If you take an array and do the following: arr = []; arr[100] = 1; The length will be 101, which makes sense due to 0-99 being set as undefined Now if we sort that array: arr.sort() it will look like this: [1, undefined x100] since keys are not preserved. However, the length is still 101, since the undefined have all been moved to the end, instead of removed. Is this behavior intentional and if so: is there a built-in function that removes undefined and recalculates and why is it intentional? I am not asking how to write my own function to recalculate length. A sorted array's length can easily be forced with for (x = 0; arr[x] != undefined; x++);arr.length = x;