palindromize a word if possible in O(n), if already a palindrom return it, otherwise return null

palindromize.js

function palindromize(word) {
    if (Array.isArray(word)) {
        return word.map(palindromize);
    };
    
    if (!word) return null;        
    
    word = "" + word;
    
    // single letter words? really? i don't know..
    if (word.length === 1) return word; 
    
    
    var hash = {};
    
    var half = Math.floor(word.length / 2);
    var isPalindrom = true;

    for (var i = 0; i < word.length; i++) {
       hash[word[i]] = hash[word[i]] || 0;
       hash[word[i]]++;

       // since we're iterating, mind as well check if it's already a palindrom
       if (i < half && isPalindrom && word[i] !== word[word.length - 1 - i]) {
          isPalindrom = false;
       }
    }
    if (isPalindrom) return word;
    
    var letters = Object.keys(hash);
    
    var oddLetter;        
    var left = "";
    var right = "";
    
    letters.every(function(letter) {
       var count = hash[letter];
       if (count % 2 !== 0) {
           if (oddLetter) {
              left = "";
              return false;
           }
           oddLetter = letter;
           count--;                             
       }
       var add = Array(count / 2 + 1).join(letter);
       left = add + left;
       right += add;
       return true;
    });
    
    return left ? left + (oddLetter || "") + right : null;
}