shallow

Shallow equality comparison function for Preact 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 component re-renders.

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 (including symbols) using Object.is

Usage

Default Behavior

import { useStore } from '@tanstack/preact-store'
import { store } from './store'

// shallow is used by default
function Component() {
  const user = useStore(store, (state) => state.user)
  return <div>{user.name}</div>
}

Explicit Usage

import { useStore, shallow } from '@tanstack/preact-store'
import { store } from './store'

function UserCard() {
  const user = useStore(
    store,
    (state) => ({
      name: state.name,
      email: state.email,
      role: state.role
    }),
    { equal: shallow }
  )

  return (
    <div>
      <h2>{user.name}</h2>
      <p>{user.email}</p>
    </div>
  )
}

Selecting Object Slices

import { useStore, shallow } from '@tanstack/preact-store'
import { store } from './store'

function Settings() {
  const settings = useStore(
    store,
    (state) => ({
      theme: state.settings.theme,
      language: state.settings.language,
      notifications: state.settings.notifications
    }),
    { equal: shallow }
  )

  return (
    <div>
      <p>Theme: {settings.theme}</p>
      <p>Language: {settings.language}</p>
    </div>
  )
}

Reference Equality Alternative

import { useStore } from '@tanstack/preact-store'
import { store } from './store'

// Use reference equality for primitive values or immutable references
function Counter() {
  const count = useStore(
    store,
    (state) => state.count,
    { equal: (a, b) => a === b }
  )

  return <div>Count: {count}</div>
}

Array Comparisons

import { useStore, shallow } from '@tanstack/preact-store'
import { store } from './store'

function FilteredList() {
  // Shallow comparison checks array length and element references
  const activeItems = useStore(
    store,
    (state) => state.items.filter(item => item.active),
    { equal: shallow }
  )

  return (
    <ul>
      {activeItems.map(item => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  )
}

Map and Set Support

import { useStore, shallow } from '@tanstack/preact-store'
import { store } from './store'

function TagCloud() {
  // shallow handles Map and Set comparisons
  const tags = useStore(
    store,
    (state) => state.tags, // Map or Set
    { equal: shallow }
  )

  return (
    <div>
      {Array.from(tags).map(tag => (
        <span key={tag}>{tag}</span>
      ))}
    </div>
  )
}

Deep Equality Alternative

import { useStore } from '@tanstack/preact-store'
import { store } from './store'

// Custom deep equality for nested structures
const deepEqual = (a, b) => JSON.stringify(a) === JSON.stringify(b)

function ComplexData() {
  const data = useStore(
    store,
    (state) => state.complexNested,
    { equal: deepEqual }
  )

  return <pre>{JSON.stringify(data, null, 2)}</pre>
}

Performance Optimization

import { useStore, shallow } from '@tanstack/preact-store'
import { store } from './store'

function OptimizedList() {
  // Only re-render when the selected properties change
  const listData = useStore(
    store,
    (state) => ({
      items: state.items,
      filter: state.filter,
      sort: state.sort
    }),
    { equal: shallow }
  )

  return (
    <div>
      {listData.items
        .filter(listData.filter)
        .sort(listData.sort)
        .map(item => <div key={item.id}>{item.name}</div>)}
    </div>
  )
}

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
• Includes symbol properties in comparison (unlike basic React shallow)
• More efficient than deep equality for most use cases
• Prevents unnecessary re-renders when object contents are equivalent
• Works with Preact’s virtual DOM diffing for optimal performance