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.
To read the file into a Buffer instance, first use .arrayBuffer() to consume the file as an ArrayBuffer, then use Buffer.from() to create a Buffer from the ArrayBuffer.
const path = "/path/to/package.json";
const file = Bun.file(path);
const arrbuf = await file.arrayBuffer();
const buffer = Buffer.from(arrbuf);
Refer to Binary data > Buffer for more information on working with Buffer and other binary data formats in Bun.