# Variable

## Function

 variable is used to name the value for subsequent reading, modification and transfer. BT is a dynamically typed language, and variables can be reassigned to different types of values during runtime.

## Syntax

```bt
name = 'Nina'
age = 26
sex = 'Girl'
```

## Naming rules

| Rules | Description |
| ------ | ------ |
| The first character | can be lowercase English letters, `_` or `$`, and cannot be numbers or uppercase letters. |
| The following characters | can be English letters, numbers, `_` or `$`. |
| Case | Case-sensitive, `name` and `Name` have different meanings. |

Names matching `[A-Z][A-Za-z0-9_]*` will be recognized as constants, such as `Name`, `Config`, `NAME`, `Name_1`. If the first letter is capitalized but contains `$`, such as `User$`, an error will be reported directly.

## let local declaration

`let` is used to declare local variables within the current function. The uninitialized variable value is `empty`. `let` is only a variable and cannot be used to declare constants.

```bt
fn demo() {
    let count = 1
    count++
    count
}
```

## Return Value

| Type | Description |
| ------ | ------ |
| Any | Assignment expression returns the value of the expression on the right. |

## Code Examples

```bt
data = 'Nina'
data = 26
data = true

first second third

println data
println is_empty(first)
```

## Notes

- Multiple statements can be written on new lines or separated by commas.
- `first second third` This null declaration initializes the variable to `empty`.
 - Variable names cannot begin with a capital letter; use constant naming rules when non-rewritable bindings are required, and define them directly with assignment syntax.