10 84 35 4F 3E A3 CE E1, when converted to bytes, is not zlib data. It does not start with a zlib header, and it is invalid if interpreted as a raw deflate stream.

I recommend you look at the data immediately after it is compressed on the other end, and compare that to what you are receiving.

Using Python 3.5:

import zlib

s = '789ccbc80400013b00d2'
b = bytearray.fromhex(s)
z = zlib.decompress(b)
print(z)

Thanks to @GordonAitchJay for the tip there. Got there in the end.

On the Python side we have

json_object = ["a", "b", "c"]
base64.b64encode(zlib.compress(bytes(json.dumps(json_object), "utf-8"))).decode("ascii")

On the JS side we have

let compressedData = Uint8Array.from(atob(input), (c) => c.charCodeAt(0));
let decompressedData = pako.inflate(compressedData, { to: "string" });
let jsonObject = JSON.parse(decompressedData);