A constant variable is an immutable variable that can be fully evaluated at parse-time. These are useful with commands that need to know the value of an argument at parse time, like [`source`](/commands/docs/source.md), [`use`](/commands/docs/use.md) and [`plugin use`](/commands/docs/plugin_use.md). See [how nushell code gets run](how_nushell_code_gets_run.md) for a deeper explanation. They are declared using the `const` keyword

```nu
const script_file = 'path/to/script.nu'
source $script_file
```

## Choosing between mutable and immutable variables

Try to use immutable variables for most use-cases.

You might wonder why Nushell uses immutable variables by default. For the first few years of Nushell's development, mutable variables were not a language feature. Early on in Nushell's development, we decided to see how long we could go using a more data-focused, functional style in the language. This experiment showed its value when Nushell introduced parallelism. By switching from [`each`](/commands/docs/each.md) to [`par-each`](/commands/docs/par-each.md) in any Nushell script, you're able to run the corresponding block of code in parallel over the input. This is possible because Nushell's design leans heavily on immutability, composition, and pipelining.

Many, if not most, use-cases for mutable variables in Nushell have a functional solution that:

- Only uses immutable variables, and as a result ...
- Has better performance
- Supports streaming
- Can support additional features such as `par-each` as mentioned above

For instance, loop counters are a common pattern for mutable variables and are built into most iterating commands. For example, you can get both each item and the index of each item using [`each`](/commands/docs/each.md) with [`enumerate`](/commands/docs/enumerate.md):

```nu
ls | enumerate | each { |elt| $"Item #($elt.index) is size ($elt.item.size)" }
# => ╭───┬───────────────────────────╮
# => │ 0 │ Item #0 is size 812 B     │
# => │ 1 │ Item #1 is size 3.4 KiB   │
# => │ 2 │ Item #2 is size 11.0 KiB  │
# => │ 3 │ ...                       │
# => │ 4 │ Item #18 is size 17.8 KiB │
# => │ 5 │ Item #19 is size 482 B    │
# => │ 6 │ Item #20 is size 4.0 KiB  │
# => ╰───┴───────────────────────────╯
```

You can also use the [`reduce`](/commands/docs/reduce.md) command to work in the same way you might mutate a variable in a loop. For example, if you wanted to find the largest string in a list of strings, you might do:

```nu
[one, two, three, four, five, six] | reduce {|current_item, max|
  if ($current_item | str length) > ($max | str length) {