Disallow sparse arrays.
Sparse arrays are arrays with empty slots, they are denoted by extra commas, such as:
let foo = [,,]; let foo = [bar,, baz];
Sparse elements will be filled in as undefined elements and count towards array length. This is often a typo or is hard to comprehend and an explicit method should be used.
# Invalid Code Examples
let foo = [,]; let bar = [foo,, bar];
More incorrect examples
More correct examples