narrow

The method narrows the variable model doc type. It’s available on Doc.

const ghAccount = account.narrow<GitHubAccount>(
  (data) => data.type === "github" && data,
);
//=> Doc<GitHubAccount> | undefined

The method accepts a function that has to return the data of the target type or undefined.

It allows checking the data structure on the runtime and also asserts the type. If the data structure checks aren’t sufficient, it will result in a type error.

Beware of data inconsistency

Failing to write a proper check will result in a type error, however it won’t prevent the code from running, which might lead to data inconsistency.

→ Read more about variable models.