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
  await Deno.truncate("my_file.txt");

  // truncate part of the file
  const file = await Deno.makeTempFile();
  await Deno.writeFile(file, new TextEncoder().encode("Hello World"));
  await Deno.truncate(file, 7);
  const data = await Deno.readFile(file);
  console.log(new TextDecoder().decode(data));  // "Hello W"

Requires allow-write permission.