Learn Advanced Array Methods by Building a Statistics Calculator - Step 27

Tell us what’s happening:

I’m not getting what it means by fallback value. loving the course and flying through it so far!

Your code so far

I’ve tried:
array.forEach(el => counts[el] ? counts[el]++ : counts[el] = 1;
and the equivelant
array.forEach(el => {if (counts[el] === undefined) {counts[el] = 0;} counts[el]++;)
the latter I found after banging my head against the wall a few times.

<!-- file: index.html -->
<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <link rel="stylesheet" href="./styles.css" />
    <script src="./script.js"></script>
    <title>Statistics Calculator</title>
    <h1>Statistics Calculator</h1>
    <p>Enter a list of comma-separated numbers.</p>
    <form onsubmit="calculate(); return false;">
      <label for="numbers">Numbers:</label>
      <input type="text" name="numbers" id="numbers" />
      <button type="submit">Calculate</button>
    <div class="results">
        The <dfn>mean</dfn> of a list of numbers is the average, calculated by
        taking the sum of all numbers and dividing that by the count of numbers.
      <p class="bold">Mean: <span id="mean"></span></p>
        The <dfn>median</dfn> of a list of numbers is the number that appears in
        the middle of the list, when sorted from least to greatest.
      <p class="bold">Median: <span id="median"></span></p>
        The <dfn>mode</dfn> of a list of numbers is the number that appears most
        often in the list.
      <p class="bold">Mode: <span id="mode"></span></p>
        The <dfn>range</dfn> of a list of numbers is the difference between the
        largest and smallest numbers in the list.
      <p class="bold">Range: <span id="range"></span></p>
        The <dfn>variance</dfn> of a list of numbers measures how far the values
        are from the mean, on average.
      <p class="bold">Variance: <span id="variance"></span></p>
        The <dfn>standard deviation</dfn> of a list of numbers is the square
        root of the variance.
      <p class="bold">
        Standard Deviation: <span id="standardDeviation"></span>
/* file: styles.css */
body {
  margin: 0;
  background-color: rgb(27, 27, 50);
  text-align: center;
  color: #fff;

button {
  cursor: pointer;
  background-color: rgb(59, 59, 79);
  border: 3px solid white;
  color: white;

input {
  background-color: rgb(10, 10, 35);
  color: white;
  border: 1px solid rgb(59, 59, 79);

.bold {
  font-weight: bold;
/* file: script.js */
const getMean = (array) => array.reduce((acc, el) => acc + el, 0) / array.length;

const getMedian = (array) => {
  const sorted = array.sort((a, b) => a - b);
  const median =
    array.length % 2 === 0
      ? getMean([sorted[array.length / 2], sorted[array.length / 2 - 1]])
      : sorted[Math.floor(array.length / 2)];
  return median;

// User Editable Region

const getMode = (array) => {
  const counts = {};
  array.forEach(el => counts?[el]++ counts[el] = 1)


// User Editable Region

const calculate = () => {
  const value = document.querySelector("#numbers").value;
  const array = value.split(/,\s*/g);
  const numbers = array.map(el => Number(el)).filter(el => !isNaN(el));
  const mean = getMean(numbers);
  const median = getMedian(numbers);

  document.querySelector("#mean").textContent = mean;
  document.querySelector("#median").textContent = median;

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36 OPR/

Challenge Information:

Learn Advanced Array Methods by Building a Statistics Calculator - Step 27

the value of counts[el] can be undefined or a number, as you can’t sum to undefined you need to fallback to 0 if the value of counts[el] is undefined. You can do this using the OR logical operator

Thanks for the handy tip @ilenia !

I came up with this solution for the forEach() loop however it is not being marked as an accepted answer.

const getMode = (array) => {
  const counts = {};
  array.forEach(el => {
    counts[el] ||= 0;

I added a console.log to print out the value of counts and am seeing this (based on a form input of “1,2,2,3,3,3”:

{ '1': 1, '2': 2, '3': 3 }

This looks correct to me, but i must be missing something. Any ideas?

Please create your own topic to ask for help

Anyway, good job there!, but the tests are particularly string, and don’t accept this even if it’s working.

Only one line in the forEach, in which you both assign the new value, and use the fallback if needed

1 Like