node.js library for extracting words from a text
The top part looks clean, personally I would


Not compare with null all the time, just check word.length and have word be an array at all times.
Not initialize word and words separately
Not use String.fromCharCode(code), I would use text[index] instead
I would use the ~ operator instead of comparing to -1
I would first deal with finding a match , and then with not finding a match ( switch the if blocks in other words ), my mind had to do a double take when I was reading your code
if would add a space to the end of text, so that I would not need the last if statement


All that would give me something like:

module.exports = function (text){
 text += " ";  

 var words = [], 
     word  = [], 
     limit = text.length, code, index;

    for (index = 0; index &lt; limit; index += 1) {
        code = text.charCodeAt(index);
        if (~_.indexOf(letters, code, true)) {
          word.push( text[index] )
        } else {
          if (word.length) {
            words.push(word.join(''));
            word = [];
          }
        }
    }
    return words;
};


Finally, I think collecting char-codes in the 2nd script, then taking chars, converting those to char-codes and then use the chars again might not be the best approach.

Personally, I would letters be an object where each letter ( not the char-code ) would be a property of the object set to true. No more char-code conversions, and most likely it would beat the sorted lookup table. ( To be tested.. )