The error is here:
hasLetter("a",words[]);
You are passing the first item of words, instead of the array.
Instead, pass the array to the function:
hasLetter("a",words);
Problem solved!
Here's a breakdown of what the problem was:
I'm guessing in your browser (chrome throws a different error), words[] == words[0], so when you call hasLetter("a",words[]);, you are actually calling hasLetter("a",words[0]);. So, in essence, you are passing the first item of words to your function, not the array as a whole.
Of course, because words is just an empty array, words[0] is undefined. Therefore, your function call is actually:
hasLetter("a", undefined);
which means that, when you try to access d[ascii], you are actually trying to access undefined[0], hence the error.