solidjs-reactivity

name	solidjs-reactivity
description	Implements SolidJS reactive patterns using signals, effects, and memos. Use when writing or reviewing SolidJS components, state management, or reactive code. Covers the core mental model: components run once, signals track in reactive scopes, and props must not be destructured.

SolidJS Reactivity

Core Mental Model

SolidJS is NOT React. The fundamental differences:

1. Components run once — A component function executes a single time to set up the reactive graph. It never re-runs.
2. Reactivity is automatic — Signal reads inside tracking scopes create subscriptions. No dependency arrays.
3. Fine-grained updates — Only the specific DOM nodes or computations that depend on a signal update when it changes. No virtual DOM diffing.

// The component function runs ONCE. Only the {count()} expression re-evaluates.
function Counter() {
  const [count, setCount] = createSignal(0);
  console.log("This logs once, not on every update");
  return <button onClick={() => setCount(c => c + 1)}>{count()}</button>;
}

createSignal

Creates a reactive value with a getter (accessor) and setter.

import { createSignal } from "solid-js";

const [count, setCount] = createSignal(0);

// Read: call the getter — signals are functions
count(); // 0

// Write: call the setter
setCount(5);
setCount(prev => prev + 1);

Options:

// Custom equality — skip update if values are "equal"
const [data, setData] = createSignal(initialData, {
  equals: (prev, next) => prev.id === next.id,
});

// Always notify — useful for triggering effects on every set
const [trigger, setTrigger] = createSignal(undefined, { equals: false });

Key rule: Signal getters must be called (with ()) to read the value. count is a function, count() is the value.

Tracking Scopes

Signals are only tracked when read inside a tracking scope. These are:

1. JSX expressions — <div>{count()}</div>
2. createEffect — createEffect(() => console.log(count()))
3. createMemo — createMemo(() => count() * 2)

Reading a signal outside these scopes does NOT create a subscription:

function MyComponent() {
  const [count, setCount] = createSignal(0);

  // NOT tracked — runs once during component setup
  console.log(count());

  // Tracked — re-runs when count changes
  createEffect(() => {
    console.log(count()); // tracked
  });

  // Tracked — JSX expressions are tracking scopes
  return <div>{count()}</div>;
}

createEffect

Runs side effects when dependencies change. Tracks signals automatically.

import { createSignal, createEffect, onCleanup } from "solid-js";

function Timer() {
  const [count, setCount] = createSignal(0);

  createEffect(() => {
    const id = setInterval(() => setCount(c => c + 1), 1000);
    onCleanup(() => clearInterval(id)); // cleanup on re-run or disposal
  });

  return <span>{count()}</span>;
}

Execution timing:

• Initial run: after synchronous component code, before browser paint
• Subsequent runs: after tracked dependencies change
• Effects run AFTER all pure computations (memos) in the same update cycle
• Effects never run during SSR

Cleanup: Use onCleanup inside an effect to clean up before re-execution or disposal.

createMemo

Creates a cached derived value. Only recalculates when dependencies change.

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

function FilteredList() {
  const [query, setQuery] = createSignal("");
  const [items] = createSignal(["apple", "banana", "cherry"]);

  // Recalculates only when query() or items() change
  const filtered = createMemo(() =>
    items().filter(item => item.includes(query()))
  );

  return <ul><For each={filtered()}>{item => <li>{item}</li>}</For></ul>;
}

Memo vs derived signal (plain function):

// Derived signal — recalculates every time it's read
const doubled = () => count() * 2;

// Memo — caches result, recalculates only when count changes
const doubled = createMemo(() => count() * 2);

Use createMemo when:

• The computation is expensive
• The result is read in multiple places (prevents duplicate computation)
• You need to suppress downstream updates when the result hasn't changed

Use a plain derived signal when:

• The computation is cheap
• It's only read in one place

batch

Groups multiple signal updates so downstream computations run once.

import { batch } from "solid-js";

batch(() => {
  setFirstName("John");
  setLastName("Doe");
  setAge(30);
}); // Effects/memos that depend on these signals update once here

Automatic batching: Updates inside createEffect, onMount, and store setters are already batched. You rarely need explicit batch().

Important: If batch callback is async, only updates before the first await are batched.

untrack

Reads a signal without creating a tracking dependency.

import { untrack } from "solid-js";

createEffect(() => {
  // count() is tracked, name() is NOT tracked
  console.log(count(), untrack(() => name()));
});

Use untrack when you need a signal's current value without subscribing to changes.

Anti-Patterns — See PITFALLS.md

The most common mistakes when writing SolidJS code are documented in detail in the PITFALLS.md companion file. The top issues:

1. Destructuring props (breaks reactivity)
2. Conditional access outside JSX (breaks reactivity)
3. Setting signals inside derived computations
4. Forgetting () on signal getters
5. Accessing signals outside tracking scopes
6. Event handler reactivity assumptions
7. stopPropagation with delegated events
8. Async breaking tracking context
9. Creating signals/effects outside component scope
10. Assuming effect execution order