ensuring-type-safety

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)

1. critical-any-escape-hatch — Avoid any, use unknown or proper types
2. critical-null-undefined-confusion — Distinguish null from undefined correctly

High Priority (5 rules)

3. 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.)
4. high-strict-mode-disabled — Always enable strict: true in tsconfig
5. high-implicit-any — All function parameters must have explicit types
6. high-type-narrowing — Use discriminated unions instead of type assertions for safe narrowing
7. high-optional-chaining-abuse — Don't use ?. to hide type issues

Medium Priority (3 rules)

8. medium-generic-constraints — Constrain generic types properly
9. medium-index-signatures — Prefer mapped types or Record<> over loose index signatures
10. 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.)

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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.