Qwik Framework Patterns

Quick Guide: Qwik is resumable - it serializes application state on the server and resumes on the client without re-executing framework code (no hydration). Every $ suffix marks a lazy-loading boundary where the optimizer splits code into separate chunks. Only the code for the interaction the user triggers gets downloaded. Use component$ for all components, useSignal/useStore for state, routeLoader$ for server data, routeAction$ for mutations, and server$ for ad-hoc server RPC. The critical mental model: anything crossing a $ boundary must be serializable.

---

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST wrap every component in component$() - plain functions cannot be lazy-loaded, cannot use hooks, and cannot use <Slot />)

(You MUST ensure all values captured in a $ closure are serializable - non-serializable captures pass type-checking but fail at runtime)

(You MUST use routeLoader$ for initial server data instead of fetching in useTask$ or useResource$ - loaders run before render and integrate with SSR streaming)

(You MUST use preventdefault:click as a JSX attribute instead of calling event.preventDefault() - event handlers load asynchronously so synchronous Event APIs are unavailable)

(You MUST export routeLoader$ and routeAction$ from route files (index.tsx or layout.tsx in src/routes/) - unexported or misplaced loaders/actions silently do nothing)

(You MUST NOT destructure store properties at the top level - destructuring breaks reactivity because you lose the Proxy reference)

</critical_requirements>

---

Auto-detection: Qwik, component$, useSignal, useStore, useTask$, useVisibleTask$, useComputed$, useResource$, routeLoader$, routeAction$, server$, sync$, QRL, noSerialize, @builder.io/qwik, @builder.io/qwik-city, Qwik City, $(), onClick$, onInput$, Slot, q:slot, preventdefault, stoppropagation, useStylesScoped$, resumable, resumability

When to use:

• Building web apps where instant interactivity matters (zero hydration delay)
• Apps with complex interactivity that would ship too much JS with traditional hydration
• Projects needing fine-grained lazy loading without manual code-splitting
• Full-stack apps with server loaders, actions, and RPC via server$
• Progressive enhancement where forms work without JavaScript

When NOT to use:

• Static content sites with minimal interactivity (use a static site generator)
• Projects where the team is deeply invested in React ecosystem libraries that have no Qwik equivalents
• Apps that rely heavily on non-serializable runtime state (class instances, closures with side effects)

Key patterns covered:

• Resumability mental model and the $ suffix convention
• Component definition with component$, props, and <Slot />
• Reactive state: useSignal, useStore, useComputed$
• Lifecycle: useTask$, useVisibleTask$, useResource$
• Event handling: onClick$, preventdefault:click, sync$
• Qwik City routing: file-based routes, layouts, dynamic params
• Server data: routeLoader$, routeAction$, server$
• Serialization rules and the $ boundary

Detailed Resources:

• For decision frameworks and anti-patterns, see reference.md

Core patterns:

• examples/core.md - Components, signals, stores, tasks, events, slots
• examples/routing.md - File-based routing, routeLoader$, routeAction$, server$, middleware
• examples/serialization.md - Serialization rules, $ boundary, non-serializable patterns

---

<philosophy>

Philosophy

Qwik is built on resumability - the idea that the server can serialize the entire application state (component tree, listeners, state) into HTML, and the client can resume exactly where the server left off without re-executing any framework code.

How it differs from hydration frameworks:

Traditional SSR frameworks render HTML on the server, then re-execute all component code on the client to attach event listeners and rebuild the component tree. This is hydration - the client replays the server's work.

Qwik skips this entirely. The server serializes everything into the HTML. When a user clicks a button, only the click handler's code downloads and executes. The framework itself, the component tree, and all other handlers stay unloaded until needed.

The $ suffix is the core mechanism. Every function ending in $ is a lazy-loading boundary. The Qwik optimizer splits code at each $ marker into separate chunks. This means:

• component$() - the component's render function loads only when needed
• onClick$() - the click handler loads only when the user clicks
• routeLoader$() - the loader runs server-side only
• useTask$() - the task loads when its tracked dependencies change

The tradeoff: Because code must be serializable to cross $ boundaries, you cannot capture non-serializable values (class instances, functions, DOM nodes) in $ closures. This constraint is the price of instant interactivity.

