You can code it in Rust then compile into WASM. You can show the original Rust source code to your users, including the hash string (checksum) of the WASM file. Users can verify that the running WASM file is indeed compiled from the source code that you provide them by using the hash string.

There are also some other ways called Zero-Knowledge Proof (ZK). There are many ZK libs already. Some requires trusted setup, and some others don't. Check this repo from Microsoft: https://github.com/microsoft/Spartan which doesn't require trusted setup. There are also some ZK libs written in pure JS but their proofs are much heavier than the piece of information they prove, so not so efficient.