References vs Values

(The following is basically a short version of this article)

Primitives are accessed directly. Primitives are:

• String
• Number
• Boolean
• null
• undefined

Other data types are passed by reference (and handled as Objects in JavaScript):

• Array
• Function
• Object

When a primitive is assigned to a variable, its value is copied. For example:

var x = 10
var a = x
var x = 'abc'
 
console.log(x) // => 'abc'
console.log(a) // => 10

Changes made to x after assigning x to a are not reflected in a, because its value copied and the variables have no connection whatsoever to on another.

In contrast, Objects are references to an address. If an Object is assigned to a variable, that variable contains a reference to the address of the Object. Therefore, changes made to the object are reflected in all variables referencing that address.

function invalidatePerson(person) {
	person.valid = false
}
 
var jenkins = {
	age: 25,
	valid: true
}
 
var invalidPerson = invalidatePerson(jenkins)
 
console.log(jenkins.valid) // => false
console.log(invalidPerson.valid) // => false