Seek a resource ID (rid) to the given offset under mode given by whence. The call resolves to the new position within the resource (bytes from the start).

const file = await Deno.open('hello.txt', {read: true, write: true, truncate: true, create: true});
await Deno.write(file.rid, new TextEncoder().encode("Hello world"));
// advance cursor 6 bytes
const cursorPosition = await Deno.seek(file.rid, 6, Deno.SeekMode.Start);
console.log(cursorPosition);  // 6
const buf = new Uint8Array(100);
await file.read(buf);
console.log(new TextDecoder().decode(buf)); // "world"

The seek modes work as follows:

// Given file.rid pointing to file with "Hello world", which is 11 bytes long:
// Seek 6 bytes from the start of the file
console.log(await Deno.seek(file.rid, 6, Deno.SeekMode.Start)); // "6"
// Seek 2 more bytes from the current position
console.log(await Deno.seek(file.rid, 2, Deno.SeekMode.Current)); // "8"
// Seek backwards 2 bytes from the end of the file
console.log(await Deno.seek(file.rid, -2, Deno.SeekMode.End)); // "9" (e.g. 11-2)