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:

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:

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