Treat `null`/`undefined` and “absence vs presence” as first-class concerns. - **TypeScript:** Keep null safety effective by ensuring `strictNullChecks` is enabled and not overridden by extended configs (explicitly set it to `true` if needed). - **Zod runtime:** Decide what should happen when input is `undefined` vs when it is a wrong value or `null`. - Use **`.default(value)`** for **absent** inputs (typically `undefined`). - Use **`.catch(fallback)`** for **parsing failures** (including wrong strings), not as a substitute for handling `undefined`. - **Form inputs:** Don’t assume “required field” means “non-empty”. Browsers/DOM often send `""` for an “empty” required field; `z.string()` will accept `""` as a valid string. If you require non-empty content, validate it explicitly (e.g., `.min(1)`). Example: ```ts import { z } from "zod"; const boolSchema = z.string().boolean(); // absent (undefined) -> fallback const withDefault = boolSchema.default(false); // wrong value (including "other" / null if you pass it in) -> fallback const withCatch = boolSchema.catch(false); // non-empty string required by UI const requiredText = z.string().min(1, "Required"); ``` Apply these consistently so validation behavior matches what your types and UI expectations actually mean.