When to use Qwik:

• Interactive apps where time-to-interactive matters
• Large apps where traditional hydration downloads too much JS upfront
• Full-stack apps leveraging routeLoader$/routeAction$/server$ for server logic
• Progressive enhancement (Qwik forms work without JS)

When NOT to use Qwik:

• Static content sites with little interactivity
• Projects heavily dependent on React-specific libraries without Qwik equivalents
• Apps requiring extensive non-serializable runtime state

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Components with component$

Every Qwik component must be wrapped in component$(). This is not optional - it enables lazy loading, hooks, and <Slot />.

import { component$, useSignal } from "@builder.io/qwik";

interface CounterProps {
  initial?: number;
  label: string;
}

export const Counter = component$<CounterProps>(({ initial = 0, label }) => {
  const count = useSignal(initial);

  return (
    <div>
      <span>
        {label}: {count.value}
      </span>
      <button onClick$={() => count.value++}>+</button>
    </div>
  );
});

Why good: component$ enables the optimizer to split this into a lazy chunk, typed props via generic, useSignal for reactive state, onClick$ handler loads only on click

// BAD: Plain function component
export const Counter = (props: { label: string }) => {
  // Cannot use hooks here - useSignal will throw
  // Cannot use <Slot /> - only works inside component$
  return <div>{props.label}</div>;
};

Why bad: Without component$ wrapper, hooks throw at runtime, <Slot /> breaks, optimizer cannot split the code, component is not resumable

---

Pattern 2: Reactive State - useSignal vs useStore

useSignal holds a single reactive value accessed via .value. useStore holds a reactive object with deep tracking by default.

import { component$, useSignal, useStore } from "@builder.io/qwik";

export const UserProfile = component$(() => {
  // useSignal for primitives and flat values
  const isEditing = useSignal(false);
  const selectedTab = useSignal<"profile" | "settings">("profile");

  // useStore for objects/arrays - deep reactivity by default
  const user = useStore({
    name: "Alice",
    email: "alice@example.com",
    preferences: {
      theme: "dark",
      notifications: true,
    },
  });

  return (
    <div>
      <h1>{user.name}</h1>
      {isEditing.value ? (
        <input
          value={user.name}
          onInput$={(_, el) => {
            user.name = el.value;
          }}
        />
      ) : (
        <button
          onClick$={() => {
            isEditing.value = true;
          }}
        >
          Edit
        </button>
      )}
    </div>
  );
});

Why good: useSignal for simple toggles/selections (accessed via .value), useStore for structured data (mutate properties directly), deep reactivity tracks user.preferences.theme changes automatically

// BAD: Destructuring a store
const { name, email } = useStore({ name: "Alice", email: "a@b.com" });
// name and email are now plain strings - NOT reactive
// Changing them does nothing to the UI

Why bad: Destructuring extracts primitive values from the Proxy, breaking reactivity - you must keep the store reference intact and access store.name directly

---

Pattern 3: Computed Values with useComputed$

useComputed$ derives values from signals/stores. It re-runs only when dependencies change. Synchronous only.

import { component$, useSignal, useComputed$ } from "@builder.io/qwik";

const TAX_RATE = 0.08;
const FREE_SHIPPING_THRESHOLD = 100;

export const CartSummary = component$(() => {
  const subtotal = useSignal(85);

  const tax = useComputed$(() => subtotal.value * TAX_RATE);
  const shipping = useComputed$(() =>
    subtotal.value >= FREE_SHIPPING_THRESHOLD ? 0 : 9.99,
  );
  const total = useComputed$(() => subtotal.value + tax.value + shipping.value);

  return (
    <div>
      <p>Subtotal: ${subtotal.value.toFixed(2)}</p>
      <p>Tax: ${tax.value.toFixed(2)}</p>
      <p>Shipping: ${shipping.value.toFixed(2)}</p>
      <p>Total: ${total.value.toFixed(2)}</p>
    </div>
  );
});

Why good: Automatic dependency tracking (no dependency arrays), read-only signal prevents accidental mutation, recomputes only when subtotal changes, named constants for magic numbers

---

Pattern 4: Tasks and Lifecycle

useTask$ runs before render (server + client). useVisibleTask$ runs after render (browser only). Use track() to declare reactive dependencies.

import {
  component$,
  useSignal,
  useTask$,
  useVisibleTask$,
} from "@builder.io/qwik";
import { server$ } from "@builder.io/qwik-city";

const DEBOUNCE_MS = 300;

export const SearchBox = component$(() => {
  const query = useSignal("");
  const results = useSignal<string[]>([]);

  // Runs before render, re-runs when query changes
  useTask$(({ track, cleanup }) => {
    const searchTerm = track(() => query.value);
    if (!searchTerm) {
      results.value = [];
      return;
    }

    const debounceTimer = setTimeout(async () => {
      const data = await fetchResults(searchTerm);
      results.value = data;
    }, DEBOUNCE_MS);

    cleanup(() => clearTimeout(debounceTimer));
  });

  return (
    <div>
      <input
        value={query.value}
        onInput$={(_, el) => {
          query.value = el.value;
        }}
      />
      <ul>
        {results.value.map((r) => (
          <li key={r}>{r}</li>
        ))}
      </ul>
    </div>
  );
});

const fetchResults = server$(async function (term: string) {
  // Runs on server only - safe to access DB, env vars, etc.
  const db = this.env.get("DATABASE_URL");
  // ... query database
  return ["result1", "result2"];
});

