The Bun.file() function accepts a path and returns a BunFile instance. BunFile extends Blob, so you can read the file lazily in a variety of formats. Use .json() to read and parse the contents of a .json file as a plain object.
Bun sets the MIME type of the BunFile accordingly.
index.ts
const path = "/path/to/package.json";
const file = Bun.file(path);
const contents = await file.json();
// { name: "my-package" }
file.type; // => "application/json;charset=utf-8";