JS: Hallar percentil de un número

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