code: function n2bts(n_) {
    var neg = n_ < 0;
    var n = Math.abs(n_);
    var s = '';
    do {
      var digit = n % 3;
  
      // balance the ternary http://stackoverflow.com/questions/26456597/how-to-convert-from-unbalanced-to-balanced-ternary
      if (digit === 2) {
        digit = -1;
        ++n;
      }
  
      //console.log('digit',digit,n,n_,s);
  
      // if number has negative sign, flip all digits
      if (neg) {
        digit = -digit;
      }
  
      s = N_TO_BT_DIGIT[digit] + s;
      n = ~~(n / 3);    // truncate, not floor! negatives
  
    } while(n);
    //console.log('n2bts',n_,s);
    return s;
  }
docs: signed integer to balanced ternary string