They're using different demo functions, and their return values are consistent with those functions. Returning the sum is just a common example use case for [].reduce.

[].reduce starts at the beginning of the array and works its way to the end. [].reduceRight starts at the end and works its way to the beginning. I think the example for [].reduceRight used in the pen might be mildly confusing for those still learning since it reduces nested arrays. Here's an example of both using the same function; note how the output differs:

['a', 'b', 'c', 'd'].reduce((current, carry) => current+carry); // ➡ abcd
['a', 'b', 'c', 'd'].reduceRight((current, carry) => current+carry); // ➡ dcba

Yeah @QWp6t you're right, I should make that a little more clear and easy to understand for beginners, since that's who this resource is for.

@sdras - hello! Can I work on this issue? 🙂