Apply Frontend Typescript Standards

Apply Frontend TypeScript Standards

Purpose

Prevent runtime UI bugs by enforcing consistent TypeScript patterns and avoiding unsafe shortcuts.

When to use

Use this skill when you are:

• Adding new components, hooks, or API clients
• Fixing type errors and improving type safety
• Introducing new feature modules with public exports
• Reviewing PRs for TypeScript quality

Inputs

• Existing TypeScript configuration and lint rules
• Feature types/DTOs and API shapes
• Known type pain points (implicit any, unions, nullability)

Outputs

• Typed props and module public APIs
• Safe utility types and helper functions
• Reduced reliance on any and type assertions

Rules

• any SHOULD be avoided; prefer unknown + refinement when needed.
• Public module APIs MUST be typed and stable.
• Narrowing MUST be explicit for unknown values.
• Prefer discriminated unions for complex state machines.

Steps

1. Define types at the boundary:
   • API DTOs
   • component props
2. Prefer inference where safe; add annotations at boundaries.
3. Replace unsafe casts with:
   • runtime checks
   • schema validation (if available)
4. Use discriminated unions for complex UI state.
5. Verify:
   • TypeScript build passes
   • no new unsafe escapes introduced

Verification

• TypeScript build passes with no errors
• No new any types introduced without justification
• Public module APIs are typed and exported correctly
• unknown values are narrowed before use
• Discriminated unions are used for complex state
• Type assertions are avoided or justified

Boundaries

• MUST NOT use any without explicit justification
• MUST NOT use type assertions (as) to bypass type checking
• MUST NOT export untyped public APIs
• SHOULD NOT ignore TypeScript errors with @ts-ignore
• SHOULD NOT use ! (non-null assertion) without runtime guarantee
• SHOULD NOT define complex types inline (extract to named types)

Included assets

• Examples: ./examples/ includes safe patterns for unknown and discriminated unions.

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