[javascript] Javascript Uncaught TypeError: Cannot read property '0' of undefined

I know there's plenty of questions related to this error and I've checked most of them and none help me solve my issue. (Which seems so easy to debug...)

I have an array (which is empty aat first):

var words = [];

And my function hasLetter, checks if we find a letter (object) in the array (that I call here: d) words.

function hasLetter(letter,d){

// if words[0] not null should return object of letter "a", here we getting
// the index of the letter (since ascii of "a" is 97, I substract 97)
var ascii = letter.charCodeAt(0)-97;

//Trying to not get an error with this but still creates an err
if(typeof d[ascii ] !== "undefined" && d[ascii ] !== null && d[ascii ].length > 0){
    if(d[ascii].letter == letter){
        return true;
    }
}
return false; }

and I have a function called addLetter which checks if hasLetter returns true/false and then creates or not accordingly a new node.

function addLetter(letter,d){
var ascii = letter.charCodeAt(0)-97;
if(!hasLetter(letter,d)){
    document.write("This letter" + letter + " hasn't been found in words.");
    d[ascii] = new Node(letter);
}
    document.write("This letter " + letter + " already exists in words.");
    document.write(d[ascii].letter);

}

and if I test:

addLetter("a",words);

it returns:

Uncaught TypeError: Cannot read property '0' of undefined

I don't know what to do to say "if it's undefined then don't look into it or something along those lines...

Thanks

There is no error when I use your code,

but I am calling the hasLetter method like this:

hasLetter("a",words);