TypeScript Best Practices

When to Use

Use this skill when:

• Writing new TypeScript code
• Reviewing TypeScript pull requests
• Refactoring JavaScript to TypeScript

Triggers

Activated when editing .ts or .tsx files in the project.

Rules

Always

• Always use const for variables that won't be reassigned
• Always use explicit return types on exported functions
• Always prefer interface over type for object shapes

Never

• Never use any — use unknown instead
• Never use var — use const or let
• Never ignore TypeScript errors with @ts-ignore

Examples

// Good: explicit return type
export function calculateTotal(items: Item[]): number {
  return items.reduce((sum, item) => sum + item.price, 0);
}

// Good: discriminated union
type Result<T> =
  | { success: true; data: T }
  | { success: false; error: Error };

Boundaries

• Do not modify tsconfig.json without explicit permission
• Do not add new dependencies without checking existing utilities
• Focus only on TypeScript patterns, not runtime behavior