Truncates (or extends) the file to reach the specified len. If len is not specified, then the entire file contents are truncated.

Truncate the entire file

const file = await Deno.open("my_file.txt", { write: true });
await file.truncate();
file.close();

Truncate part of the file

// if "my_file.txt" contains the text "hello world":
const file = await Deno.open("my_file.txt", { write: true });
await file.truncate(7);
const buf = new Uint8Array(100);
await file.read(buf);
const text = new TextDecoder().decode(buf); // "hello w"
file.close();