> For the complete documentation index, see [llms.txt](https://structure.js.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://structure.js.org/coercion/arrays-and-array-subclasses.md).

# 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 attribute definition 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', 1],
});

libraryOne.books; // BooksCollection ['Brave New World'] => coerced the array to BooksCollection
libraryOne.users; // ['John', '1'] => new instance of Array with coerced items
```

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`.

## Observations

Structure only does array **items** coercion during instantiation, so mutating an array (using push, for example) won't coerce the new item:

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

const library = new Library({
  books: [1984],
});

library.books; // ['1984'] => coerced number to string

library.books.push(42);

library.books; // ['1984', 42] => new item was not coerced
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## 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, and the optional `goal` query parameter:

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

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

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.