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strictEqualityCheck

Checks that the current value is strictly equal (===) to the previous value. Type-safe for primitive types and functions only.

When to Use​

Use strictEqualityCheck when deriving primitive values or functions from facets:

Type Safety

strictEqualityCheck has a generic constraint <T extends Immutable | Function> that restricts it to only work with primitives and functions. TypeScript will give a compile error if you try to use it with objects or arrays.

Signature​

typescript
function strictEqualityCheck<T extends Immutable | Function>(): (current: T) => boolean
// Where Immutable = boolean | number | string | undefined | null

Basic Usage​

tsx
import { strictEqualityCheck } from '@react-facet/core'
 
const equalityCheck = strictEqualityCheck()
 
equalityCheck(0)
 
console.log(equalityCheck(0)) // true
console.log(equalityCheck(1)) // false
console.log(equalityCheck(1)) // true

Usage with Facets​

tsx
import { useFacetMap, useFacetWrap, strictEqualityCheck } from '@react-facet/core'
 
const UserGreeting = () => {
  const userFacet = useFacetWrap({ firstName: 'Steve', lastName: 'Miner' })
 
  // Derive first name as a string
  const firstNameFacet = useFacetMap(
    (user) => user.firstName,
    [],
    [userFacet],
    strictEqualityCheck, // Prevents updates when firstName hasn't changed
  )
 
  return (
    <div>
      Hello, <fast-text text={firstNameFacet} />!
    </div>
  )
}

Behavior Details​

First Call​

Always returns false on the first call (no previous value to compare):

tsx
import { strictEqualityCheck } from '@react-facet/core'
 
const equalityCheck = strictEqualityCheck()
 
console.log(equalityCheck(42)) // false - first call
console.log(equalityCheck(42)) // true - same value

Strict Equality​

Uses JavaScript's strict equality operator (===):

tsx
import { strictEqualityCheck } from '@react-facet/core'
 
const equalityCheck = strictEqualityCheck<number>()
 
equalityCheck(0)
 
console.log(equalityCheck(0)) // true
console.log(equalityCheck(0.0)) // true
console.log(equalityCheck(-0)) // true (0 === -0 in JavaScript)
console.log(equalityCheck(NaN)) // false (NaN !== NaN in JavaScript)

Common Patterns​

Deriving String Values​

tsx
import { useFacetMap, strictEqualityCheck } from '@react-facet/core'
const displayNameFacet = useFacetMap(
  (user) => `${user.firstName} ${user.lastName}`,
  [],
  [userFacet],
  strictEqualityCheck,
)

Deriving Numeric Values​

tsx
import { useFacetMap, strictEqualityCheck } from '@react-facet/core'
const percentageFacet = useFacetMap(
  (current, max) => (current / max) * 100,
  [],
  [currentFacet, maxFacet],
  strictEqualityCheck,
)

Deriving Boolean Values​

tsx
import { useFacetMap, strictEqualityCheck } from '@react-facet/core'
const isValidFacet = useFacetMap(
  (email) => email.includes('@') && email.includes('.'),
  [],
  [emailFacet],
  strictEqualityCheck,
)

Comparison with defaultEqualityCheck​

Both use === for primitives, but they have important differences:

strictEqualityCheck:

  • ✅ Type-safe - TypeScript enforces primitive/function types only
  • ✅ Explicit - Makes intent clear in code
  • ✅ Works with functions (function reference comparison)
  • ❌ Slightly slower - Not optimized like defaultEqualityCheck

defaultEqualityCheck:

  • ✅ Performance optimized - Inlined in mapping functions for speed
  • ✅ Default behavior - Used automatically when no check specified
  • ✅ Accepts any type (but always returns false for objects/arrays)
  • ❌ Not type-safe - Allows any type at compile time
tsx
// strictEqualityCheck - explicit, type-safe, slightly slower
useFacetMap((x) => x * 2, [], [numberFacet], strictEqualityCheck)
// defaultEqualityCheck - implicit, optimized, default behavior
useFacetMap((x) => x * 2, [], [numberFacet]) // automatically uses defaultEqualityCheck
Performance Tip

For primitives, the defaultEqualityCheck (used by default) has a special performance optimization that makes it faster than explicitly providing strictEqualityCheck. Unless you need the type safety or function comparison, you can omit the equality check parameter.

See Also​