| name | ensuring-type-safety |
| description | Enforces TypeScript type system best practices. Use to detect unsafe type usage, verify strict mode, and prevent runtime type errors. |
| metadata | {"sources":["Effective TypeScript (Dan Vanderkam)","TypeScript Handbook (Official)","Programming TypeScript (Boris Cherny)"]} |
Type Safety - TypeScript Best Practices
Ensure type-safe code by avoiding any, using proper type guards, and enabling strict mode.
When to Use
- Writing new TypeScript code
- Reviewing code for type safety issues
- Migrating JavaScript to TypeScript
- Debugging type-related runtime errors
Trigger Phrases:
- "Check for type safety issues"
- "Review TypeScript types"
- "Find unsafe type usage"
- "Is this type-safe?"
Rules Overview
Detailed guidance for these rules is available in RULES.md.
Critical (2 rules)
- critical-any-escape-hatch — Avoid
any, use unknown or proper types
- critical-null-undefined-confusion — Distinguish
null from undefined correctly
High Priority (5 rules)
- high-unsafe-type-assertion — Avoid unsafe
as assertions (especially as any, unvalidated API data). Safe uses exist (DOM APIs after null-check, as const). (Heuristic: the problem is unvalidated assertions, not every as.)
- high-strict-mode-disabled — Always enable
strict: true in tsconfig
- high-implicit-any — All function parameters must have explicit types
- high-type-narrowing — Use discriminated unions instead of type assertions for safe narrowing
- high-optional-chaining-abuse — Don't use
?. to hide type issues
Medium Priority (3 rules)
- medium-generic-constraints — Constrain generic types properly
- medium-index-signatures — Prefer mapped types or
Record<> over loose index signatures
- medium-type-vs-interface — Use interfaces for object shapes, types for unions. (Team heuristic: both work for object shapes. The distinction matters mainly for declaration merging and extends/implements.)
Summary
Type safety is the foundation of TypeScript's value. These 10 rules ensure:
- ✅ No
any escape hatches
- ✅ Safe type assertions (not blanket prohibition of
as)
- ✅ Strict mode enabled
- ✅ Proper generic constraints
- ✅ Clear null/undefined semantics
Refer to the Detailed Rule Catalog for examples and detection criteria.
