Frontend Architecture (portable, module-based)

> Portable skill — readable by Claude Code, OpenCode, Codex, Cursor, Windsurf, and others. > This skill describes a structure and a set of rules, not a component library, a state library, or a visual style. > It is deliberately global: the same module/page/state model maps onto > Next.js (App Router), React + Vite (SPA), Remix, and Expo / React Native, and it works > with any state-management and styling stack.

The goal: a codebase where any contributor can instantly answer three questions — "where does this code live?", "what is allowed to import what?", and "is this server state or UI state?" — without asking anyone. The structure makes the answers obvious.

0. The five core ideas

1. Feature modules own their world. Each feature is a self-contained modules/{feature}/ folder with its own pages, components, hooks, state, types, and a single public barrel.
2. Pages/screens are directories, not files. A route is a folder that co-locates its component, its styles, and the components/hooks used only by it.
3. State is split by origin. Server data lives in a query/cache layer. UI/client state lives in a store. They never overlap — regardless of which libraries you pick.
4. Imports cross boundaries only through barrels. Reaching into another module's internals is forbidden; you import from @/modules/{feature} and nothing deeper.
5. Code is promoted, not pre-placed. It starts as local as possible and moves outward only when a second consumer appears.

Everything below is the mechanical application of these five ideas. None of it is tied to a specific library — pick your stack in Sections 4 and 6.

1. Directory layout

The shape is identical across frameworks; only the routing layer on top differs (see Section 7).

src/
├── app/ or routes/ or navigation/   ← framework routing layer (thin — see §7)
├── modules/                         ← feature modules (the heart of the app)
│   └── {feature}/
│       ├── index.ts                 ← PUBLIC BARREL — the only cross-module entry point
│       ├── README.md                ← what this module owns, its routes, its data deps
│       ├── components/              ← components reused by 2+ pages IN THIS MODULE
│       ├── pages/                   ← page/screen directories (one per route)
│       │   └── {page}/
│       │       ├── {page}.tsx               ← the page/screen component
│       │       ├── {page}.styles.ts         ← ALL styling for this page
│       │       ├── index.ts                 ← re-exports the page component
│       │       ├── components/              ← components used ONLY by this page
│       │       ├── hooks/                   ← hooks used ONLY by this page
│       │       ├── constants/
│       │       └── README.md                ← route, params, permissions, data deps
│       ├── hooks/                   ← data hooks (query/mutation) + module hooks
│       ├── stores/                  ← UI/client state store(s) — never server data
│       ├── services/                ← data-access (API calls) for this feature
│       ├── utils/                   ← pure module utilities (co-located *.test.ts)
│       ├── constants/
│       └── types/                   ← module request/response + view-model types
└── shared/                          ← cross-module building blocks
    ├── components/                  ← components used by 2+ MODULES
    ├── hooks/                       ← cross-cutting hooks
    ├── api-client/                  ← one typed client; the only place that talks to the network
    ├── store/                       ← root store wiring (if your state lib needs one — see §4)
    ├── utils/                       ← formatters, cn()/clsx, helpers
    ├── constants/
    └── types/

Every folder that can be empty at scaffold time keeps a .gitkeep so the structure is visible from day one.

2. Feature modules

A module is a vertical slice of the product (e.g. auth, billing, dashboard, settings). It contains everything that feature needs and exposes a deliberately small surface.

2.1 The barrel (index.ts) is the contract

modules/{feature}/index.ts is the only thing other modules and the routing layer may import from. It re-exports:

• Page/screen components the router mounts.
• Data hooks other features legitimately need.
• The store hook/slice and its public types.
• Shared constants / types other features depend on.

ts
// CORRECT — consume the public surface
import { InvoiceListPage, useInvoiceList } from "@/modules/invoice";

// WRONG — reaching into internals couples you to private structure
import { InvoiceListPage } from "@/modules/invoice/pages/invoice-list/invoice-list";

Keep the barrel curated. If something isn't exported, it's private by design. Group exports with short comments (pages, hooks, store, types) — future readers use the barrel as the module's API docs.

2.2 One module = one bounded context

Don't create utils modules or components modules. Modules map to product capabilities, not to technical layers. Technical building blocks live in shared/.

2.3 Module README

Each module's README.md states: what it owns, which routes render its pages, its data dependencies (which endpoints/hooks), and any cross-module rules. This is the first thing a new contributor reads.

