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JavaScript has **three ways to declare a variable**: `var`, `let`, and `const`.
They differ in **scope**, **whether they can be redeclared**, and **hoisting behavior**.
Let's go through them in detail.

---

## **1.** `var` **- the old way (ES5 and earlier)**

```javascript
var name = 'Tim';
```

### Features:

- It can be **redeclared** and **reassigned**:

```javascript
  var x = 10;
  var x = 20; // does not cause an error
  ```
- **Function scope** (ignores `{}` blocks)

```javascript
  if (true) {
      var a = 5;
  }
  console.log(a); // 5 - the variable "escaped" outside the block
  ```
- **Hoists** - the variable is accessible before its declaration, but its value is `undefined`:

```javascript
  console.log(user); // undefined
  var user = 'Alex';
  ```

> Using `var` in modern code **is not recommended**.

---

## **2.** `let` **- the modern way for mutable variables**

```javascript
let age = 25;
age = 26; // can be changed
```

### Features:

- The value **can be changed**, but it cannot be redeclared in the same block:

```javascript
  let a = 1;
  // let a = 2; Error
  ```
- **Block scope**:

```javascript
  if (true) {
      let x = 10;
  }
  console.log(x); // ReferenceError - the variable is not visible outside the block
  ```
- Not accessible before its declaration (unlike `var`):

```javascript
  console.log(a); // ReferenceError
  let a = 5;
  ```

> Use `let` when the variable's value will **change**.

---

## **3.** `const` **- a constant (immutable reference)**

```javascript
const PI = 3.14;
```

### Features:

- It cannot be **reassigned** or **redeclared**:

```javascript
  const x = 10;
  x = 20; // TypeError
  ```
- But if it is an **object or array**, its internal data can be changed:

```javascript
  const user = { name: 'Tim' };
  user.name = 'Alex'; // allowed
  // user = {} cannot reassign the reference itself
  ```
- **Block scope** - same as `let`.

> Use `const` **by default**, and `let` only if the variable actually needs to change.

---

## Quick comparison:

| Feature | `var` | `let` | `const` |
|---|---|---|---|
| Scope | Function | Block | Block |
| Can be redeclared | Yes | No | No |
| Can be reassigned | Yes | Yes | No |
| Hoisting | Yes, with value `undefined` | Yes, but cannot be used before declaration | Yes, but cannot be used before declaration |
| Modern standard | No, deprecated | Yes | Yes |

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