code: function percentRank(arr, v) {
      if (typeof v !== 'number') throw new TypeError('v must be a number');
      for (var i = 0, l = arr.length; i < l; i++) {
          if (v <= arr[i]) {
              while (i < l && v === arr[i]) {
                  i++;
              }
              if (i === 0) return 0;
              if (v !== arr[i-1]) {
                  i += (v - arr[i-1]) / (arr[i] - arr[i-1]);
              }
              return i / l;
          }
      }
      return 1;
  }
docs: Returns the percentile of the given value in a sorted numeric array.
  
  @param {Number[]} arr
  A sorted numeric array to examine.
  @param {Number} v
  The value at which to return the percentile.
  
  @return {Number}
  The percentile at the given value in the given array.