You could use Array#forEach, which skips sparse elements.
let array = new Array(99999),
count = 0;
array[30] = undefined;
array.forEach(_ => count++);
console.log(count);The same with Array#reduce
let array = new Array(99999),
count = 0;
array[30] = undefined;
count = array.reduce(c => c + 1, 0);
console.log(count);For filtering non sparse/dense elements, you could use a callback which returns for every element true.
Maybe this link helps a bit to understand the mechanic of a sparse array: JavaScript: sparse arrays vs. dense arrays.
let array = new Array(99999),
nonsparsed;
array[30] = undefined;
nonsparsed = array.filter(_ => true);
console.log(nonsparsed);
console.log(nonsparsed.length);