> ## 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.

# shallow

> Shallow equality comparison function for Svelte Store

## Reference

### `shallow`

```ts theme={null}
function shallow<T>(objA: T, objB: T): boolean
```

A utility function that performs shallow equality comparison between two values. Used as the default equality function in `useStore` to prevent unnecessary reactivity updates.

<ParamField path="objA" type="T">
  The first value to compare.
</ParamField>

<ParamField path="objB" type="T">
  The second value to compare.
</ParamField>

### Returns

<ResponseField name="boolean" type="boolean">
  `true` if the values are shallowly equal, `false` otherwise.
</ResponseField>

## Comparison Behavior

The `shallow` function handles various data types:

* **Reference equality**: Returns `true` if `Object.is(objA, objB)`
* **Primitives/null**: Returns `false` if either value is not an object
* **Map**: Compares size and all entries using `Object.is`
* **Set**: Compares size and all values for presence
* **Date**: Compares timestamps using `getTime()`
* **Objects**: Compares own properties using `Object.is`

## Usage

### Default Behavior

```svelte theme={null}
<script>
import { useStore } from '@tanstack/svelte-store'
import { store } from './store'

// shallow is used by default
const user = useStore(store, (state) => state.user)
</script>

<div>{user.current.name}</div>
```

### Explicit Usage

```svelte theme={null}
<script>
import { useStore, shallow } from '@tanstack/svelte-store'
import { store } from './store'

const user = useStore(
  store,
  (state) => ({
    name: state.name,
    email: state.email
  }),
  { equal: shallow }
)
</script>

<div>
  <h2>{user.current.name}</h2>
  <p>{user.current.email}</p>
</div>
```

### Reference Equality

```svelte theme={null}
<script>
import { useStore } from '@tanstack/svelte-store'
import { store } from './store'

// Use strict reference equality
const items = useStore(
  store,
  (state) => state.items,
  { equal: (a, b) => a === b }
)
</script>

<ul>
  {#each items.current as item (item.id)}
    <li>{item.name}</li>
  {/each}
</ul>
```

### Selecting Object Slices

```svelte theme={null}
<script>
import { useStore, shallow } from '@tanstack/svelte-store'
import { store } from './store'

const config = useStore(
  store,
  (state) => ({
    theme: state.config.theme,
    language: state.config.language,
    notifications: state.config.notifications
  }),
  { equal: shallow }
)
</script>

<div>
  <p>Theme: {config.current.theme}</p>
  <p>Language: {config.current.language}</p>
</div>
```

### Array Filtering

```svelte theme={null}
<script>
import { useStore, shallow } from '@tanstack/svelte-store'
import { store } from './store'

// Shallow comparison prevents updates when filtered array content matches
const activeUsers = useStore(
  store,
  (state) => state.users.filter(u => u.active),
  { equal: shallow }
)
</script>

<div>
  <p>Active users: {activeUsers.current.length}</p>
  <ul>
    {#each activeUsers.current as user (user.id)}
      <li>{user.name}</li>
    {/each}
  </ul>
</div>
```

### With Derived Values

```svelte theme={null}
<script>
import { useStore, shallow } from '@tanstack/svelte-store'
import { store } from './store'

const stats = useStore(
  store,
  (state) => ({
    users: state.users.length,
    posts: state.posts.length,
    comments: state.comments.length
  }),
  { equal: shallow }
)

const total = $derived(
  stats.current.users + stats.current.posts + stats.current.comments
)
</script>

<div>Total items: {total}</div>
```

### Custom Comparison

```svelte theme={null}
<script>
import { useStore } from '@tanstack/svelte-store'
import { store } from './store'

// Deep equality for complex nested objects
const deepEqual = (a, b) => JSON.stringify(a) === JSON.stringify(b)

const preferences = useStore(
  store,
  (state) => state.preferences,
  { equal: deepEqual }
)
</script>
```

## Notes

* Used as the **default equality function** in `useStore`
* Only compares **own properties** at the first level (shallow)
* For nested objects, only the reference is compared
* Handles special object types: Map, Set, Date
* Optimized for Svelte 5's runes-based reactivity
* Prevents unnecessary `$state` updates when object contents are equivalent
* More efficient than deep equality for most reactive scenarios