Solid JS 2x API Changes And Best Practices

SolidJS 2.x API Changes And Best Practices

Use this skill for Solid 2.x beta or next work. Treat the current beta migration guide and RFCs as the source of truth, and be explicit that some APIs or package boundaries may still shift before final stable release.

Goals

• Help the user write correct Solid 2.x code, not just mechanically renamed 1.x code.
• Explain changed semantics, especially where old 1.x intuitions are now wrong.
• Surface likely ecosystem lag or package-version caveats instead of pretending everything is already settled.

First step

Determine whether the user wants:

• fresh 2.x code
• explanation of a 2.x API or warning
• review of existing 2.x code
• comparison between 1.x and 2.x behavior

If the user is actually upgrading old code, prefer the dedicated migration skill unless they only want a narrow API explanation.

How to use this skill

Read this skill in passes instead of treating every section as equally important.

1. Start with Important framing, Primitive selection guide, and Core behavior changes to internalize.
2. Use Core API guide for fresh code, reviews, and narrow API questions.
3. Use Reference guide when the task is concentrated in one area such as async data, ownership, stores, or DOM behavior.
4. Use Task-focused loading and Common mistakes to catch for code review, warning triage, and implementation guidance.

Important framing

• Target is Solid 2.x beta or next.
• If the local project dependencies disagree with the skill's guidance, trust the installed versions and package exports in the repo.
• Prefer explaining the new mental model, because many 2.x mistakes come from carrying over 1.x habits.
• For code generation, prefer the exact primitives and package boundaries described in this skill and the official Solid 2.0 docs. Do not substitute neighboring framework helpers or unofficial aliases just because they sound plausible.
• In particular, do not introduce createAsync, mount, router helpers, or Start-specific patterns unless the user is explicitly in that ecosystem and the local repo confirms those exports.

Scope boundary

• Use this skill for fresh 2.x code, narrow 2.x API explanations, 2.x warning triage, and review of code that already targets the new semantics.
• If the user is upgrading an existing 1.x app or library, prefer the dedicated migration skill unless they only need a narrow explanation of one changed API.
• Do not answer Start-specific route, query, action, middleware, or deployment questions from generic Solid assumptions. Use the SolidStart skill alongside this one when the app is a Start app.

Reference guide

Keep the main body of this skill as the working guide. Read the deeper references when the task is concentrated in one area.

• for batching, split effects, top-level read warnings, onSettled, or ownership: read references/reactivity-effects-and-ownership.md
• for Loading, isPending, latest, refresh, action, or optimistic UI: read references/async-data-and-mutations.md
• for createStore, createProjection, storePath, snapshot, merge, or omit: read references/stores-and-helpers.md
• for For, Repeat, function-child accessors, DOM attributes, ref directive factories, or context providers: read references/control-flow-dom-and-context.md

Primitive selection guide

Choose primitives with this bias:

1. createSignal for local scalar state
2. createMemo for derivation, including async derivation when it matches the problem
3. createStore for nested state
4. Loading and Errored for async and error boundaries
5. isPending, latest, and refresh for async coordination
6. action plus optimistic helpers for mutations
7. createProjection or function-form createStore for projection-oriented derived collections and keyed reconciliation
8. createEffect only for bridging to the outside world
9. For, Show, Switch, Match, and Repeat for control flow
10. ref factories for imperative DOM hooks

Core API guide

Keep this dense and practical. Bias toward use it for, watch out for, and the most important 2.x-specific behavior.

createSignal

• Use it for core local state.
• In 2.x, writes are batched until flush, so immediate reads do not observe the new value.
• Function form can express derived-but-writable state when that shape is truly intended:

const [value, setValue] = createSignal(() => props.something);

For readonly derivation, prefer createMemo.

createMemo

• Use it for derivation, including many async derivations in 2.x.
• Tracked reads must happen before await.
• For Solid core 2.x examples, prefer an explicitly async computation shape instead of drifting into neighboring helpers like createAsync.

const profile = createMemo(async () => {
  const id = userId();
  return fetchProfile(id);
});

• Consumers still read the accessor normally; unresolved async values suspend through the graph until a Loading boundary handles them.

