learn how to make our variables take in values from outside sources

let, var, and const

A variable definition consists of three parts: a variable-defining keyword (let,var, or const), a name, and a value. Let’s start with the difference between let, var, or const. Here you can see some examples of variables using the different keywords:  

 let nr1 = 12;
var nr2 = 8;
const PI = 3.14159;

let and var are both used for variables that might have a new value assigned to them
somewhere in the program. The difference between let and var is complex. It is
related to scope.

Note: If you understand the following sentences on scope, that is great, but it is totally fine if you do not get it. You will understand it soon enough as you keep
working your way through the blog.

var has global scope and let has block scope. var’s global scope means that you can use
the variables defined with var in the entire script. On the other hand, let’s
block scope means you can only use variables defined with let in the specific
block of code in which they were defined. Remember, a block of code will always
start with { and end with), which is how you can recognize them.

On the other hand, const is used for variables that only get a value assigned once-
for example, the value of pi, which will not change. If you try reassigning a
value declared with const, you will get an error:

const someConstant = 3 
someConstant = 4;

This will result in the following output:

Uncaught TypeError: Assignment to constant variable.

We will be using let in most of our examples for now, trust us that you should use let
in most cases.  

Naming variables

When it comes to naming variables, there are some conventions in place:

• Variables start with a lowercase letter, and they should be descriptive. If something holds an age, do not call it x, but age. This way, when you read your script later, you can easily understand what you did by just reading your code
• Variables cannot contain spaces, but they can use underscores. If you use a space, JavaScript doesn’t recognize it as a single variable.

NOTE: We will be using camel case here. This means that when we want to use multiple words to describe a variable, we will start with a lowercase word, then use a capital for every new word after the first word-for example: ageOfBuyer.
Whatever the convention is in the place you are working, the key is consistency. If all naming is done in a similar format, the code will look cleaner and more readable, which makes it a lot easier to make a small change later
The value of your variable can be anything. Let's start with the easiest thing variables can be primitives.