> For the complete documentation index, see [llms.txt](https://modelico.javiercejudo.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://modelico.javiercejudo.com/quickstart/immutability.md).

# Immutability

Following the [nested types example](/quickstart/nested_types_example.md):

```javascript
const person2 = person1.set('givenName', 'Javi')

// person2 is a clone of person1 with the givenName
// set to 'Javi', but person1 is not mutated
person2.fullName() // => 'Javi Cejudo'
person1.fullName() // => 'Javier Cejudo'

const person3 = person1.setIn(['pets', 0, 'name'], 'Bane')

person3.pets().get(0).name() // => 'Bane'
person1.pets().get(0).name() // => 'Robbie'
```

The same principle applies across all Modélico classes. In the case of `M.List`, the inner array is frozen. At the time of writing, there is no canonical way of freezing the other inner structures, so a copy is returned each time `.inner()` is called.

```javascript
person1.pets().inner().shift().speak()
// => TypeError: Cannot add/remove sealed array elements
```