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.