no-sparse-arrays
Disallows sparse arrays.
Sparse arrays are arrays that contain empty slots, which later could be
handled either as undefined value or skipped by array methods, and this may
lead to unexpected behavior:
[1, , 2].join(); // => '1,,2'
[1, undefined, 2].join(); // => '1,,2'
[1, , 2].flatMap((item) => item); // => [1, 2]
[1, undefined, 2].flatMap((item) => item); // => [1, undefined, 2]
Invalid:
const items = ["foo", , "bar"];
Valid:
const items = ["foo", "bar"];