Solid Core Reactivity Model

solid-core-reactivity-model

Quick Reference

The Fundamental Mental Model

SolidJS has NO virtual DOM. Components run exactly once to set up a reactive dependency graph. Only reactive expressions (effects, memos, JSX bindings) re-execute when their dependencies change. The DOM is updated directly through reactive bindings — no diffing, no reconciliation, no re-rendering.

Tracking Scope Matrix

Reactive Dependency Graph

Signal (source)          Memo (derived)         Effect (sink)
  [count]  ──────────►  [double]  ──────────►  [DOM update]
     │                                              ▲
     └──────────────────────────────────────────────┘
                    (direct subscription)

• Signals are sources — they store values and notify subscribers
• Memos are derived nodes — they cache computed values and propagate changes
• Effects are sinks — they perform side effects, NEVER produce reactive values
• Dependencies are tracked automatically when a signal/memo getter is called inside a tracking scope

Execution Models

Critical Warnings

NEVER assume components re-render — SolidJS component functions run ONCE. Only reactive expressions inside tracking scopes re-execute. Placing logic in the component body expecting it to re-run is the #1 cause of broken SolidJS code.

NEVER destructure props or signal results outside a tracking scope — this captures a static snapshot and permanently breaks reactivity.

NEVER use dependency arrays — SolidJS tracks dependencies automatically. There is no equivalent to React's useEffect([deps]) or useMemo([deps]).

NEVER return cleanup functions from effects — use onCleanup() as a separate call instead. Returning a function from createEffect does NOT register cleanup.

NEVER access signals conditionally or after early returns inside effects — signals accessed after an early return or inside an if branch are only tracked when that code path executes, causing intermittent tracking failures.

ALWAYS call signal getters as functions (count(), not count) — the getter function call is what registers the dependency in the tracking scope.

ALWAYS access props directly (props.name) or use splitProps/mergeProps — NEVER destructure props at the function parameter level.

How JSX Compilation Works

SolidJS compiles JSX into direct DOM creation calls with reactive bindings. There is no virtual DOM tree, no diff algorithm, no reconciliation.

// What you write:
const el = <div class={className()}>{count()}</div>;

// What the compiler produces (conceptual):
const el = document.createElement("div");
createRenderEffect(() => el.className = className());
createRenderEffect(() => el.textContent = count());

Each {expression} in JSX becomes a fine-grained reactive binding. When count() changes, ONLY that specific text node updates — the div element, its attributes, and all sibling nodes remain untouched.

Ownership Tree and Disposal

SolidJS organizes reactive computations into an ownership tree. Every computation (effect, memo) is owned by the scope that created it. When a scope disposes, all child computations are automatically cleaned up.

Key Primitives

Ownership Hierarchy

createRoot (application root)
  └── Component A (runs once)
        ├── createEffect (owned by A)
        │     └── onCleanup (runs on re-execute or dispose)
        ├── createMemo (owned by A)
        └── Child Component B (owned by A)
              └── createEffect (owned by B)

When Component A unmounts, ALL its owned computations (effects, memos, child components) are disposed automatically.

When to Use createRoot

ALWAYS use createRoot when creating reactive computations outside of a component tree — for example, in a standalone reactive system, a test harness, or a module-level subscription.

// WRONG: Effect created outside ownership — memory leak, no cleanup
createEffect(() => console.log(globalSignal()));

// CORRECT: createRoot provides ownership and disposal
const dispose = createRoot((dispose) => {
  createEffect(() => console.log(globalSignal()));
  return dispose;
});
// Later: dispose() to clean up

runWithOwner for Async Contexts

Async callbacks lose their tracking owner. Use runWithOwner to restore it:

function AsyncComponent() {
  const owner = getOwner();

  onMount(async () => {
    const data = await fetchData();
    // WRONG: createEffect here has no owner (async broke the scope)
    // CORRECT: restore the owner explicitly
    runWithOwner(owner!, () => {
      createEffect(() => console.log(data, someSignal()));
    });
  });
}

Synchronous vs Batched Execution

SolidJS 1.x: Synchronous by Default

Signal updates propagate immediately. Each setSignal() call triggers all dependent computations synchronously before the next line of code executes.

const [a, setA] = createSignal(1);
const [b, setB] = createSignal(2);

createEffect(() => console.log(a() + b()));

setA(10); // Effect runs immediately: logs 12
setB(20); // Effect runs immediately: logs 30

Use batch() to defer propagation:

batch(() => {
  setA(10);
  setB(20);
}); // Effect runs ONCE: logs 30

SolidJS 2.x: Microtask-Batched by Default

All signal updates are batched automatically. Reads do NOT reflect updates until the microtask batch flushes. Use flush() when immediate propagation is needed.

// 2.x behavior:
setA(10);
setB(20);
// Effects have NOT run yet — batched until microtask flush

flush(); // Force immediate propagation

Version-Specific Changes

Key 2.x Reactivity Changes

• Microtask batching: Default behavior — no manual batch() needed for most cases
• flush(): Forces immediate propagation when synchronous behavior is required
• Async first-class: Computations can return Promises; the graph handles suspension automatically
• onMount renamed to onSettled: Can return a cleanup function
• Dev guardrails: Warnings for accidental top-level reads and reactive scope writes
• createSignal(fn): Derived-but-writable signals (function argument creates derived initial value)

Decision Trees

"Should this code be in a tracking scope?"

1. Does the code read a signal/memo value? → If NO → tracking scope not needed
2. Should the code re-run when that value changes? → If NO → use untrack() or read in component body
3. Does the code produce a derived value? → If YES → use createMemo
4. Does the code perform a side effect? → If YES → use createEffect
5. Does the code update the DOM directly? → If YES → use createRenderEffect

"Why is my value not updating?"

1. Are you calling the signal getter as a function? → count() not count
2. Is the access inside a tracking scope? → Component body is NOT tracked
3. Did you destructure props or store values? → Destructuring breaks tracking
4. Is there an early return BEFORE the signal access? → Move signal reads before returns
5. Is the access inside untrack()? → This explicitly suppresses tracking
6. Is the signal inside a conditional branch? → Access unconditionally, use value conditionally

Reference Links

• references/methods.md — API signatures for createRoot, getOwner, runWithOwner, tracking scope primitives
• references/examples.md — Reactive graph examples, tracking demos, ownership patterns
• references/anti-patterns.md — React mental model violations: re-render assumption, VDOM thinking, dependency arrays

Official Sources

• https://docs.solidjs.com/concepts/intro-to-reactivity
• https://docs.solidjs.com/concepts/signals
• https://docs.solidjs.com/concepts/effects
• https://docs.solidjs.com/reference/basic-reactivity/create-signal
• https://docs.solidjs.com/reference/basic-reactivity/create-effect
• https://docs.solidjs.com/reference/basic-reactivity/create-memo
• https://docs.solidjs.com/reference/reactive-utilities/batch
• https://docs.solidjs.com/reference/reactive-utilities/untrack
• https://github.com/solidjs/solid/releases