Track before await:

const filtered = createMemo(async () => {
  const q = query();
  await warmCache(q);
  return search(q);
});

Do not read query() for the first time after an await.

createEffect

• Use it as a split-phase effect: compute first, apply second.

• Old 1.x single-callback intuition often produces the wrong shape or warning-prone code.

• Drive this distinction hard:

• compute is for reading reactive inputs and deciding what changed

• apply is for mutating the outside world

• cleanup belongs to apply

• if you are only deriving data, you probably want createMemo, not createEffect

Example:

createEffect(
  () => [title(), enabled()],
  ([nextTitle, nextEnabled]) => {
    document.title = nextTitle;
    widget.setEnabled(nextEnabled);
    return () => widget.dispose();
  }
);

onSettled

• Use it instead of onMount.
• It runs once current activity settles and can return cleanup.
• Treat onMount migration as a semantic review, not a blind rename.

Loading

• Use it as the async readiness boundary.
• It is primarily for initial readiness, not every later refresh.
• Pair it with an async computation from createMemo(async () => ...) or another clearly derived 2.x primitive. Do not answer with old Suspense code unless the user asked for comparison or migration context.

Errored

• Use it as the 2.x error boundary primitive.
• It can render a fallback callback with the error and reset function.
• Pair it with the 2.x async and effect model instead of carrying forward old boundary assumptions.

isPending, latest, and refresh

• Use them as async coordination helpers.
• isPending(() => expr) is for refresh or stale-while-revalidate states, not first load.
• latest(fn) lets you inspect the most recent in-flight value.
• refresh(...) recomputes derived reads after writes.

Example:

const users = createMemo(async () => fetchUsers(filter()));
const usersPending = () => isPending(() => users());
const latestUsers = () => latest(users);

action, createOptimistic, createOptimisticStore

• Use action(...) for mutation flows.
• Actions run inside the transition model and are the intended place to sequence optimistic writes, async work, and refreshes.
• Use optimistic primitives when immediate UI feedback matters and the real source of truth still needs refresh afterward.
• Prefer this over inventing extra pending or mirror state by hand.

Example:

const [todos] = createStore(() => api.listTodos(), []);
const [optimisticTodos, setOptimisticTodos] = createOptimisticStore(() => todos(), []);

const addTodo = action(function* (text: string) {
  const optimistic = { id: crypto.randomUUID(), text, pending: true };
  setOptimisticTodos(list => list.push(optimistic));
  const saved = yield api.addTodo(text);
  refresh(todos);
  return saved;
});

Keep the real source of truth explicit and refresh it after the mutation settles.

createProjection

• Use it when the derived result is better modeled as a store-shaped projection than a plain accessor.
• It fits projection, keyed reconciliation, and selection-like patterns that 1.x users may have modeled with selectors or ad-hoc effects.
• If it returns list or map data, unchanged keyed entries can keep identity via reconciliation.

createStore

• Use it for nested state, now from solid-js.
• Prefer draft-first setters in fresh 2.x code.
• Function-form createStore(fn, initial) still exists for derived-store patterns when that shape is the right fit.
• snapshot replaces unwrap, and returning a value from a setter performs a top-level shallow replacement or diff.
• Old path-style habits still exist, but they are no longer the center of the API.

Preferred update shape:

setStore(s => {
  s.user.address.city = "Paris";
});

storePath

Compatibility helper for path-style store updates. Useful during migration, but not the default style for fresh 2.x code.

Example:

setStore(storePath("user", "address", "city", "Paris"));

merge and omit

General helpers replacing mergeProps and splitProps. merge treats undefined as an explicit value, not as "missing". omit is the preferred way to exclude keys without split-style copying.

