# Arrays coercion

It's also possible to coerce values to `Array` or some other class that extends `Array`. On these circumstances Structure will use the `itemType` value of the type descriptor on the schema to coerce the items as well. Note that, when coercing arrays, it'll always create a new instance of the type and then push each item of the passed value to the new instance:

```javascript
class BooksCollection extends Array { }

const Library = attributes({
  books: {
    type: BooksCollection,
    itemType: String
  },
  users: {
    type: Array,
    itemType: String
  }
})(class Library { });

const libraryOne = new Library({
  books: ['Brave New World'],
  users: ['John', 'Jane']
});

libraryOne.books; // BooksCollection ['Brave New World'] => coerced the array to BooksCollection
libraryOne.users; // ['John', 'Jane'] => new instance of Array
```

The passed raw value have to be non-null and have a `length` attribute or implement the `Symbol.iterator` method, otherwise it'll fail to coerce and throw a `TypeError`.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://structure.js.org/v1/coercion/arrays-and-array-subclasses.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.