Index of elisions in JavaScript arrays

Question

What is the difference between the undefined elements in JavaScript arrays:

1. created directly,
2. created with elision?

Elements of case 1. and 2. both have undefined types and values, but the ones born in elision don't have indexes (or any enumerable properties, their array has a correct Array.length that's clear). Can you give some background on this?

Example:

// case 1.
const undefineds = [undefined, undefined, undefined].map((el, i) => i)
console.log(undefineds, undefineds.length, 'undefineds')

// case 2.
const empties = [,,,].map((el, i) => i)
console.log(empties, empties.length, 'empties')

Output:

Array [0, 1, 2] 3 "undefineds"
Array [undefined, undefined, undefined] 3 "empties"

Note: In case 2. I am aware of not just the indexes i would be undefined, but anything returned from the iteration, e.g.: [,,,].map((el, i) => 'puppies') => [undefined, undefined, undefined].