Why is there a whitespace after a '\w' and before ']'?

I have the same problem.

We are mutating (reducing) the items arrays' length every time we execute splice method.
Modulo is used to check if our index value is bigger than an arrays' length.
If it is we just use the remainder as an index, because it goes at the begining of an array again.
Think of it like a circle, it will go back to where it began.

Why are we writing '% 256' in these two lines:

'+': function() { data[pos] = ((data[pos] || 0) + 1) % 256 },
'-': function() { data[pos] = ((data[pos] || 0) + 255) % 256 },

And why did he make an 'l' variable inside of for loop? I don't think it's necessary?

Thank You Guys so much, I finally get it!

Can someone please explain this kata to me?

I have spent 30 minutes reading the description and just can't figure out what I need to do!

Basically you are asigning value to your key inside object count.
(count[s] || 0) + 1 means that if there is no value(count) of that specific letter, value 0 is being assigned to it and then it is increased by 1.
If we encounter a letter which count is bigger than 1 (not 0 or undefined) than we reach that value and add 1 to it.

Can someone explain this a bit to me?
I do not understand!