Example:

const merged = merge({ a: 1, b: 2 }, { b: undefined });

snapshot and deep

Use snapshot(store) for serialization or interop. Use deep(store) only when deep observation is truly intended.

Example:

const plain = snapshot(store);

For, Show, Switch, Match, Repeat

• Use them as control-flow primitives with more explicit accessor semantics.
• Function children often receive accessors.
• For replaces Index via keyed={false}.
• Repeat handles count or range rendering without list diffing.

createContext

Use the context object itself as the provider component.

Example:

const ThemeContext = createContext("light");

<ThemeContext value="dark">
  <Page />
</ThemeContext>

Prefer this over carrying forward Context.Provider.

Ownership and createRoot

• A createRoot(...) created inside an owned scope is owned by its parent by default.
• Disposal follows the parent unless you detach explicitly.
• This reduces accidental unowned graphs and cleanup surprises.

If the user genuinely needs detached lifetime, make that explicit:

const singleton = runWithOwner(null, () => {
  const [value, setValue] = createSignal(0);
  return { value, setValue };
});

createComputed removal

• createComputed is removed.
• Replace it with split createEffect for side effects, function-form createSignal or createStore for derived state with a setter, or createMemo for readonly derivation.
• 2.x wants derived state, effects, and ownership to be more explicit and easier to reason about.

refs and directive factories

Use ref factories for imperative DOM hooks and directive-like behavior. Do not translate use: literally.

Core behavior changes to internalize

Batching is microtask-based

• Setters do not immediately change what reads return.
• Updated values become visible after the microtask flush.
• Use flush() only when a synchronous settled point is truly needed, especially in tests or imperative DOM code.

This interacts directly with async derivations and split effects. Do not reason about 2.x as though setters synchronously rewire every downstream read.

Effects are split into compute and apply phases

Prefer the 2.x shape:

createEffect(
  () => source(),
  value => {
    sideEffect(value);
    return () => cleanup();
  }
);

The compute function tracks dependencies. The apply function performs effects and can return cleanup.

Use this bias:

• compute reads
• apply mutates the outside world
• cleanup belongs with apply

Be unusually strict about this. If the user asks about createEffect, explain the split even if the immediate bug looks small, because this is one of the main 2.x mental-model changes.

Top-level reactive reads warn

In component bodies, avoid reading reactive values at top level unless the read is intentionally wrapped in untrack. This includes common patterns like destructuring reactive props.

Prefer reading inside JSX expressions, memos, or effect compute functions.

Writes inside owned reactive scope warn

Do not use memos or tracked scopes to write application state back into signals or stores. Prefer:

• derived state with createMemo
• event handlers
• actions
• narrow pureWrite: true only for valid internal cases

Do not recommend pureWrite: true as a generic warning silencer.

Async derivation patterns

Solid 2.x can derive async values directly. Show that this is possible, but do not force it as the one true answer for every async problem.

Use this guidance:

• for async derived reads, createMemo(async () => ...) is one viable 2.x pattern
• for keyed or collection-oriented derived state, createProjection(...) or function-form createStore(...) may fit better than a plain memo
• for initial readiness, pair async values with Loading
• for stale-while-refreshing UI, use isPending and optionally latest
• for user-facing mutations where immediate feedback matters, prefer action plus optimistic primitives over hand-rolled pending flags

Important distinctions:

• unresolved async reads suspend through the graph until a Loading boundary handles them
• Loading is for first readiness
• isPending is for background revalidation when usable UI already exists
• async reads and async mutations are different jobs; computations handle reads, actions handle mutations

Async memo example

const profile = createMemo(async () => {
  const currentId = userId();
  return fetchProfile(currentId);
});

<Loading fallback={<Spinner />}>
  <ProfileView profile={profile()} />
</Loading>

The accessor is still read normally. The async part is expressed by suspension and the boundary, not by adding a separate resource API.

