Primitive types vs objects in JavaScript

First, let's define what are primitive types.

Primitive types in JavaScript are

• strings
• numbers (Number and BigInt)
• booleans (true or false)
• undefined
• Symbol values

null is a special primitive type. If you run typeof null you'll get 'object' back, but it's actually a primitive type.

Everything that is not a primitive type is an object.

Functions are objects, too. We can set properties and method on functions. typeof will return 'function' but the Function constructor derives from the Object constructor.

The big differences between primitive types and objects are

• primitive types are immutable, objects only have an immutable reference, but their value can change over time
• primitive types are passed by value. Objects are passed by reference
• primitive types are copied by value. Objects are copied by reference
• primitive types are compared by value. Objects are compared by reference

If we copy a primitive type in this way:

let name = 'Flavio'
let secondName = name

Now we can change the name variable assigning it a new value, but secondName still holds the old value, because it was copied by value:

name = 'Roger'
secondName //'Flavio'

If we have an object:

let car = {
  color: 'yellow'
}

and we copy it to another variable:

let car = {
  color: 'yellow'
}

let anotherCar = car

in this case anotherCar points to the same object as car. If you set

car.color = 'blue'

also

anotherCar.color

will be 'blue'.

The same works for passing around objects to functions, and for comparing.

Say we want to compare car to anotherCar:

anotherCar === car //true

This is true because both variables point to exactly the same object.

But if anotherCar was an object with the same properties as car, comparing them would give a false result:

let car = {
  color: 'yellow'
}

let anotherCar = {
  color: 'yellow'
}

anotherCar === car //false