Gmail currently restricts certain file attachments (e.g. .exe, .msc, and .bat) for security reasons, and starting on February 13, 2017, Gmail will not allow .js file attachments as well. Similar to other restricted file attachments, you will not be able to attach a .js file and an in-product warning will appear, explaining the reason why.
For inbound mail, senders will get a bounce message explaining why the email was blocked.
Release track: Launching to Rapid release on February 13, 2017, with Scheduled release coming 2 weeks afterward
Rollout pace: Full rollout
Impact: All end users