check a string for unique characters javascript

function checkForUnique(str){
    var hashtable = {};
    for(var i=0,len=str.length;i<len;i++){
        if (hashtable[str[i]] != null){
            hashtable[str[i]] = 1;
            return false;
        }else{
            hashtable[str[i]] = 0;
        }
    }
    return true;
}