> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/TanStack/store/llms.txt
> Use this file to discover all available pages before exploring further.

# Atom

> The Atom type represents a writable reactive state container

# Atom

The `Atom<T>` type represents a writable reactive state container that can be read, written, and subscribed to for changes.

## Type Definition

```typescript theme={null}
interface Atom<T> extends BaseAtom<T> {
  /** Sets the value of the atom using a function. */
  set: ((fn: (prevVal: T) => T) => void) & ((value: T) => void)
}

interface BaseAtom<T> extends Subscribable<T>, Readable<T> {}
```

## Properties

### `set`

Sets the value of the atom. Can be called with either a new value directly or a function that receives the previous value and returns the new value.

**Type:**

```typescript theme={null}
((fn: (prevVal: T) => T) => void) & ((value: T) => void)
```

### `get`

Inherited from `Readable<T>`. Returns the current value of the atom.

**Type:**

```typescript theme={null}
() => T
```

### `subscribe`

Inherited from `Subscribable<T>`. Subscribes to changes in the atom's value.

**Type:**

```typescript theme={null}
((observer: Observer<T>) => Subscription) &
  ((
    next: (value: T) => void,
    error?: (error: any) => void,
    complete?: () => void,
  ) => Subscription)
```

## Usage Examples

### Setting values directly

```typescript theme={null}
import { atom } from '@tanstack/store'

const counterAtom = atom(0)

// Set a new value directly
counterAtom.set(5)

console.log(counterAtom.get()) // 5
```

### Setting values with a function

```typescript theme={null}
import { atom } from '@tanstack/store'

const counterAtom = atom(0)

// Update based on previous value
counterAtom.set((prev) => prev + 1)

console.log(counterAtom.get()) // 1
```

### Reading and subscribing

```typescript theme={null}
import { atom } from '@tanstack/store'

const counterAtom = atom(0)

// Subscribe to changes
const subscription = counterAtom.subscribe((value) => {
  console.log('Counter changed:', value)
})

// Update the atom
counterAtom.set(1) // Logs: "Counter changed: 1"
counterAtom.set(2) // Logs: "Counter changed: 2"

// Clean up
subscription.unsubscribe()
```

### Working with complex state

```typescript theme={null}
import { atom } from '@tanstack/store'

interface User {
  name: string
  age: number
}

const userAtom = atom<User>({ name: 'John', age: 30 })

// Update specific properties
userAtom.set((prev) => ({ ...prev, age: prev.age + 1 }))

console.log(userAtom.get()) // { name: 'John', age: 31 }
```

## Related Types

* [ReadonlyAtom](/api/types/readonly-atom) - A read-only version of Atom
* [Observer](/api/types/observer) - The observer pattern for subscriptions
* [Subscription](/api/types/subscription) - The subscription object returned by subscribe
* [AtomOptions](/api/types/atom-options) - Configuration options for creating atoms