SolidJS Core Knowledge

Full Reference: See advanced.md for WebSocket primitive, context provider, chat component, room management, Socket.IO integration, and store patterns.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: solid for comprehensive documentation.

When NOT to Use This Skill

Skip this skill when:

Working with React (use frontend-react - APIs are different)
Building Vue applications (use vue-composition)
Using Svelte (use svelte)
Working with Angular (use angular)
Need server-side only logic (no framework needed)

Component Structure

import { createSignal, createEffect, createMemo } from 'solid-js';

interface Props {
  name: string;
  count?: number;
}

function Counter(props: Props) {
  const [localState, setLocalState] = createSignal('');
  const doubled = createMemo(() => (props.count ?? 0) * 2);

  createEffect(() => {
    console.log('Count changed:', props.count);
  });

  return (
    <div>
      <h1>Hello {props.name}</h1>
      <p>Doubled: {doubled()}</p>
      <button onClick={() => setLocalState('clicked')}>
        Click
      </button>
    </div>
  );
}

Reactivity Primitives

API	Purpose
`createSignal`	Reactive state
`createMemo`	Cached computation
`createEffect`	Side effects
`createResource`	Async data fetching
`createStore`	Nested reactive objects

Key Differences from React

No Virtual DOM - fine-grained updates
Props are getters - access via props.name
No dependency arrays - auto-tracking
Components run once - not on every render
JSX compiles differently - expressions are reactive

Control Flow

import { Show, For, Switch, Match } from 'solid-js';

<Show when={isLoggedIn()} fallback={<Login />}>
  <Dashboard />
</Show>

<For each={items()}>{(item) => <Item data={item} />}</For>

Anti-Patterns

Anti-Pattern	Why It's Bad	Correct Approach
Destructuring props	Loses reactivity	Access via `props.name`
Using React patterns	Different paradigm	Use Solid primitives
Not using `<Show>` component	Manual conditional logic	Use `<Show when={}>`
Recreating signals in components	Components run once	Create outside or use stores
Using `innerHTML` without sanitization	XSS vulnerability	Use DOMPurify
Not cleaning up in `onCleanup`	Memory leaks	Add cleanup logic

Quick Troubleshooting

Issue	Likely Cause	Solution
Props not reactive	Destructured props	Access via `props.name`
Signal not updating	Forgot to call setter	Use `setCount(newValue)`
Effect not running	Not tracking signal	Call signal inside effect: `count()`
Component re-running	Treating like React	Components run once, use signals
List not updating	Using array methods	Use `produce()` from solid-js/store
Memory leaks	No cleanup	Use `onCleanup()`

Production Readiness

Error Handling

import { ErrorBoundary } from 'solid-js';

function App() {
  return (
    <ErrorBoundary
      fallback={(err, reset) => (
        <div>
          <p>Error: {err.message}</p>
          <button onClick={reset}>Retry</button>
        </div>
      )}
    >
      <MainContent />
    </ErrorBoundary>
  );
}

Performance

// Lazy loading components
const HeavyComponent = lazy(() => import('./HeavyComponent'));

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <HeavyComponent />
    </Suspense>
  );
}

// Batch updates (rarely needed)
import { batch } from 'solid-js';

batch(() => {
  setCount(count() + 1);
  setName('New Name');
});

Store Patterns

import { createStore, produce } from 'solid-js/store';

const [state, setState] = createStore({
  users: [] as User[],
  filters: { active: true },
});

// Immutable-style updates with produce
function addUser(user: User) {
  setState(produce((s) => {
    s.users.push(user);
  }));
}

// Path-based updates
setState('users', (users) => [...users, newUser]);
setState('filters', 'active', false);

Testing

import { render, screen } from '@solidjs/testing-library';
import userEvent from '@testing-library/user-event';
import Counter from './Counter';

describe('Counter', () => {
  it('increments on click', async () => {
    const user = userEvent.setup();
    render(() => <Counter initial={0} />);

    const button = screen.getByRole('button');
    await user.click(button);

    expect(screen.getByText('Count: 1')).toBeInTheDocument();
  });
});

Security

// innerHTML - only with trusted content
<div innerHTML={sanitizedHtml} />

// Prefer text content
<div>{userInput}</div> // Safe - auto-escaped

// XSS prevention
import DOMPurify from 'dompurify';

function SafeHtml(props: { html: string }) {
  const clean = () => DOMPurify.sanitize(props.html);
  return <div innerHTML={clean()} />;
}

Monitoring Metrics

Metric	Target
Bundle size	< 30KB
First Contentful Paint	< 1s
Time to Interactive	< 1.5s
Memory usage	Stable

Checklist

Reference Documentation

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: solid for comprehensive documentation.