Defense-in-Depth Validation

Add validation at every layer the data passes through: entry point checks, business logic validation, environment guards, and debug instrumentation. Each layer catches cases the others miss, making the bug structurally impossible.

Defense-in-depth validation is a pattern where you place checks at multiple layers of the data flow instead of a single fix. Entry validation catches most bugs, business logic catches edge cases, environment guards block context-specific dangers, and debug logging aids forensics.

A single check can be bypassed by different code paths, mocks in tests, or later refactoring. Multiple layers ensure that even if one check is skipped, another catches the invalid data before it causes damage.

Yes, the pattern is language-agnostic and applies to any codebase. The examples use TypeScript, but the four layers of entry validation, business logic, environment guards, and debug instrumentation translate to any stack.

Use environment guards to prevent dangerous operations in specific contexts, such as refusing git init outside temp directories during test runs. They protect against context-specific risks that entry and business logic validation cannot anticipate.