TypeScript Best Practices

Goal

Make code safer and easier to change. Use TypeScript to model real constraints without adding noise.

Rules

• Prefer unknown over any.
• Let inference work for local values.
• Add explicit types at boundaries: exports, API input/output, public functions.
• Enable strict compiler options when project allows.
• Use discriminated unions for state variants.
• Narrow unknown data before use.
• Prefer interface for object shapes.
• Prefer type for unions and utility types.
• Avoid type assertions unless crossing a known boundary.
• Keep runtime validation near external input.

Common Fixes

• Replace any with unknown plus guards.
• Replace boolean state pairs with a discriminated union.
• Replace broad string with a union when valid values are finite.
• Infer types from schemas instead of duplicating them.
• Move reusable complex types to named aliases.

Avoid

• Non-null assertions used to silence real uncertainty.
• Large anonymous object types repeated in many places.
• as chains.
• Deep conditional types for app-level code.
• Types that are stricter than runtime validation.

References

• context/code-patterns.md: type system, functions, async patterns, and guard clauses.
• context/organization.md: file naming, barrel files, and project configuration.
• context/testing-performance.md: DI, type guards, null handling, and performance.
• W3Schools TypeScript Best Practices
• TypeScript Handbook
• TypeScript Performance Wiki

Output

• Type safety issue.
• Simpler type model.
• Boundary validation plan.
• Compiler or lint check when relevant.