I acknowledge that there is a pretty common need to serialise BigInts in order to store them, communicate them between systems, etc., but I do not think we need a standard transcoder built in to the language, at least not now. There are many, many different "VLQ"/"varint" encodings of arbitrary-magnitude integers used in a variety of systems: MIDI popularised one, protobuffs popularised another, and git uses yet another. Each of them would be a valid way to encode BigInts, but none is so overwhelmingly popular among JavaScript users (yet) or hard enough to implement via libraries to justify inclusion in the language. For example, the one I've built (https://github.com/michaelficarra/bigint-serialiser) uses yet another "VLQ"/"varint" variant that makes the tradeoffs that I felt were appropriate for general usage, and its implementation is not particularly large or difficult to get correct, as you can see from the source and tests.