Synchronously truncates or extends the specified file, to reach the specified len. If len is not specified then the entire file contents are truncated.

  // truncate the entire file
  Deno.truncateSync("my_file.txt");

  // truncate part of the file
  const file = Deno.makeTempFileSync();
  Deno.writeFileSync(file, new TextEncoder().encode("Hello World"));
  Deno.truncateSync(file, 7);
  const data = Deno.readFileSync(file);
  console.log(new TextDecoder().decode(data));

Requires allow-write permission.