calculates the average character width in Open Sans 11px

Font Width.js

function quickselect_median(arr) {
   const L = arr.length, halfL = L/2;
   if (L % 2 == 1)
      return quickselect(arr, halfL);
   else
      return 0.5 * (quickselect(arr, halfL - 1) + quickselect(arr, halfL));
}

function quickselect(arr, k) {
   // Select the kth element in arr
   // arr: List of numerics
   // k: Index
   // return: The kth element (in numerical order) of arr
   if (arr.length == 1)
      return arr[0];
   else {
      const pivot = arr[0];
      const lows = arr.filter((e)=>(e<pivot));
      const highs = arr.filter((e)=>(e>pivot));
      const pivots = arr.filter((e)=>(e==pivot));
      if (k < lows.length) // the pivot is too high
         return quickselect(lows, k);
      else if (k < lows.length + pivots.length)// We got lucky and guessed the median
         return pivot;
      else // the pivot is too low
         return quickselect(highs, k - lows.length - pivots.length);
   }
}
function getTextWidth(text, font) {
    // re-use canvas object for better performance
    const canvas = getTextWidth.canvas || (getTextWidth.canvas = document.createElement("canvas"));
    const context = canvas.getContext("2d");
    context.font = font;
    const metrics = context.measureText(text);
    return metrics.width;
}
// from https://fonts.google.com/specimen/Open+Sans#standard-styles
const glyphs = "ABCČĆDĐEFGHIJKLMNOPQRSŠTUVWXYZŽabcčćdđefghijklmnopqrsštuvwxyzžАБВГҐДЂЕЁЄЖЗЅИІЇЙЈКЛЉМНЊОПРСТЋУЎФХЦЧЏШЩЪЫЬЭЮЯабвгґдђеёєжзѕиіїйјклљмнњопрстћуўфхцчџшщъыьэюяΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩαβγδεζηθικλμνξοπρστυφχψωάΆέΈέΉίϊΐΊόΌύΰϋΎΫΏĂÂÊÔƠƯăâêôơư1234567890‘?’“!”(%)[#]{@}/&\<-+÷×=>®©$€£¥¢:;,.*"

const characterSizes = [...glyphs].map(c => getTextWidth(c, "11px Open Sans"))
console.log([...glyphs])
console.log(characterSizes)
const sum = (x, y) => x+y
const avg = characterSizes.reduce(sum, 0)/characterSizes.length
const max = Math.max(...characterSizes)
const min = Math.min(...characterSizes)
const median = quickselect_median(characterSizes)
console.log({avg,max,min,median})