Why is Object.keys([,1,,1]) different from Object.keys([,1,undefined,1])?

Question

I was shocked that the result of

document.writeln(Object.keys([,1,,1]));

is

1,3

which filters the index of an array that doesn't contains anything automatically. But when I try to print out Object.keys([,1,undefined,1]):

document.writeln(Object.keys([,1,undefined,1]));

the result is:

1,2,3

which is different from the previous one, and I try to print the value of index 2 at the first one:

document.writeln([,1,,1][2]);

which is undefined actually. Why would Object.keys([,1,,1]) and Object.keys([,1,undefined,1]) return different results?

Answer 1

In Object.keys([, 1, undefined, 1]) case, you are explicitly defining the third element to be undefined, but in Object.keys([, 1, , 1]), there is no element associated with the index 2. So, 2 is recognized as an array index in the first case, but not in the second case.

Just to represent the holes in the array, when you actually reference it, undefined is returned. That is why [, 1, , 1][2] is undefined.