init: common-skills v1

skills/typescript/SKILL.md

@@ -0,0 +1,47 @@
+---
+name: typescript-skill
+description: TypeScript coding patterns and type safety guide. Use when writing, reviewing, or debugging TypeScript code.
+---
+
+# TypeScript Skill
+
+## Key Rules
+
+### Always Prefer Explicit Types
+```typescript
+// Bad
+const process = (data: any) => data.value;
+
+// Good
+interface GameState { value: number; }
+const process = (data: GameState): number => data.value;
+```
+
+### Use `unknown` Instead of `any`
+```typescript
+// Bad
+function parse(input: any) { return input.name; }
+
+// Good
+function parse(input: unknown): string {
+  if (typeof input === 'object' && input !== null && 'name' in input) {
+    return String((input as { name: unknown }).name);
+  }
+  throw new Error('Invalid input');
+}
+```
+
+### Prefer `const` Assertions for Literals
+```typescript
+const DIRECTIONS = ['up', 'down', 'left', 'right'] as const;
+type Direction = typeof DIRECTIONS[number]; // 'up' | 'down' | 'left' | 'right'
+```
+
+## Common Patterns
+
+| Pattern | Use Case |
+|---------|----------|
+| `type` | Unions, intersections, primitives |
+| `interface` | Object shapes (extendable) |
+| `enum` | Named constants (prefer `as const` for simple cases) |
+| `generic <T>` | Reusable, type-safe utilities |