code: function boll(close, n = 20) {
      let mb = []
      let up = []
      let bn = []
      let len = close.length
      if (len === 0) {
          return {mb, up, bn}
      }
      mb = ma(close, n)
      up.push(mb[0])
      bn.push(mb[0])
      for (let i = 1; i < len; i++) {
          let sum = 0
          for (let j = i; j > -1 && j > i - n; j--) {
              sum += Math.pow(close[j] - mb[i], 2)
          }
          let sd = Math.sqrt(sum / Math.min(i + 1, n))
          up.push(mb[i] + 2 * sd)
          bn.push(mb[i] - 2 * sd)
      }
  
      return {mb, up, bn}
  }
docs: Bollinger Bands