3. Pages/screens as directories

A page is a route the router mounts (a "screen" in React Native). It is always a folder, never a loose file — even when it starts as a single component. This keeps growth in place: when the page needs a sub-component or a hook, there is already a home for it.

pages/{page}/
├── {page}.tsx          ← the page/screen component
├── {page}.styles.ts    ← every style for this page (no inline styles — see §5)
├── index.ts            ← export { PageComponent } from "./{page}"
├── components/         ← used ONLY by this page
├── hooks/              ← used ONLY by this page
├── constants/
└── README.md           ← route, params, permissions, data deps

The page README is short and high-signal: route path, expected params, required permissions/auth, and the hooks it depends on. It is the contract between the page and the rest of the app.

Why folders from the start: a page that begins as one file inevitably grows a sub-row component, a derived-totals hook, a styles file. If the page is a file, those land in arbitrary places. If the page is a folder, they have an obvious home and the diff stays readable.

4. State: split by origin (non-negotiable, library-agnostic)

Two kinds of state, two homes. Mixing them is the most common architectural failure this skill exists to prevent. The split is mandatory; the libraries are your choice.

| State kind | Examples | Lives in | | --------------------- | --------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------- | | Server state | fetched entities, lists, aggregates — anything the API owns | a query/cache layer (e.g. TanStack Query, RTK Query, SWR, Apollo) | | UI / client state | open dialogs, table filters/sort, wizard step, draft being typed, preview toggles | a client store (e.g. Zustand, Redux Toolkit, MobX, Jotai, Valtio, or React Context) |

4.1 Hard rules (independent of library)

• Never mirror server responses into the client store. No copying fetched entities into Zustand/Redux/MobX. The query/cache layer is the single source of truth for server data.
• Never fetch inside components. Components read server data from a data hook and UI state from a store selector. They don't call the network client directly.
• Never drive continuous values through re-render state. Scroll progress, pointer position, drag offset — use refs / animation values, not render state (it re-renders the tree every frame).
• One store boundary per module. Whatever library you use, give each module one cohesive store unit (a Zustand hook, a Redux slice, a MobX class, a Jotai atom group) accessed via the module barrel. Components subscribe to the smallest slice they need to avoid needless re-renders.

4.2 Choosing a client-state library — same shape, different syntax

Pick one per project and stay consistent. Each maps onto "one store unit per module" cleanly. Note the I interface-naming convention: state interfaces are prefixed with I (e.g. IFeatureUiState).

Zustand — modules/{feature}/stores/{feature}.store.ts

ts
import { create } from "zustand";

export interface IFeatureUiState {
  isPreviewOpen: boolean;
  filter: string;
  togglePreview: () => void;
  setFilter: (filter: string) => void;
  reset: () => void;
}

const INITIAL_STATE = { isPreviewOpen: false, filter: "" } as const;

export const useFeatureUiStore = create<IFeatureUiState>()((set) => ({
  ...INITIAL_STATE,
  togglePreview: () => set((s) => ({ isPreviewOpen: !s.isPreviewOpen })),
  setFilter: (filter) => set({ filter }),
  reset: () => set({ ...INITIAL_STATE }),
}));

Redux Toolkit — modules/{feature}/stores/{feature}.slice.ts (registered in shared/store/)

ts
import { createSlice, type PayloadAction } from "@reduxjs/toolkit";

export interface IFeatureUiState {
  isPreviewOpen: boolean;
  filter: string;
}

const initialState: IFeatureUiState = { isPreviewOpen: false, filter: "" };

export const featureUiSlice = createSlice({
  name: "featureUi",
  initialState,
  reducers: {
    togglePreview: (s) => {
      s.isPreviewOpen = !s.isPreviewOpen;
    },
    setFilter: (s, action: PayloadAction<string>) => {
      s.filter = action.payload;
    },
    reset: () => initialState,
  },
});

MobX — modules/{feature}/stores/{feature}.store.ts

ts
import { makeAutoObservable } from "mobx";

export interface IFeatureUiState {
  isPreviewOpen: boolean;
  filter: string;
}

export class FeatureUiStore implements IFeatureUiState {
  isPreviewOpen = false;
  filter = "";
  constructor() {
    makeAutoObservable(this);
  }
  togglePreview = () => {
    this.isPreviewOpen = !this.isPreviewOpen;
  };
  setFilter = (filter: string) => {
    this.filter = filter;
  };
  reset = () => {
    this.isPreviewOpen = false;
    this.filter = "";
  };
}

Jotai — modules/{feature}/stores/{feature}.atoms.ts

ts
import { atom } from "jotai";
export const isPreviewOpenAtom = atom(false);
export const filterAtom = atom("");

> Whichever you choose, keep the rules in §4.1 constant. The skill cares that server and UI state are separated and that each module owns one store unit — not which library draws the box.

4.3 Data layer (server state)

All network access goes through one typed client in shared/api-client/. Modules wrap it in query/mutation hooks and a key factory so caches and invalidation stay consistent.

ts
// modules/invoice/hooks/invoiceKeys.ts — hierarchical key factory (TanStack Query style)
export const invoiceKeys = {
  all: ["invoices"] as const,
  lists: () => [...invoiceKeys.all, "list"] as const,
  list: (params: IListParams) => [...invoiceKeys.lists(), params] as const,
  details: () => [...invoiceKeys.all, "detail"] as const,
  detail: (id: string) => [...invoiceKeys.details(), id] as const,
} as const;

Invalidating lists() refreshes every filtered page; detail(id) targets one entity. (RTK Query/SWR/Apollo express the same idea with tags/keys.) Components never write raw fetch() — they call useInvoiceList() / useCreateInvoice().

5. Styling: co-located, no inline styles (styling-library agnostic)

Keep styling out of JSX and out of the component body. Each page or component has a co-located styles file. The rule is constant; the syntax follows your styling stack.

• Tailwind (web): {name}.styles.ts exports named class strings composed with cn() (clsx + tailwind-merge); variants via cva. JSX references styles.header.
• CSS Modules / vanilla-extract: a co-located {name}.module.css / {name}.css.ts; JSX references styles.header.
• styled-components / Emotion: a co-located {name}.styles.ts exporting styled components.
• Tamagui (web + native): a co-located {name}.styles.ts exporting styled(...) components or a createStyledContext / useStyle token set; reference Tamagui tokens ($background, $space.4) — never hardcoded values inline. Tamagui is the recommended choice when you target both web and React Native from one codebase.
• React Native StyleSheet / Nativewind: a co-located {name}.styles.ts exporting StyleSheet.create({...}) (or Nativewind classnames). JSX references styles.header.

ts
// invoice-list.styles.ts (Tailwind example)
export const invoiceListStyles = {
  page: "flex flex-col gap-8",
  header: "flex flex-col gap-1.5",
  title: "text-3xl font-semibold tracking-tight",
} as const;

ts
// invoice-list.styles.ts (Tamagui example — works on web AND native)
import { styled, YStack, Text } from "tamagui";

export const InvoiceListPage = styled(YStack, { flex: 1, gap: "$8" });
export const InvoiceListHeader = styled(YStack, { gap: "$1.5" });
export const InvoiceListTitle = styled(Text, {
  fontSize: "$8",
  fontWeight: "600",
});

No inline style={{...}} literals in the component body, on any stack. Why: styling drifts and duplicates when it lives inline. A co-located styles file gives one place to audit spacing rhythm, theme correctness, and responsive behavior per surface. Document non-obvious choices (accent locks, breakpoints) in comments there.

This skill does not dictate the _visual_ design — pair it with a design/component skill for that. It dictates only _where styling lives_.

6. Naming conventions

Consistent naming makes the structure self-describing.

• Interfaces are prefixed with I — IFeatureUiState, IInvoiceListParams, IUserProfile. Type aliases (unions, mapped types, primitives) are not prefixed (type SortDirection = "asc" | "desc").
• Components: PascalCase files and exports — InvoiceListPage.tsx, LineItemRow.tsx.
• Pages/screens: kebab-case directories, the component file matches — pages/invoice-list/invoice-list.tsx.
• Hooks: useCamelCase — useInvoiceList, useFeatureUiStore.
• Stores: {feature}.store.ts (Zustand/MobX), {feature}.slice.ts (Redux), {feature}.atoms.ts (Jotai). Hook is use{Feature}{Purpose}Store.
• Styles: {name}.styles.ts co-located with its owner.
• Constants: SCREAMING_SNAKE_CASE values; kebab-case or camelCase files.
• Barrels: always index.ts.

7. Framework adapters

The module/page/state model is constant. Only the thin routing layer on top changes. Pages always live in modules/; the routing layer just mounts them.

7.1 Next.js (App Router)

• src/app/ holds route segments and route groups ((marketing), (app), (public)) for layout/auth boundaries. Route files are thin: import a page component from a module barrel and render it.
• Default to Server Components; mark interactive leaves "use client". Providers (query client, store, theme) live in a "use client" boundary.

tsx
// app/(app)/invoices/page.tsx — thin route file
import { InvoiceListPage } from "@/modules/invoice";
export default function Page() {
  return <InvoiceListPage />;
}

7.2 React + Vite (SPA)

• A src/routes/ (or single router.tsx) declares the route table (React Router / TanStack Router) and maps paths to module page components. Everything is client-side. Wrap the tree once with the query-client and store/theme providers at the app root.

