Anyone attempting to send a .js file using Gmail will automatically have it blocked before the email is sent. Trying to put the .js file inside an archive, e.g. a .zip file, will also be detected and blocked. This auto-blocking won’t start happening until February 13, though, and the feature rollout could take as long as three days meaning you may not see it in action until February 16.
Of course, some users need to send one of the blocked file types over email. So instead of attaching it to an email, Google recommends uploading it to Google Drive. Alternatively, you could use one of the many other cloud storage services available to upload the file and then link to it in the email.