It's common to have code and types that are needed on both the frontend and the backend. It's important that you write this code in a particularly defensive way because it's limited by what both environments support:

For example, you cannot use the Deno keyword. For imports, you can't use npm: specifiers, so we reccomend https://esm.sh because it works on the server & client. You can use TypeScript because that is transpiled in /backend/index.ts for the frontend. Most code that works on the frontend tends to work in Deno, because Deno is designed to support "web-standards", but there are definitely edge cases to look out for.