The Bun.file() function accepts a path and returns a BunFile instance. The BunFile class extends Blob and allows you to lazily read the file in a variety of formats. Use .text() to read the contents as a string.
const path = "/path/to/file.txt";
const file = Bun.file(path);
const text = await file.text();
// string
Any relative paths will be resolved relative to the project root (the nearest directory containing a package.json file).
const path = "./file.txt";
const file = Bun.file(path);