Problem

Currently many of our frontend tests are written in JavaScript. This leaves us vulnerable to inaccurate tests that could give false confidence in our code due to potentially-subtle bugs relating to inaccurate types.

Description

Convert these files to TypeScript, and make any changes necessary in our actual implementation code if errors are revealed in the type-safe tests.

If a typed test reveals significant refactorings necessary, please extract it into its own PR.

Alternatives

Additional context

This work was previously prevented due to some poor type coverage of Jest internals.