Why good: track() explicitly declares what triggers re-runs, cleanup() prevents timer leaks, server$ keeps the fetch server-side

When to use each:

Hook	Runs	Use for
useTask$	Server + client, before render	Data init, side effects on state change
useVisibleTask$	Browser only, after render	DOM manipulation, browser APIs, animations
useComputed$	Synchronous, auto-tracked	Derived values (formatting, filtering)
useResource$	Server + client, non-blocking	Async data that shouldn't block render

---

Pattern 5: Event Handling

Event handlers use the on{Event}$ convention. Because handlers load asynchronously, synchronous Event APIs (preventDefault, stopPropagation, currentTarget) are NOT available - use declarative attributes instead.

import { component$, useSignal, $ } from "@builder.io/qwik";

export const LoginForm = component$(() => {
  const email = useSignal("");

  // Extracted handler - wrap with $() for reuse
  const handleSubmit = $((e: SubmitEvent) => {
    // Submit email.value to server
  });

  return (
    <form preventdefault:submit onSubmit$={handleSubmit}>
      <input
        type="email"
        value={email.value}
        onInput$={(_, el) => {
          email.value = el.value;
        }}
      />
      <button type="submit">Login</button>
    </form>
  );
});

Why good: preventdefault:submit replaces e.preventDefault() declaratively, second parameter of onInput$ gives the element directly (avoiding async currentTarget issues), extracted handler uses $() wrapper

// BAD: Calling synchronous Event APIs
<form onSubmit$={(e) => {
  e.preventDefault(); // WRONG - handler is async, this is a no-op
  e.stopPropagation(); // WRONG - same reason
}}>

Why bad: Event handlers are lazy-loaded asynchronously, so preventDefault() and stopPropagation() execute too late to have any effect - use preventdefault:submit and stoppropagation:submit attributes instead

---

Pattern 6: Content Projection with Slot

<Slot /> projects child content. Named slots use the q:slot attribute. Only works inside component$().

import { component$, Slot } from "@builder.io/qwik";

export const Card = component$<{ variant?: "default" | "outlined" }>(
  ({ variant = "default" }) => {
    return (
      <div class={`card card-${variant}`}>
        <header class="card-header">
          <Slot name="header" />
        </header>
        <div class="card-body">
          <Slot /> {/* Default slot */}
        </div>
        <footer class="card-footer">
          <Slot name="footer" />
        </footer>
      </div>
    );
  },
);

// Usage
export const Page = component$(() => {
  return (
    <Card variant="outlined">
      <h2 q:slot="header">Card Title</h2>
      <p>This goes in the default slot.</p>
      <div q:slot="footer">
        <button>Action</button>
      </div>
    </Card>
  );
});

Why good: Named slots via q:slot attribute, default slot for main content, parent and child render independently

Gotcha: q:slot must be on a direct child of the component. Wrapping slotted content in an intermediate element breaks projection.

---

Pattern 7: routeLoader$ for Server Data

routeLoader$ runs on the server before the page renders. It must be exported from a route file. Returns a read-only signal.

// src/routes/products/[id]/index.tsx
import { component$ } from "@builder.io/qwik";
import { routeLoader$ } from "@builder.io/qwik-city";

export const useProduct = routeLoader$(async (requestEvent) => {
  const productId = requestEvent.params.id;
  const product = await db.products.findById(productId);

  if (!product) {
    return requestEvent.fail(404, {
      errorMessage: `Product ${productId} not found`,
    });
  }

  return product;
});

export default component$(() => {
  const product = useProduct(); // ReadonlySignal

  return product.value.failed ? (
    <p>{product.value.errorMessage}</p>
  ) : (
    <div>
      <h1>{product.value.name}</h1>
      <p>${product.value.price}</p>
    </div>
  );
});

Why good: Server-only execution, runs before render (no loading states during SSR), type-safe error handling with fail(), read-only signal prevents accidental client-side mutation

---

Pattern 8: routeAction$ for Mutations

routeAction$ handles form submissions and mutations. Supports Zod validation. Must be exported from route files.

