code

var line = "the word in the word list are the best word in the word list a word could word for";
var banned = "word";
var arr = string.split(line, " ");
var counts = {};
var hold = {};
for(var i = 0; i < arr.length; i++){
    var s = arr[i];
    if(s==banned){
    continue;
    }
    if(counts[s] != -1){
        if(counts[s] == undefined){
            counts[s] = 1;
            hold[s] = 1;
        }
        if(counts[s] == 1){
            counts[s] = -1;
            delete hold[s]
        }
    }
}

return Object.Keys(hold)[0];