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Deep dive into TypeScript's type system—narrowing, discriminated unions, mapped types, conditional types, type guards, branding, and escape hatches.
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SOC 職業分類に基づく
Enforce and guide the Mikado Method when a developer is refactoring, restructuring, or dealing with legacy code. Use this skill whenever the user mentions refactoring, technical debt, legacy code, code restructuring, dependency untangling, or "breaking everything" when making changes. Also trigger when the user wants to make a large change safely, asks how to split a big refactoring task, wants to work on main branch without a long-lived feature branch, or asks how to incrementally improve a codebase. The skill enforces the full Mikado loop: goal → naive attempt → map prerequisites → revert → implement leaves → commit → repeat.
Evaluate TypeScript/JavaScript code against Dave Thomas's OO principles and the Tell Don't Ask rule. Trigger when: class has only static methods; class has a single public method + constructor; code uses GoF pattern names (Decorator/Factory/Strategy/Builder/Command/Observer); class is invalid until setters are called; data class with only constructor + properties; external code reads getters to make decisions (Tell Don't Ask); user asks "is this a code smell?", "should this be a class or a function?", or "why does my code feel procedural?".
Detect code smells and apply refactoring techniques to improve clarity, testability, and maintainability.
Guide users through TDD and TCRDD (Test && Commit || Revert + Test Driven Development). Use this skill whenever a user mentions TDD, test-driven development, writing tests before code, red-green-refactor cycles, or unit testing workflows. Also trigger when a user asks about TCRDD, TCR, "test commit revert", "git gamble", or wants a strict TDD workflow with automatic commits and reverts. Trigger when the user asks to implement a feature, fix a bug, or write a class/function and mentions tests, TDD, or "test first". If the user shares code and asks for a review with any testing angle, consult this skill.
Master testing strategy — philosophy, approach, architecture decisions, test quality audit, and BDD review practices.
Design patterns for TypeScript—Factory, Strategy, Builder, Decorator, Singleton, Mixin, and more with problem-oriented and GoF triggers.
| name | typescript-type-system |
| description | Deep dive into TypeScript's type system—narrowing, discriminated unions, mapped types, conditional types, type guards, branding, and escape hatches. |
| triggers | ["unknown vs any","type narrowing","discriminated unions","exhaustiveness checking","mapped types","conditional types","infer keyword","user-defined type guards","type branding","companion object pattern","as const","escape hatches","type assertion","how do I narrow a type","how do I write a type predicate","how to create nominal types","how to prevent structural aliasing"] |
| Problem / Topic | Concept |
|---|---|
| Safely handle API responses / JSON.parse | unknown vs any |
| Narrow a union based on runtime checks | Type narrowing & refinement |
| Route on message type with different shapes | Discriminated unions |
| Fail at compile time when a case is missed | Exhaustiveness / assertNever |
Create Partial, Readonly, or custom maps | Mapped types |
Unwrap Promise<T>, filter union members | Conditional types + infer |
| Carry narrowing across function boundaries | User-defined type guards |
Prevent mixing UserId and SessionToken | Type branding (nominal types) |
| Pair a type and utility under one import | Companion object pattern |
| Preserve literal types in config objects | as const / type widening |
| Last-resort override of TypeScript's checks | Escape hatches (as T, !) |
unknown vs any — any disables checking; unknown forces narrowing. Use for external data.
Type narrowing — TypeScript narrows types through typeof, instanceof, in, equality, truthiness, Array.isArray.
Discriminated unions — Tag field enables dispatch via switch. Essential for Redux actions, WebSocket messages, API variants.
Exhaustiveness checking — assertNever(value: never) forces compile error if new union member isn't handled.
Mapped types — Transform object keys: { [K in keyof T]?: T[K] }. Built-ins: Partial, Required, Readonly, Pick, Record.
Conditional types — Type-level ternary: T extends Promise<infer U> ? U : T. Built-ins: Exclude, Extract, Awaited.
User-defined type guards — Return value is T to carry refinement across function boundaries.
Type branding — type UserId = string & { readonly _brand: unique symbol } prevents structural aliasing at zero cost.
Companion object pattern — Bind the same name to both a type and const value. One import covers annotation and utilities.
as const — Freeze values to literal types. Use on configs/arrays to derive union types.
Escape hatches — as T, !, !: override TypeScript checks. Last resort; frequent use signals refactoring needed.
→ Full examples with runnable code: references/type-system.md
