percentile

Calculates the percentage of numbers in the given array that are less or equal to the given value.

• Use Array.prototype.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.

const percentile = (arr, val) =>
  (100 *
    arr.reduce(
      (acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0),
      0
    )) /
  arr.length;

percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55