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The testkit wires up the TypeScript language with the native parser and the English locale, so a test only has to say which plugins it needs.

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narrate

Narrates a snippet and returns plain text. path defaults to snippet.ts. Set it to a .tsx name when the snippet contains JSX, or to a path such as app/blog/[slug]/page.tsx when a plugin reads meaning from it. If a plugin rule throws, narrate throws too, with the plugin and rule in the message, even though a real narrator would carry on with a plain reading. That way a bug in a rule fails its test instead of hiding behind a fallback. Syntax errors don’t throw, so snippets may use a top-level return. The testkit doesn’t load dependency manifests, so a plugin that checks e.usesLibrary only applies when the snippet imports its library. Start test snippets with the import, as real files do.
narrate(code,

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narrateWithStats

Like narrate, and also returns how many nodes fell back to raw code. Assert it is 0 to make sure your snippet is fully covered.

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phrasebookGaps

Compares the key paths of two phrasebooks. Use the English phrasebook as the reference. missing lists phrases the translation doesn’t have yet, and extra lists keys that don’t exist in the reference, which usually means a typo.