
Referential Equality
Tended:
Status: decay
Primitives
If you compare primitive types; null
, undefined
, string
, number
, boolean
or symbol
using the equality operator ===
, the result is what you would except:
Console
const bool1 = true const bool2 = true console.log(bool1 === bool1) const num1 = 1 const num2 = 1 console.log(num1 === num2) const letter1 = `a` const letter2 = `a` console.log(letter1 === letter2)
Objects
If you compare objects object
, array
, function
(array
and function
are a sub-class of object
), the result isn't as intuitive:
Console
const person1 = { name: `Deckard` } const person2 = { name: `Deckard` } console.log(person1 === person2) const letters1 = [`a`, `b`] const letters2 = [`a`, `b`] console.log(letters1 === letters2) const function1 = () => null const function2 = () => null console.log(function1 === function2)
person1
and person2
are both the same type, object
.
They have the same properties and values.
It would makes sense if the equality operator returned true
when comparing them.
But it doesn't.
This is because when you compare objects, the operator is testing reference equality, not value equality.
Testing whether they are the same instance, not whether they are the same value.
person1
and person2
may have the same value, but they are 2 different object instances.
Thus, when compared using the equality operator, it returns false
.

Value vs Reference
When assigning or passing variables of a primitive, a copy of the value is created. When assigning or passing variables of an object, a copy of the reference is created.
Console
// 'a' holds a copy of the value 2. // 'b' is assigned a copy of the value 2. // Any changes to 'b' will not effect 'a'. var a = 2; var b = a; b++; console.log(a)
Console
// 'c' holds a reference to the shared value '[1, 2, 3]' // 'd' holds a copy of the reference to shared value '[1, 2, 3]' // When using the 'd' reference to modify the shared value, 'c' will be effected. var c = [1,2,3]; var d = c; d.push(4); console.log(c);