There is a unique object in JavaScript that calls `const`. 

> Note
>
> `const` - is a constant value that can be used like a variable but cannot be changed.

You need to use the `const` keyword to define constant:

```javascript
const CONSTANT_NAME = 10;
```

> Note
>
> Constants should be named in **UPPER_SNAKE_CASE** style. The **snake_case** means that words will be tied by the `_` symbol. The **UPPER** means that words will be written in **uppercase**.

Look at the example with a changeable variable:

// a variable changing
let a = 5;
console.log(a)

a = 10;
console.log(a)

Trying to change the value of the constant should cause the `TypeError`:

// a constant changing
const B = 7;
console.log(B);

B = 8;
console.log(B);

Also, you cannot define a const without a value: it will cause the `SyntaxError`:

# Usage
Constants are used like unchangeable variables. You can define constant one time and use it multiple times.

Constants provide data integrity with the ability to refactor quickly.

> Note
>
> **Refactor** is a change code structure. (Change names of variables, values, etc.)

For example, `MAX_HEIGHT` for site elements. You can release the site update by changing the maximum height of the elements by modifying only one value in the code. However, during runtime, it is not possible to change the maximum height, which ensures the integrity of data:

const MAX_HEIGHT = 250;
console.log(MAX_HEIGHT - 15);
console.log(MAX_HEIGHT - 12);
console.log(MAX_HEIGHT - 5);
console.log(MAX_HEIGHT - 13);
console.log(MAX_HEIGHT - 22);
console.log(MAX_HEIGHT - 52);

Constants can receive every value that variables can receive.

const A = "Hi!";
const B = "I'm";
const C = 15;
const D = "years old";

console.log(A, B, C, D);

First, let's get acquainted with JavaScript and look at its syntax. In this section, you will get a basic familiarity with the JavaScript syntax to make it easier to learn in detail later on.

Now it's time to learn about the data types that JavaScript works with. We'll start by looking at the difference between literals and variables, and then delve into the various data types.

We have become familiar with the various data types. Now, let's learn how to work with them by learning the operations we can perform with each data type.

Next, we will learn how to control the flow of execution. It's a straightforward process: we can tell the interpreter exactly what to do under certain conditions.

Next, we will explore how to shorten code using repetitive looped actions. We will examine the types of loops available in JavaScript and learn how to work with them.

Finally, to be able to write simple programs, we need to learn about functions that can make our lives much easier. We will learn how to create and use our functions.

Usage

Define a variable:

Define a constant: