Solid Impl State Patterns

solid-impl-state-patterns

Quick Reference

State Primitives Overview

Critical Warnings

NEVER use createEffect to synchronize derived state — ALWAYS use createMemo instead. Effects are for side effects (DOM, network, logging), not for computing values from other signals.

NEVER store signal values in variables outside tracking scopes — the variable captures a snapshot, not a reactive binding. ALWAYS call the getter function where you need the current value.

NEVER destructure props or stores — destructuring reads values once and breaks reactive tracking. ALWAYS access properties through the proxy object.

NEVER use Redux, useReducer, or Zustand patterns — SolidJS has built-in primitives (signals, stores, context) that are more efficient and idiomatic.

NEVER use global mutable variables for shared state — ALWAYS use createContext + createStore for type-safe, reactive, scoped state sharing.

State Selection Decision Tree

Need state? → What kind of data?
│
├─ Single primitive value (string, number, boolean)
│  → createSignal
│
├─ Object with nested properties / array of items
│  → createStore
│
├─ Derived from other state (computed value)
│  → createMemo (NEVER createEffect + createSignal)
│
├─ Shared across multiple components
│  │
│  ├─ Parent → children (1-2 levels deep)
│  │  → Pass as props (direct)
│  │
│  └─ Across distant components / app-wide
│     → createContext + createStore + Provider
│
├─ Async data from server/API
│  → createResource (SolidJS 1.x) or createAsync (with @solidjs/router)
│
└─ External reactive source (RxJS, custom)
   → from() utility

When to Use Signals vs Stores

Global State Pattern: Context + Store

The canonical SolidJS pattern for shared state combines createContext, createStore, and a Provider component with a custom hook.

import { createContext, useContext, ParentProps } from "solid-js";
import { createStore, SetStoreFunction } from "solid-js/store";

// 1. Define state shape
interface AppState {
  user: { name: string; email: string } | null;
  theme: "light" | "dark";
  notifications: { id: string; message: string }[];
}

// 2. Define context type (state + actions)
type AppContextValue = [
  state: AppState,
  actions: {
    setUser: (user: AppState["user"]) => void;
    toggleTheme: () => void;
    addNotification: (message: string) => void;
    removeNotification: (id: string) => void;
  }
];

// 3. Create context (no default value — enforce Provider usage)
const AppContext = createContext<AppContextValue>();

// 4. Provider component with store + actions
export function AppProvider(props: ParentProps) {
  const [state, setState] = createStore<AppState>({
    user: null,
    theme: "light",
    notifications: [],
  });

  const actions = {
    setUser: (user: AppState["user"]) => setState("user", user),
    toggleTheme: () =>
      setState("theme", (prev) => (prev === "light" ? "dark" : "light")),
    addNotification: (message: string) =>
      setState("notifications", (prev) => [
        ...prev,
        { id: crypto.randomUUID(), message },
      ]),
    removeNotification: (id: string) =>
      setState("notifications", (prev) => prev.filter((n) => n.id !== id)),
  };

  return (
    <AppContext.Provider value={[state, actions]}>
      {props.children}
    </AppContext.Provider>
  );
}

// 5. Custom hook with safety check
export function useApp() {
  const context = useContext(AppContext);
  if (!context) throw new Error("useApp must be used within AppProvider");
  return context;
}

Usage in components:

function Header() {
  const [state, { toggleTheme }] = useApp();
  return (
    <header>
      <span>{state.user?.name ?? "Guest"}</span>
      <button {state.theme}</button>
    </header>
  );
}

Derived State with createMemo

ALWAYS use createMemo for values computed from other reactive state. Memos cache results and only recompute when dependencies change.

import { createSignal, createMemo } from "solid-js";

const [items, setItems] = createSignal<{ price: number; qty: number }[]>([]);
const [taxRate, setTaxRate] = createSignal(0.21);

// CORRECT: Derived values via createMemo
const subtotal = createMemo(() =>
  items().reduce((sum, item) => sum + item.price * item.qty, 0)
);
const tax = createMemo(() => subtotal() * taxRate());
const total = createMemo(() => subtotal() + tax());

// Memos chain: items/taxRate → subtotal → tax → total
// Changing taxRate recalculates tax and total, but NOT subtotal

Derived State Anti-Pattern

// WRONG: Using createEffect to sync derived state
const [count, setCount] = createSignal(0);
const [double, setDouble] = createSignal(0);

createEffect(() => {
  setDouble(count() * 2); // Unnecessary signal + effect cycle
});

