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---
name: typescript-strict
description: Use TypeScript strictness, narrowing, and validation for safe boundaries.
---
# TypeScript strictness
Keep strict mode valuable by treating escapes as design decisions.
## Unknown data
Use `unknown` for untrusted values, then narrow with control flow, discriminants, predicates, or a
schema validator. Validate external input at the boundary before it enters domain code.
## Avoid unsafe escapes
Avoid `any`, unsafe casts, and non-null assertions. If a value can be absent, model that possibility
and handle it. Prefer checks that prove the value exists over assertions that silence the compiler.
## Optional semantics
When exact optional behavior matters, distinguish an omitted property from a property whose value is
`undefined`. Do not use optional fields as a substitute for a clear variant model.
