How to get median and quartiles/percentiles of an array in JavaScript (or PHP)?

Question

This question is turned into a Q&A, because I had struggle finding the answer, and think it can be useful for others

I have a JavaScript array of values and need to calculate in JavaScript its Q2 (50th percentile aka MEDIAN), Q1 (25th percentile) and Q3 (75th percentile) values.

Answer 1

I updated the JavaScript translation from the first answer to use arrow functions and a bit more concise notation. The functionality remains mostly the same, except for std, which now computes the sample standard deviation (dividing by arr.length - 1 instead of just arr.length)

// sort array ascending const asc = arr => arr.sort((a, b) => a - b);  const sum = arr => arr.reduce((a, b) => a + b, 0);  const mean = arr => sum(arr) / arr.length;  // sample standard deviation const std = (arr) => {     const mu = mean(arr);     const diffArr = arr.map(a => (a - mu) ** 2);     return Math.sqrt(sum(diffArr) / (arr.length - 1)); };  const quantile = (arr, q) => {     const sorted = asc(arr);     const pos = (sorted.length - 1) * q;     const base = Math.floor(pos);     const rest = pos - base;     if (sorted[base + 1] !== undefined) {         return sorted[base] + rest * (sorted[base + 1] - sorted[base]);     } else {         return sorted[base];     } };  const q25 = arr => quantile(arr, .25);  const q50 = arr => quantile(arr, .50);  const q75 = arr => quantile(arr, .75);  const median = arr => q50(arr); 

Answer 2

After searching for a long time, finding different versions that give different results, I found this nice snippet on Bastian Pöttner's web blog, but for PHP. For the same price, we get the average and standard deviation of the data (for normal distributions)...

PHP Version

//from https://blog.poettner.de/2011/06/09/simple-statistics-with-php/  function Median($Array) {   return Quartile_50($Array); }  function Quartile_25($Array) {   return Quartile($Array, 0.25); }  function Quartile_50($Array) {   return Quartile($Array, 0.5); }  function Quartile_75($Array) {   return Quartile($Array, 0.75); }  function Quartile($Array, $Quartile) {   sort($Array);   $pos = (count($Array) - 1) * $Quartile;    $base = floor($pos);   $rest = $pos - $base;    if( isset($Array[$base+1]) ) {     return $Array[$base] + $rest * ($Array[$base+1] - $Array[$base]);   } else {     return $Array[$base];   } }  function Average($Array) {   return array_sum($Array) / count($Array); }  function StdDev($Array) {   if( count($Array) < 2 ) {     return;   }    $avg = Average($Array);    $sum = 0;   foreach($Array as $value) {     $sum += pow($value - $avg, 2);   }    return sqrt((1 / (count($Array) - 1)) * $sum); } 

Based on the author's comments, I simply wrote a JavaScript translation that will certainly be useful, because surprisingly, it is nearly impossible to find a JavaScript equivalent on the web, and otherwise requires additional libraries like Math.js

JavaScript Version

//adapted from https://blog.poettner.de/2011/06/09/simple-statistics-with-php/ function Median(data) {   return Quartile_50(data); }  function Quartile_25(data) {   return Quartile(data, 0.25); }  function Quartile_50(data) {   return Quartile(data, 0.5); }  function Quartile_75(data) {   return Quartile(data, 0.75); }  function Quartile(data, q) {   data=Array_Sort_Numbers(data);   var pos = ((data.length) - 1) * q;   var base = Math.floor(pos);   var rest = pos - base;   if( (data[base+1]!==undefined) ) {     return data[base] + rest * (data[base+1] - data[base]);   } else {     return data[base];   } }  function Array_Sort_Numbers(inputarray){   return inputarray.sort(function(a, b) {     return a - b;   }); }  function Array_Sum(t){    return t.reduce(function(a, b) { return a + b; }, 0);  }  function Array_Average(data) {   return Array_Sum(data) / data.length; }  function Array_Stdev(tab){    var i,j,total = 0, mean = 0, diffSqredArr = [];    for(i=0;i<tab.length;i+=1){        total+=tab[i];    }    mean = total/tab.length;    for(j=0;j<tab.length;j+=1){        diffSqredArr.push(Math.pow((tab[j]-mean),2));    }    return (Math.sqrt(diffSqredArr.reduce(function(firstEl, nextEl){             return firstEl + nextEl;           })/tab.length));   }