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
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