Smart one! ^^

Clever code, cool!

In 'sum' the currentValue is already set to 0 should there be no 'x' or 'y' to add.

While the first if statement is redundant, perhaps it's helpful for readability.

If you initialize your reducer to 0 you don't need to check the array length.

indent your code and that's it, i like your easy to read solution.
Maybe one line in the first if.

no.

you could use str.isupper()

I like that you simply spelled out the sequence A through Z and did the string[i] in z. I could see doing this in other problems of similar nature.

This is easy to read.

Holding out just a little while longer without using regular expressions ;)

Does that account for capital letters as well?

using map (instead of for) is event better ;)