The reference solution was wrong, it was then fixed.

The assert answer is not right! No way that -61 is in between -66 and -96! Please correct it.

This will incorrectly return 0 if arr[0] happens to be 0:

alternateSqSum([0, 5, 7, 9, 12]); // returns 0 instead of 125

What you wanted to do was check that arr isn't empty, to avoid reduce throwing an error. You could use arr.length to determine that it's not empty. However, if you just seed reduce with an initial value, then it can process an array of length 0, so you won't need to check the array's length at all.