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.