Here is my suggestion.
Download the JetPack install ZIP package to your drive and do a scan of the ZIP file with your Antivirus scanner and also use Malwarebytes to scan it as well.
Also, if you know where the JS file is located in the folder structure of the plugin, access the ZIP and see if it is there.
I do this to determine if the plugin install package itself contains the malware. If it does not, then someone inserted it and you were hacked.
If you were hacked, I strongly recommend WordFence install into your WordPress. Then scan your website with that and it will detect malware and malicious hacks. It also blocks hackers from getting in.
Website hosts do not protect your WordPress site from being hacked, not even with SiteLock, which a lot of them now use. They will protect their own server files, but your site protection is all up to you. I had a client whose site was hacked and the SiteLock did not stop it.
I do strongly suggest to use Malwarebytes. Anti-virus software as Microsoft Security Essentials or any other are never 100% reliable. Using two dependable scanners increases the reliability of detection.
In the event you don’t use Malwarebytes, here is the link to download it: https://www.malwarebytes.com/