7.3 Remix

• Route modules in app/routes/ stay thin and re-export/mount module page components; loaders/actions delegate to the module's services/. Module boundaries are unchanged.

7.4 Expo / React Native

• Routing is Expo Router (file-based, in app/) or React Navigation (navigation/). Route/screen files are thin and import screen components from module barrels.
• "Pages" are "screens" — same directory pattern: pages/{screen}/{screen}.tsx + {screen}.styles.ts.
• Query layer + client store run unchanged (TanStack Query, Zustand, Redux, MobX, Jotai all work in RN). The typed api-client is shared logic and works as-is.
• Styling uses Tamagui (recommended for shared web+native), StyleSheet, or Nativewind. Keep module logic DOM-free.

tsx
// app/invoices/index.tsx (Expo Router) — thin screen file
import { InvoiceListScreen } from "@/modules/invoice";
export default InvoiceListScreen;

7.5 Sharing across web + native

If you target both web and Expo, push framework-free code (types, validators, formatters, the API client contract) into a shared package consumed by both apps, and prefer Tamagui for components that must render on both. Module boundaries stay the same on both sides.

8. Conventions checklist (enforce in review)

• ] New feature → new modules/{feature}/ with index.ts + README.md, not files scattered into shared/.
• ] New route → a page/screen directory ({page}.tsx + {page}.styles.ts + index.ts + README.md), not a loose file.
• ] Cross-module imports go through the barrel (@/modules/{feature}) — no deep internal paths.
• ] Server data is in the query/cache layer; UI state is in the module store; neither leaks into the other (whatever libraries are chosen).
• ] No fetch() in components — only typed data hooks built on the shared client.
• ] No inline styles — co-located {name}.styles.ts (Tailwind/CSS Modules/Tamagui/StyleSheet/styled-components).
• ] Components/hooks/utils placed at the narrowest scope; promoted only when a 2nd consumer appears.
• ] One store unit per module, accessed via the barrel, with selectors and a reset.
• ] Interfaces use the I prefix; components/hooks/files follow §6.
• ] Query keys/tags come from a per-module factory; invalidation is hierarchical.
• ] Routing files are thin — they mount module pages and own only layout/auth boundaries.
• ] Module/page READMEs updated when routes, params, or data deps change.

9. Component promotion (start local, move outward)

A component is born in the narrowest scope that uses it and is promoted only when a second consumer appears. Never pre-place a component "because it might be reused."

| A component used by… | Lives in | Imported as | | ---------------------- | --------------------------------- | ---------------------------------- | | Only one page | pages/{page}/components/ | relative path within the page | | 2+ pages in one module | modules/{feature}/components/ | @/modules/{feature} (via barrel) | | 2+ modules | shared/components/ | @/shared/... | | 2+ apps / repos | a published design-system package | the package name |

The same ladder applies to hooks, utils, and constants: local → module → shared → package. Promotion is a deliberate move (update the import sites), not a guess made up front.

10. How to apply this skill

Scaffolding a new app: create src/modules/, src/shared/, and the framework routing layer (§7). Add the shared api-client, the query layer, and your chosen client-store provider. Drop a store template into the first module.

Adding a feature: create modules/{feature}/ with the full subfolder set (pages/ components/ hooks/ stores/ services/ utils/ constants/ types/), a curated index.ts, and a README.md. Build the first screen as a page directory.

Deciding where code goes: ask "who consumes this?" → narrowest scope wins (§9). Ask "where did this data come from?" → server = query layer, UI = store (§4).

Reviewing structure: run the checklist in §8. The most valuable catches are state-origin leaks (server data in the client store) and deep cross-module imports (bypassing the barrel) — both erode the architecture fastest.

Publishing / installing this skill

This skill follows the Anthropic SKILL.md format and is portable across agents. To make it installable and discoverable (e.g. on skills.sh / npx skills):

1. Put this folder under a skills/ directory in a public GitHub repo (path like skills/frontend-architecture/SKILL.md).
2. Keep the frontmatter name and a high-signal description (above) — that description is what discovery indexes match against.
3. Install from any project with: npx skills add <org>/<repo> --skill "frontend-architecture".
4. Non-SKILL.md agents can be pointed here from AGENTS.md / CLAUDE.md; Kiro can mirror it as a steering file.