Users will no longer be able to attach .JS files to emails in Gmail, regardless of whether they attach them directly or they include them in archives like .gz, .bz2, .zip or .tgz. For those rare cases when such files need to be shared via email, users can upload them to a storage service like Google Drive and then share the link.
The .JS file extension will be added an existing list of other banned file attachments that includes: .ADE, .ADP, .BAT, .CHM, .CMD, .COM, .CPL, .EXE, .HTA, .INS, .ISP, .JAR, .JSE, .LIB, .LNK, .MDE, .MSC, .MSP, .MST, .PIF, .SCR, .SCT, .SHB, .SYS, .VB, .VBE, .VBS, .VXD, .WSC, .WSF and .WSH. Most of these file types have long been abused by cybercriminals to send malware via email.
Other scripting files like .VBS (VBScript), .VBE (VBScript Encoded), .WSH (Windows Script Host Settings File) and .WSF (Windows Script File) -- all of which Gmail already blocks -- can be used in a similar way.
The general rule is that if you don't recognize a file type and don't know what it does, you should never open it. If you are expecting files in unusual formats, you should always double check the source before opening them and it would be best not to share them via email.