> a = [1, undefined, undefined, undefined, 3]
  [1, undefined, undefined, undefined, 3]
> b = [1,,,,3]
  [1, undefined × 3, 3]
> 1 in a
  true
> 1 in b
  false

b = [1,,,,3];

console.log(Object.getOwnPropertyNames(b));
# [ '0', '4', 'length' ]
console.log(Object.getOwnPropertyNames(a));
# [ '0', '1', '2', '3', '4', 'length' ]

console.log(a.indexOf(1));
# 0
console.log(b.indexOf(1));
# 0
console.log(b.indexOf(5));
# -1

console.log(a.indexOf(1) !== -1);
# true
console.log(b.indexOf(1) !== -1);
# true
console.log(a.indexOf(5) !== -1);
# false