.JS has been added to the long list of file types that are blocked by Gmail for security reasons. The full list: .ADE, .ADP, .BAT, .CHM, .CMD, .COM, .CPL, .EXE, .HTA, .INS, .ISP, .JAR, .JS (NEW), .JSE, .LIB, .LNK, .MDE, .MSC, .MSI, .MSP, .MST, .NSH .PIF, .SCR, .SCT, .SHB, .SYS, .VB, .VBE, .VBS, .VXD, .WSC, .WSF, .WSH. "To prevent against potential viruses, Gmail doesn't allow you to attach certain types of files, including: certain file types (listed above), including their compressed form (like .gz or .bz2 files) or when found within archives (like .zip or .tgz files), documents with malicious macros, archives whose listed file content is password protected, archives whose content includes a password protected archive."
The GSuite Blog informs that "for inbound mail, senders will get a bounce message explaining why the email was blocked. If you still need to send .js files for legitimate reasons, you can use Google Drive, Google Cloud Storage, or other storage solutions to share or send your files."