use num > -num as the condition to correct the negative 0

just use the spoiler flag?

Классный код! Каждый элемент взаимосвязан. Очень умное использование цикла while и метода reduce.

I love this code, though it took me awhile to understand it completely.
Very neat implementation of recursion.

The value 10 is the last elemnt in the array (the client with the most items).
As he (the last client) would come to the window last, it would take him 10 seconds to process his items and plus 2 seconds from the previous client.

Why do the results have to be 10 or 12?
In both the cases first two clients would occupy both the windows, thus adding 2 seconds or 6 seconds respectively.
When both the clients are finished, the last client would start, adding 10 more seconds.
Thus, the result would be 12 or 16 seconds respectively.

That's true. Though in JavaScript for all purposes apart from bitwise operations this is the same as the value 0.

If you pass 0.5 to the function, it won't return the negative value;

This code returns -0, if you pass 0 as a parameter.