Utility functions to calculate percentiles and percent ranks in a JavaScript array.

percentile.js

// Returns the value at a given percentile in a sorted numeric array.
// "Linear interpolation between closest ranks" method
function percentile(arr, p) {
    if (arr.length === 0) return 0;
    if (typeof p !== 'number') throw new TypeError('p must be a number');
    arr.concat().sort((a,b) => a-b);
    if (p <= 0) return arr[0];
    if (p >= 1) return arr[arr.length - 1];

    var index = (arr.length - 1) * p,
        lower = Math.floor(index),
        upper = lower + 1,
        weight = index % 1;

    if (upper >= arr.length) return arr[lower];
    return arr[lower] * (1 - weight) + arr[upper] * weight;
}

// Returns the percentile of the given value in a sorted numeric array.
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;
}