Book: Kotlin and Java to TypeScript — A Bridge for JVM Developers

Also by me: The TypeScript Library — the 5-book collection

My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools

Me: xgabriel.com | GitHub

A Kotlin developer joins a TypeScript service. Day one, they model a user.

data class User ( val id : Long , val email : String ?, )

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Two minutes of thought. email is sometimes there, sometimes not. The compiler will refuse to let anyone touch user.email.length without a null check. Done.

Day two, they translate the same shape to TypeScript and pick the literal-looking option:

interface User { id : number ; email : string | null ; }

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The service serializes a user with email = null , sends the JSON over the wire to a frontend, and the frontend reads user.email and gets undefined . The form initializes blank. Validation passes.…