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:

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.