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.