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.