Combined example: stream, projection, optimistic update, pending state

const [feed] = createStore(async () => {
  const items: Message[] = [];

  for await (const chunk of getMessageStream(roomId())) {
    items.push(...chunk);
  }

  return items;
}, [], { key: "id" });

const [projectedFeed] = createStore(
  draft => {
    const visible = feed().filter(m => !m.hidden);
    draft.items = visible;
    draft.unread = visible.filter(m => !m.read).length;
  },
  { items: [], unread: 0 }
);

const [optimisticFeed, setOptimisticFeed] = createOptimisticStore(() => projectedFeed(), {
  items: [],
  unread: 0
});

const sendMessage = action(function* (text: string) {
  const optimistic = { id: crypto.randomUUID(), text, pending: true };
  setOptimisticFeed(s => {
    s.items.unshift(optimistic);
    s.unread += 1;
  });
  yield api.sendMessage(roomId(), text);
  refresh(feed);
  refresh(projectedFeed);
});

const feedRefreshing = () => isPending(() => projectedFeed().items);

Use this kind of shape when:

• an async source feeds a projection-backed collection
• another projection derives the UI-facing slice
• optimistic updates should appear immediately
• isPending should describe refresh state without replacing the current UI

API changes and replacements

Quick rename and removal map

• solid-js/web -> @solidjs/web
• solid-js/store -> solid-js
• solid-js/h -> @solidjs/h
• solid-js/html -> @solidjs/html
• solid-js/universal -> @solidjs/universal
• Suspense -> Loading
• ErrorBoundary -> Errored
• mergeProps -> merge
• splitProps -> omit
• createSelector -> createProjection or function-form createStore
• unwrap -> snapshot
• classList -> class
• createResource removed -> async computations plus Loading
• startTransition and useTransition removed -> built-in transitions, isPending, Loading, and optimistic primitives
• batch removed -> flush() when immediate application is truly needed
• Context.Provider removed -> use the context directly as provider component
• createComputed removed -> split createEffect, function-form createSignal or createStore, or createMemo
• use: directives removed -> ref directive factories
• attr: and bool: removed -> standard attribute behavior
• oncapture: removed
• onMount -> onSettled

Imports

Use the quick map above for package moves. The main practical change is that DOM or renderer subpaths often move to @solidjs/*, while store APIs move into solid-js.

Async and boundaries

• use isPending(() => expr) for stale-while-refreshing indicators
• use latest(fn) to inspect in-flight values when needed
• use refresh(...) to recompute reads after mutations

The rename and removal map above covers the high-level swaps. This section is about how to use the surviving async pieces together.

Control flow

• Index is removed; use <For keyed={false}>
• For child functions receive accessors, so call item() and i()
• function children in Show and related APIs may also receive accessors
• Repeat exists for count or range-based rendering

Stores and helpers

• prefer draft-first setStore(draft => { ... })
• createSelector patterns generally move to createProjection or function-form createStore
• storePath(...) exists as a compatibility helper when path-style ergonomics are still useful
• unwrap(store) -> snapshot(store)
• mergeProps -> merge
• splitProps -> omit
• deep(store) is available when deep observation is truly needed
• createSignal(fn) and createStore(fn, initial) support derived forms

DOM and directives

• use: directives are removed; use ref directive factories
• classList moves to class object or array forms
• attr: and bool: namespaces are removed
• oncapture: is removed
• built-in attributes are closer to raw HTML semantics

Context

• Context.Provider becomes using the context directly as the provider component

Best-practice guidance

When writing or reviewing 2.x code:

