The point is how they've decided to implement Array#reduce.
If you go up to exercise 16, you'll see they force the result of reducing to always be returned inside an array.
return [accumulatedValue];
They even remind you in the comment in exercises 17 and 18:
// Remember that reduce always
// returns an array with one item.
I can't say I agree with this decision (it's not the common way to do reduce), but following their own version of reduce, then exercise 19 is correct.
The point is how they've decided to implement Array#reduce. If you go up to exercise 16, you'll see they force the result of reducing to always be returned inside an array. return [accumulatedValue]; They even remind you in the comment in exercises 17 and 18: // Remember that reduce always // returns an array with one item. I can't say I agree with this decision (it's not the common way to do reduce), but following their own version of reduce, then exercise 19 is correct.