// src/routes/contact/index.tsx
import { component$ } from "@builder.io/qwik";
import { routeAction$, Form, zod$, z } from "@builder.io/qwik-city";

export const useContactAction = routeAction$(
  async (data, requestEvent) => {
    // data is validated and typed: { name: string; email: string; message: string }
    await sendEmail(data);
    return { success: true };
  },
  zod$({
    name: z.string().min(1),
    email: z.string().email(),
    message: z.string().min(10),
  }),
);

export default component$(() => {
  const action = useContactAction();

  return (
    <Form action={action}>
      <input name="name" />
      <input name="email" type="email" />
      <textarea name="message" />

      {action.value?.fieldErrors?.email && (
        <p class="error">{action.value.fieldErrors.email}</p>
      )}

      {action.value?.failed && <p class="error">{action.value.message}</p>}

      {action.value?.success && <p>Message sent!</p>}

      <button type="submit" disabled={action.isRunning}>
        {action.isRunning ? "Sending..." : "Send"}
      </button>
    </Form>
  );
});

Why good: <Form> works without JS (progressive enhancement), Zod validation runs server-side with typed errors, action.isRunning for loading state, action.value.failed discriminates success/failure

</patterns>

---

<red_flags>

RED FLAGS

High Priority Issues

• Using plain functions instead of component$() - Hooks throw, <Slot /> breaks, optimizer cannot split code, component is not resumable
• Destructuring store properties - const { name } = store extracts a plain value, breaking reactivity. Always access store.name directly
• Calling event.preventDefault() inside onClick$ - Handler loads asynchronously, so preventDefault() is a no-op. Use preventdefault:click attribute
• Putting routeLoader$/routeAction$ in non-route files without re-exporting - They silently do nothing unless exported from src/routes/**/index.tsx or layout.tsx
• Capturing non-serializable values in $ closures - Class instances, functions, DOM nodes pass type-checking but fail at runtime with serialization errors

Medium Priority Issues

• Using useVisibleTask$ when useTask$ would work - useVisibleTask$ is browser-only and runs after render; prefer useTask$ by default for better SSR
• Fetching data in useTask$ instead of routeLoader$ - Loaders integrate with SSR streaming and run before render; useTask$ blocks rendering
• Using client:load-style thinking - Qwik is not an islands framework. Every component is already lazy-loaded at the interaction level. You do not choose what to hydrate.
• Over-capturing in $ closures - Closing over an entire store when you only need one property forces Qwik to serialize the whole store

Common Mistakes

• Using useStore({ deep: true }) explicitly - Deep is already the default. Passing it is redundant. Pass { deep: false } only when you need shallow tracking
• Using arrow functions for store methods - Arrow functions lose this binding. Use regular function(){} syntax for methods on stores
• Confusing @builder.io/qwik vs @builder.io/qwik-city imports - Components, signals, tasks from @builder.io/qwik. Routing, loaders, actions, server$ from @builder.io/qwik-city
• Inline <style> tags in components - Causes double-loading (SSR + client). Use useStylesScoped$() or CSS modules instead

Gotchas & Edge Cases

• useTask$ without track() runs once on mount - Without tracking any signal, it behaves like an initialization hook, not a reactive effect
• useTask$ blocks rendering - Long async operations in useTask$ delay the component render. Use useResource$ for non-blocking async
• onInput$ second parameter - The callback receives (event, element) where element is the target. Use el.value instead of event.currentTarget.value (currentTarget is null in async handlers)
• Middleware does NOT run for server$ calls - Layout-level onRequest/onGet handlers are skipped for server$ RPC. Use plugin.ts for middleware that must run on server$ requests
• Version skew with server$ - Client and server must run the same code version. Stale client deployments cause undefined behavior
• useStylesScoped$ uses emoji-based class hashing - Scoped styles apply via emoji characters in selectors. Use :global() to break out when styling <Slot /> content
• Props are shallowly immutable - Reassigning a primitive prop from a child does nothing. Pass a Signal instead if the child needs to write back
• Deep store mutations may not trigger updates - Tracking store[key].nested requires tracking the specific property, not just the key. useStore with { deep: false } disables deep tracking
• <Slot /> does not work in inline components - Only component$() functions support <Slot />. Arrow functions or plain functions will silently fail

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST wrap every component in component$() - plain functions cannot be lazy-loaded, cannot use hooks, and cannot use <Slot />)

(You MUST ensure all values captured in a $ closure are serializable - non-serializable captures pass type-checking but fail at runtime)

(You MUST use routeLoader$ for initial server data instead of fetching in useTask$ or useResource$ - loaders run before render and integrate with SSR streaming)

(You MUST use preventdefault:click as a JSX attribute instead of calling event.preventDefault() - event handlers load asynchronously so synchronous Event APIs are unavailable)

(You MUST export routeLoader$ and routeAction$ from route files (index.tsx or layout.tsx in src/routes/) - unexported or misplaced loaders/actions silently do nothing)

(You MUST NOT destructure store properties at the top level - destructuring breaks reactivity because you lose the Proxy reference)

Failure to follow these rules will cause silent runtime failures, broken reactivity, serialization errors, or loaders/actions that never execute.

</critical_reminders>