Difference between “empty' and ”undefined" in Javascript? [duplicate]

Answer 1

empty is not a type. It is just a way of describing that there is nothing there. It isn't undefined or null. It is the complete lack of a value. You'll only see that when it tries to show you a representation of the array. If you actually look in those indices, it will return undefined.

Answer 2

Arrays are objects. That means that your array

  [undefined,undefined,undefined,empty,4]

could be written as such an object:

 {
  0: undefined,
  1: undefined,
  2: undefined,
  // 3 doesnt exist at all
  4: 4,
  length: 5,
 }

So undefined is actually a slot that exists and holds a value while 3 is not even a key-value pair. As accessing a non existing key results in undefined there is no real world difference:

    array[2] // undefined
    array[3] // undefined

But iterating over an array with map and others will skip empty ones, so then you should use .fill before iterating.

The only way to detect a difference is to check if the key exists:

  2 in array // true
  3 in array // false

To turn empty to undefined you could set the key:

  array[3] = undefined;

and to turn undefined into empty you have to remove the key:

  delete array[3]

however that rarely matters.