docs: add atom guide

docs/atoms.md

@@ -0,0 +1,138 @@
+---
+title: Atoms
+id: atoms
+---
+
+Atoms are the low-level reactive values that power TanStack Store. They are
+framework-agnostic and can be used directly when you want a small state primitive
+with `get()`, `subscribe()`, and, for writable atoms, `set()`.
+
+Use a `Store` when you want the higher-level state API with `.state`,
+`.setState()`, and optional actions. Use an atom when you want to pass a
+reactive value directly to framework adapters, compose fine-grained derived
+values, or build a smaller primitive without the store wrapper.
+
+## Writable atoms
+
+Pass an initial value to `createAtom` to create a writable atom.
+
+```typescript
+import { createAtom } from '@tanstack/store'
+
+const count = createAtom(0)
+
+console.log(count.get()) // 0
+
+count.set((prev) => prev + 1)
+console.log(count.get()) // 1
+
+count.set(10)
+console.log(count.get()) // 10
+```
+
+Writable atoms expose:
+
+- `get()` to read the current value
+- `set(value)` or `set((prev) => next)` to update the value
+- `subscribe(listener)` to react to future updates
+
+```typescript
+const subscription = count.subscribe((value) => {
+  console.log('The count is now:', value)
+})
+
+count.set((prev) => prev + 1)
+
+subscription.unsubscribe()
+```
+
+## Readonly derived atoms
+
+Pass a function to `createAtom` to create a readonly derived atom. Derived atoms
+track other atoms or stores that are read while computing their value.
+
+```typescript
+import { createAtom } from '@tanstack/store'
+
+const count = createAtom(1)
+const double = createAtom(() => count.get() * 2)
+
+console.log(double.get()) // 2
+
+count.set(5)
+console.log(double.get()) // 10
+```
+
+Readonly atoms still expose `get()` and `subscribe()`, but they do not expose
+`set()`.
+
+## Custom comparison
+
+Atoms use `Object.is` to decide whether a new value should notify subscribers.
+Pass `compare` when you need a different equality check.
+
+```typescript
+import { createAtom, shallow } from '@tanstack/store'
+
+const settings = createAtom(
+  { theme: 'dark', sidebar: 'expanded' },
+  { compare: shallow },
+)
+
+settings.set((prev) => ({ ...prev }))
+```
+
+In this example, subscribers are not notified for the last update because the
+new object is shallowly equal to the previous value.
+
+## Async atoms
+
+Use `createAsyncAtom` when the atom value comes from a promise. It returns a
+readonly atom whose value is an async state object.
+
+```typescript
+import { createAsyncAtom } from '@tanstack/store'
+
+const user = createAsyncAtom(async () => {
+  const response = await fetch('/api/user')
+  return response.json()
+})
+
+console.log(user.get()) // { status: 'pending' }
+```
+
+When the promise settles, the atom updates to either `{ status: 'done', data }`
+or `{ status: 'error', error }`.
+
+## Atoms vs stores
+
+`Store` is a small wrapper around an atom. Both can be read and subscribed to,
+but they expose different APIs:
+
+| Use case | Atom | Store |
+| --- | --- | --- |
+| Read current value | `atom.get()` | `store.state` or `store.get()` |
+| Update writable value | `atom.set(...)` | `store.setState(...)` |
+| Derived value | `createAtom(() => ...)` | `createStore(() => ...)` |
+| Actions | Compose functions around the atom | Built in through `createStore(initialValue, actions)` |
+
+If you are building app-level state and want the most ergonomic API, start with
+`createStore`. If you are building reusable reactive primitives, framework
+adapter state, or fine-grained values shared through atom hooks, use
+`createAtom`.
+
+## Framework adapters
+
+Framework packages can consume atoms directly. For example, React and Preact
+provide `useAtom` for writable atoms and `useSelector` for atoms or stores,
+while Angular provides `injectAtom`.
+
+See the Atoms examples in each framework section for adapter-specific usage.
+
+## API reference
+
+- [`createAtom`](reference/functions/createAtom.md)
+- [`createAsyncAtom`](reference/functions/createAsyncAtom.md)
+- [`Atom`](reference/interfaces/Atom.md)
+- [`ReadonlyAtom`](reference/interfaces/ReadonlyAtom.md)
+- [`AtomOptions`](reference/interfaces/AtomOptions.md)

docs/config.json

@@ -20,6 +20,10 @@
         {
           "label": "Quick Start",
           "to": "quick-start"
+        },
+        {
+          "label": "Atoms",
+          "to": "atoms"
         }
       ],
       "frameworks": [