Learn-Vim/ch26_vimscript_variables_scopes.md

---
title: "Variables and Scopes"
metaTitle: "Variables and Scopes"
metaDescription: "What are the different variable scopes in Vim?"
---

Before diving into Vimscript functions, let's learn about the different sources and scopes of Vim variables.

## `let` and `const`

You can assign a value to a variable in Vim with `let`:

```
let pancake = "pancake"
```

Later you can call that variable any time.

```
echo pancake
" returns "pancake"
```

`let` is mutable, meaning you can change the value at any time in the future.

```
let pancake = "not waffles"

echo pancake
" returns "not waffles"
```

Notice that when you want to change the value of a set variable, you still need to use `let`.

```
let beverage = "milk"

beverage = "orange juice"
" throws an error
```

You can define an immutable variable with `const` (both Vim variable setters are similar to Javascript's `let` and `const`).

```
const waffle = "waffle"

echo waffle
"return waffle

const waffle = "pancake"
" returns error

let waffle = "pancake"
" returns error
```

Before learning native Vimscript variables, it's good to learn the different sources of Vim variables that you can use. There are three more "variables" you can use inside Vim expressions: environment variable, option variable, and register variable.

### Environment Variable (`$VAR`)

Suppose that you have a `SHELL` environment variable already available in your terminal. To access it from Vim:

```
echo $SHELL
" returns $SHELL value. In my case, it returns /bin/bash
```

### Option variable

You can access Vim options with `&` (options are the settings you access with `:set`). To see what background Vim uses, you can run:

```
echo &background
" returns either "light" or "dark"
```

Alternatively, you can always run `set background?` to see the value of the `background` option.

### Register variable

You can access Vim registers (Ch. 08) with `@`. Suppose the value "chocolate" is already saved to register `a`. To access it, you can use `@a`. You can also update it with `let`.

```
echo @a
" returns chocolate

let @a .= " donut"

echo @a
" returns "chocolate donut"
```

Now when you paste from register `a` (`"ap`), it will return "chocolate donut".

The operator `.=` concatenates two strings. The expression `let @a .= " donut"` is the same as `let @a = @a . " donut"`

## Variable Scopes

There are 9 variable scopes in Vim. You can recognize them from their prepended letter:

```
g:           Global variable
{nothing}    Global variable
b:           Buffer-local variable
w:           Window-local variable
t:           Tab-local variable
s:           Sourced Vimscript variable
l:           Function-local variable
a:           Function formal parameter variable
v:           Built-in Vim variable
```

### Global variable

When you are declaring a "regular" variable:

```
let pancake = "pancake"
```

`pancake` is actually a global variable. When you define a global variable, you can call them from anywhere.

Prepending `g:` to a variable also creates a global variable.

```
let g:waffle = "waffle"
```

Both `pancake` and `g:waffle` have the same scope. You can call each of them with either syntax.

```
echo pancake
" returns "pancake"

echo g:pancake
"returns "pancake"

echo waffle
" returns "waffle"

echo g:waffle
" returns "waffle"
```

### Buffer variable

A variable preceded with `b:` is a buffer variable. A buffer variable is a variable that is local to the current buffer (Ch 02). If you have multiple buffers open, each buffer will have their own separate list of buffer variables.

In buffer 1:

```
const b:donut = "chocolate donut"
```

In buffer 2:

```
const b:donut = "blueberry donut"
```

Each `b:donut` buffer variable lives inside buffers 1 and 2, respectively.

On the side note, Vim has a *special* buffer variable `b:changedtick` that keeps track of all the changes done to the current buffer.

1. Run `echo b:changedtick` and note the number it returns..
2. Make changes in Vim.
3. Run `echo b:changedtick` again and note the number it now returns.

### Window variable

A variable preceded with `w:` is a window variable. It exists only in that window.

In window 1:

```
const w:donut = "chocolate donut"
```

In window 2:

```
const w:donut = "raspberry donut"
```

On each window, you can call `echo w:donut` to get unique values.

### Tab variable

A variable preceded with `t:` is a tab variable. It exists only in that tab.

In tab 1:

```
const t:donut = "chocolate donut"
```

In tab 2:

```
const t:donut = "blackberry donut"
```

On each tab, you can call `echo t:donut` to get unique values.

### Script variable

A variable preceded with `s:` is a script variable. These variables can only be accessed from inside scripts.

If you have an arbitrary file `script.vim` and inside it you have:

```
let s:dozen = 12

function Consume()
  let s:dozen -= 1
  echo s:dozen " is left"
endfunction
```

Source the file with `:source dozen.vim`. Now call the `Consume` function:

```
:call Consume()
" returns "11 is left"

:call Consume()
" returns "10 is left"

:echo s:dozen
" Undefined variable error
```

The `Consume` function can be called directly and it increments as expected. When you try to read `s:dozen` directly, Vim won't find it because you are out of scope. `s:dozen` script variable is only accessible from inside `script.vim`.

Each time you source the `script.vim` file, it resets the `s:dozen` counter. If you are in the middle of decrementing `s:dozen` value and you run `:source dozen.vim`, the counter resets back to 12. This can be a problem for unsuspecting users. To fix this issue, refactor the code:

```
if !exists("s:dozen")
  let s:dozen = 12
endif

function Consume()
  let s:dozen -= 1
  echo s:dozen
endfunction
```

### Function-local and Function formal parameter variable

Both the function-local variable (`l:`) and the function formal variable (`a:`) will be covered in the next chapter.

### Built-in Vim variables

A variable prepended with `v:` is a special built-in Vim variable. You cannot define these variables. You have seen some of them already. For example:

- `v:version` tells you what Vim version you are using.
- `v:key` contains the current item value when iterating through a dictionary.
- `v:val` contains the current item value when running a `map()` or `filter()` operation.
- `v:true`, `v:false`, `v:null`, and `v:none` are special data types.

There are other variables. For a list of Vim built-in variables, check out `:h vim-variable` or `:h v:`.

## Using Vim Variables The Smart Way

Being able to quickly access environment, option, and register variables give you a broad flexibility to customize your editor and terminal environment. You also learned that Vim has 9 different Vimscript variable scopes, each existing under a certain constraints. You can take advantage of these unique variable types to write your own custom plugins. But before that, let's learn how to create functions!