Data is already stored in the ZIP file deflated, so I can just send whatever is inside the ZIP file back to the client if they accept that encoding (which is pretty much always the case, given how ubiquitous deflate is).
The server parses the ZIP file and stores that information in a hash table for quicker lookup but it's otherwise not decompressing anything. This hash table is kept for a few minutes to avoid having to decode the base64-encoded data and parse the ZIP file for every request.
So decompression is happening on the client, but not at the JS level, instead you are taking advantage of browser's ability to accept zip-encoded content from the server, hence decompression is done the by browser's own behavior when it receives a "content-encoding: gzip" stream or something like that.
