# Variables and Constants

Updated on Dec 22, 2021 by [Alan Morel](/content/alanmorel/index.html)

## Table of Contents

1. [Variables](/content/classes/javascript/variables-constants#variables/index.html)
2. [Declaration](/content/classes/javascript/variables-constants#declaration/index.html)
3. [Variable Naming Guidelines](/content/classes/javascript/variables-constants#variable-naming-guidelines/index.html)
4. [Initialization](/content/classes/javascript/variables-constants#initialization/index.html)
5. [Assignment](/content/classes/javascript/variables-constants#assignment/index.html)
6. [Constants](/content/classes/javascript/variables-constants#constants/index.html)
7. [Difference between let and const](/content/classes/javascript/variables-constants#difference-between-let-and-const/index.html)
8. [Resources](/content/classes/javascript/variables-constants#resources/index.html)

Pretty much every single piece of software requires the use of **variables**. Variables are a fundamental part of any programming language, and [JavaScript](/content/classes/javascript/index.html) is no different.

## Variables

Variables are containers that you give a name to that hold any piece of information or data for you. Variables are used to store data in memory, and to access that data later.

The two main parts of a variable are the **name** and its **value**. Because they are variables, their value is meant to **vary** over time, in other words change. Let's create our first one.

## Declaration

When you want to create a variable, you need to **declare** it. This tells JavaScript "hey, I want this name to represent a variable".

Let's say you want a variable to hold the number of apples you eat every day. It may look something like this:

```javascript
let apples;
```

You have now **declared** a variable called `apples`, but have yet to give it a **value**.

## Variable Naming Guidelines

Below are the rules for how to name your variables.

1. The first character has to be a letter, an underscore, or a dollar sign.
2. After the first character, you are now free to use numbers if you'd like, however, you cannot start with a number.

Examples of valid JavaScript variable names:

```javascript
let apples;
let $apples;
let _apples;
let app_les;
let apples4eva;
let apple$;
```

And examples of invalid JavaScript variable names:

```javascript
let 1apples;
let app les;
let #apples;
let apple%;
```

Also important to note is that variable names are case-sensitive, so `Apples` and `apples` are **different variables**.

## Initialization

After you have declared a variable with a valid name, you can now **initialize** it, which is to give the variable its **initial value**. This is the value that it will have when you first declare it.

The syntax for this is simple and intuitive. Let's say you look at a modest **384 memes** a day. Here is how you would initialize your variable.

```javascript
let memes = 384;
```

Likewise, if your variable is a string, like for example, the name of something, it would look like this:

```javascript
let website = "Sabe";
```

How do you know if the variable initialization worked? Try running this in your console:

```javascript
const memes = 384;
const website = "Sabe";

console.log("I view " + memes + " memes a day.");
console.log("I use " + website + " every day.");
```

You should get this as your output:

```html
I view 384 memes a day.
I use Sabe every day.
```

## Assignment

After a variable has been declared and initialized, you can always give it a new value by **assigning** it one. Because you already declared it, you don't need to declare it again.

To give the variables `memes` and `website` new values, it would look like this:

```javascript
memes = 463;
website = "Google";
```

Thus, when you run this:

```javascript
let memes = 384;
let website = "Sabe";

console.log("I view " + memes + " memes a day.");
console.log("I use " + website + " every day.");

memes = 463; // assigning a new value to memes
website = "Google"; // assigning a new value to website

console.log("I view " + memes + " memes a day.");
console.log("I use " + website + " every day.");
```

Your output looks like this:

```html
I view 384 memes a day.
I use Sabe every day.
I view 463 memes a day.
I use Google every day.
```

## Constants

**Constants** in JavaScript are basically variables except that their values are meant to be unchanging after being initialized. Instead of using the `let` keyword, they are declared using `const`.

The rules for naming constants are the same for variables, however, it is common practice to capitalize all the letters. Here is an example of two constants:

```javascript
const MEMES = 100;
const WEBSITE = "Twitter";

console.log("I view " + MEMES + " memes a day.");
console.log("I use " + WEBSITE + " every day.");
```

The output using constants.

## Difference between let and const

You might be wondering, what is the difference between `let` and `const`? The difference between `let` and `const` is that `const` is a **constant**, meaning that it cannot be changed. This is different from `let` which is a **variable**. Once you assign a value to a `const`, you cannot change it, however, you can change the value of a `let` variable. If you know you value will never change, you can use `const` instead of `let`. The use of `var` is deprecated in JavaScript, so you should avoid using it in your code.

## Resources

- [Let - MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let)
- [Const - MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const)