Arrays quirks

There are several limitations on how you can use arrays with Firestore. To prevent runtime errors, Typesaurus incorporates those limitations into its type system.

Nested arrays

Firestore doesn’t allow an array as an array item but can be nested in an object.

To prevent runtime errors, Typesaurus doesn’t allow to set such a structure:

interface Data {
  array: (string[] | string)[];
}

await db.data.set(dataId, {
  array: ["a", "b", "c"],
});

await db.data.set(dataId, {
  array: ["a", ["b", "c"]],
  //=>         ^^^^^^^^^^
  //=> Type '[string, string]' is not assignable to type 'string'.
});

Keep in mind!

While writing nested arrays is not permitted, schema will allow you to define a collection with such a type.

Write helpers

You can’t use the write helpers (i.e., $.remove() or $.serverDate()) inside an array and all nested structures, so such a code will result in a type error:

interface Post {
  comments: Comment[];
}

interface Comment {
  createdAt: Typesaurus.ServerDate;
  text: string;
}

await db.posts.update(postId, ($) => $.field("comments", 0).set($.remove()));
//=>                                                     ^
//=> Argument of type 'number' is not assignable to parameter of type 'never'.ts(2345)

To work around it, update the array manually:

const post = await db.posts.get(postId);
post?.update(($) => $.field("comments").set(post.data.comments.slice(1)));

Or use the $.arrayUnion and $.arrayRemove helpers:

await db.posts.update(postId, ($) =>
  $.field("comments").set(
    $.arrayRemove("Join NFT giveaway! https://example.com"),
  ),
);

→ Learn about using arrays when designing schema