• prefer derivation over write-back synchronization
• preserve reactive reads inside valid tracking sites
• keep side effects in effect apply functions, not the tracking half
• treat dev warnings as useful signals, not noise
• use flush() sparingly and usually only after you can explain why it is needed
• prefer Loading for initial readiness and isPending for refresh state instead of inventing extra flags
• prefer draft-first store updates in fresh 2.x code
• prefer ref factories over trying to recreate use: mentally
• for mutations with immediate UI feedback, reach for optimistic primitives before inventing custom mirror state
• keep read concerns in computations and write concerns in actions
• keep ownership explicit; detached lifetime should be rare and deliberate
• prefer the context object as provider instead of carrying forward Context.Provider boilerplate
• avoid top-level reactive reads unless the non-tracking behavior is intentional and explicit
• avoid writing application state from owned reactive scopes
• keep examples on the Solid core boundary unless the user asked for SolidStart, router, or another framework layer
• if an exact beta import or export is uncertain, say so explicitly instead of inventing a helper
• do not replace render with unrelated mount APIs unless the local repo or installed exports show that is the right move

Task-focused loading

Writing fresh 2.x code

Load these ideas first:

• pick primitives from the Primitive selection guide
• keep derivation in createMemo
• use Loading and Errored for boundaries
• use action plus optimistic helpers for mutations
• stay on the Solid core boundary unless the task explicitly moves into Start or router APIs

Explaining warnings or behavior changes

Load these ideas first:

• microtask batching and flush()
• split createEffect
• top-level reactive read warnings
• writes inside owned reactive scope warnings
• ownership and createRoot

Common warning-to-cause mapping:

• stale immediate reads after setters usually mean the user is still assuming 1.x-style synchronous visibility
• top-level read warnings usually mean props or signals were read in setup instead of JSX or another tracked scope
• owned-scope write warnings usually mean derivation was modeled as write-back synchronization instead of derived state or an action

Reviewing existing 2.x code

Look for these first:

• derivation done through effects instead of createMemo
• top-level reactive reads or destructured reactive props
• writes to signals or stores from owned tracked scopes
• fresh-code examples using migration-only or compatibility helpers by default
• drift into Start or router helpers when the task is supposed to stay on Solid core
• nested createRoot(...) calls that assume detached lifetime without making that explicit
• control-flow callbacks that forget accessor semantics such as item() or i()

Async data and mutation design

Load these ideas first:

• createMemo(async () => ...) for reads
• Loading for first readiness
• isPending for revalidation state
• latest when inspecting current in-flight value matters
• action and optimistic primitives for writes
• refresh(...) after mutations

Keep the job split explicit:

• computations handle async reads
• actions handle writes and mutation sequencing
• refresh(...) recomputes reads after writes; it is not just a renamed refetch habit

Store and projection work

Load these ideas first:

• draft-first createStore setters
• createProjection for store-shaped derived results
• function-form createStore when derived store state fits better there
• storePath only as a compatibility helper, not the fresh-code default
• snapshot, merge, and omit instead of old 1.x helpers

DOM and directive work

Load these ideas first:

• ref directive factories instead of use:
• standard HTML-like attribute behavior
• class object or array forms instead of classList
• accessor-aware control-flow callbacks

Common mistakes to catch

Output expectations

When answering, prefer this structure:

1. Name the 2.x concept or behavior change.
2. Explain how it differs from 1.x.
3. Show the 2.x-idiomatic code.
4. Call out beta or ecosystem caveats if relevant.

Example prompts this skill should handle well

• "Write this component using Solid 2.0 beta APIs."
• "Why am I getting a top-level reactive read warning in Solid 2?"
• "What replaced createResource and Suspense in Solid 2.x?"
• "Review this Solid 2 code and point out places where I am still thinking in 1.x terms."

Additional resources

• Solid 2.0 docs index: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/README.md
• Reactivity, batching, and effects RFC: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/01-reactivity-batching-effects.md
• Signals and ownership RFC: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/02-signals-derived-ownership.md
• Control flow RFC: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/03-control-flow.md
• Stores RFC: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/04-stores.md
• Async data RFC: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/05-async-data.md
• Actions and optimistic RFC: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/06-actions-optimistic.md
• DOM RFC: https://github.com/solidjs/solid/blob/next/documentation/solid-2.0/07-dom.md