Evan Miller Ranking Items with Star Ratings, in JavaScript

starsort.js

"use strict";

// based on unutbu's stackoverflow answer
// https://stackoverflow.com/a/40958702/54829

// which is based on Evan Miller's blog post
// http://www.evanmiller.org/ranking-items-with-star-ratings.html

function starsort(ratings) {
    function sum(array) { return array.reduce((x, y) => x + y, 0) };
    function square(x) { return x * x; };

    const confidenceZ = 1.65;

    // include five fake reviews
    // 1 1-star, 1 2-star, 1 3-star, 1 4-star, 1 5-star
    const fakeRatings = ratings.map(count => count + 1);
    const N = sum(fakeRatings);

    const average = sum(fakeRatings.map((count, i) => (i + 1) * count)) / N;
    
    // Dirichlet standard deviation of average
    const x = sum(fakeRatings.map((count, i) => square(i + 1) * count)) / N;
    const standardDeviation = Math.sqrt((x - square(average)) / (N + 1));
    
    return average - confidenceZ * standardDeviation;
}

console.log(starsort([60, 80, 75, 20, 25]));

Here's a Go version, for anyone searching like I was:

// starsort.go

package main

import(
	"fmt"
	"math"
)

// 	http://www.evanmiller.org/ranking-items-with-star-ratings.html

func sum(ns []int) int {
	var total int
	for _,n := range ns {
		total += n
	}
	return total
}

func f(s []int, ns []int)float64{
	N := sum(ns)
	K := len(ns)
	ks := make([]int,K)
	for i:=0; i<5; i++ {
		ks[i] = s[i] * (ns[i]+1)
	}
	return float64(sum(ks)) / float64(N+K)
}

func starSort(ns []int) float64 {
	N := sum(ns)
	K := len(ns)
	s := []int{5, 4, 3, 2, 1}
	s2 := []int{25, 16, 9, 4, 1}
	z := float64(1.65)
	fsns := f(s, ns)
	return fsns - z * math.Sqrt(((f(s2, ns) - (fsns*fsns)) / float64(N+K+1)))
}

func main(){
	fmt.Println(starSort([]int{60, 80, 75, 20, 25}))
}