How so?

If you copy/paste it into an editor, the editor will likely complain because Medium uses fancy single quotes, but if you convert those to regular single quotes, it works exactly as intended.

The function is meant to return ‘Hello world!’, not print it.

To print ‘Hello world!’ we simply need to send the output to some render target (e.g. the console). Like this:

console.log(helloWorld());

function helloWorld() {
  return 'Hello world!';
}

If there’s something I missed, please let me know.

Thanks!