shallow

Shallow equality comparison function for Svelte Store

Reference

shallow

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.

objA · T

The first value to compare.

objB · T

The second value to compare.

Returns

boolean · boolean

true if the values are shallowly equal, false otherwise.

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

<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

<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

<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

<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

<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

<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

<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