// CORRECT: createMemo — no extra signal, no effect
const double = createMemo(() => count() * 2);

Form State Pattern

Store-Based Form (Recommended for Multi-Field Forms)

import { createStore } from "solid-js/store";
import { createSignal, Show } from "solid-js";

interface FormData {
  name: string;
  email: string;
  role: "admin" | "user" | "viewer";
}

interface FormErrors {
  name?: string;
  email?: string;
}

function UserForm() {
  const [form, setForm] = createStore<FormData>({
    name: "",
    email: "",
    role: "user",
  });
  const [errors, setErrors] = createStore<FormErrors>({});
  const [submitting, setSubmitting] = createSignal(false);

  const validate = (): boolean => {
    const newErrors: FormErrors = {};
    if (!form.name.trim()) newErrors.name = "Name is required";
    if (!form.email.includes("@")) newErrors.email = "Invalid email";
    setErrors(newErrors);
    return Object.keys(newErrors).length === 0;
  };

  const handleSubmit = async (e: SubmitEvent) => {
    e.preventDefault();
    if (!validate()) return;
    setSubmitting(true);
    try {
      await saveUser(form);
    } finally {
      setSubmitting(false);
    }
  };

  return (
    <form
      <input
        value={form.name}
        => setForm("name", e.currentTarget.value)}
      />
      <Show when={errors.name}><span class="error">{errors.name}</span></Show>

      <input
        value={form.email}
        => setForm("email", e.currentTarget.value)}
      />
      <Show when={errors.email}><span class="error">{errors.email}</span></Show>

      <select
        value={form.role}
        => setForm("role", e.currentTarget.value as FormData["role"])}
      >
        <option value="admin">Admin</option>
        <option value="user">User</option>
        <option value="viewer">Viewer</option>
      </select>

      <button type="submit" disabled={submitting()}>
        {submitting() ? "Saving..." : "Save"}
      </button>
    </form>
  );
}

State Composition Strategies

splitProps for Prop Separation

import { splitProps } from "solid-js";

function Button(props: {
  variant: "primary" | "secondary";
  size: "sm" | "md" | "lg";
  onClick: () => void;
  children: any;
  class?: string;
  disabled?: boolean;
}) {
  const [local, others] = splitProps(props, ["variant", "size", "children"]);
  return (
    <button
      {...others}
      class={`btn btn-${local.variant} btn-${local.size} ${others.class ?? ""}`}
    >
      {local.children}
    </button>
  );
}

Multiple Contexts (Feature Slicing)

ALWAYS create separate contexts for unrelated state domains. This prevents unnecessary re-computations.

// auth-context.tsx
export function AuthProvider(props: ParentProps) { /* ... */ }
export function useAuth() { /* ... */ }

// theme-context.tsx
export function ThemeProvider(props: ParentProps) { /* ... */ }
export function useTheme() { /* ... */ }

// Compose at app root
function App() {
  return (
    <AuthProvider>
      <ThemeProvider>
        <Router />
      </ThemeProvider>
    </AuthProvider>
  );
}

External State Integration with from()

import { from } from "solid-js";

// Bridge RxJS observable into SolidJS signal
const currentUser = from(userService.currentUser$);

// Bridge custom subscription
const windowWidth = from((set) => {
  const handler = () => set(window.innerWidth);
  window.addEventListener("resize", handler);
  handler(); // Set initial value
  return () => window.removeEventListener("resize", handler);
});

reconcile() for External Data Sync

import { createStore, reconcile } from "solid-js/store";

const [state, setState] = createStore({ todos: [] as Todo[] });

// Sync external data — only changed properties trigger updates
websocket.on("todos-updated", (newTodos: Todo[]) => {
  setState("todos", reconcile(newTodos));
});

Reference Links

• references/methods.md -- Pattern signatures for context+store, derived state, form state patterns
• references/examples.md -- Working code examples: global state, derived state, form management, counter/theme providers
• references/anti-patterns.md -- Redux patterns, prop drilling, global mutable variables, unnecessary state

Official Sources

• https://docs.solidjs.com/concepts/signals
• https://docs.solidjs.com/concepts/stores
• https://docs.solidjs.com/concepts/context
• https://docs.solidjs.com/reference/basic-reactivity/create-signal
• https://docs.solidjs.com/reference/basic-reactivity/create-memo
• https://docs.solidjs.com/reference/store-utilities/create-store
• https://docs.solidjs.com/reference/reactive-utilities/from
• https://docs.solidjs.com/reference/store-utilities/reconcile