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

### Tell us what’s happening:

I am getting this eror message:“Your .forEach() method should increment the count for each number. Don’t forget the fallback value.” Even though I am changing “undefined” to 1 if that is the first time to encounter that number.

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

}

``````<!-- file: index.html -->
<!DOCTYPE html>
<html lang="en">
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<script src="./script.js"></script>
<title>Statistics Calculator</title>
<body>
<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>
</form>
<div class="results">
<p>
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>
<p class="bold">Mean: <span id="mean"></span></p>
<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>
<p class="bold">Median: <span id="median"></span></p>
<p>
The <dfn>mode</dfn> of a list of numbers is the number that appears most
often in the list.
</p>
<p class="bold">Mode: <span id="mode"></span></p>
<p>
The <dfn>range</dfn> of a list of numbers is the difference between the
largest and smallest numbers in the list.
</p>
<p class="bold">Range: <span id="range"></span></p>
<p>
The <dfn>variance</dfn> of a list of numbers measures how far the values
are from the mean, on average.
</p>
<p class="bold">Variance: <span id="variance"></span></p>
<p>
The <dfn>standard deviation</dfn> of a list of numbers is the square
root of the variance.
</p>
<p class="bold">
Standard Deviation: <span id="standardDeviation"></span>
</p>
</div>
</body>
</html>
``````
``````/* 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) => {
if (counts[el] === undefined) {
counts[el] = 1;
} else {
counts[el]++;
}
});

}

/* 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;
}
``````

User Agent is: `Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36`

### Challenge Information:

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

it wants you to use the OR operator to check if `counts[el]` is undefined or not

Thanks, I solved it!!!

1 Like

Can you please explain further? I really don’t understand why to use OR in this case, thank you.

you can’t add 1 to a property object that doesn’t exist, so you use shortcircuiting to check if the property is defined before adding, if it’s not defined you sub in the value 0

shortcircuiting: the OR operator checks the value of the first element, if it’s truthy it gives back that value, if it’s falsy it gives back the value of the second element

``````let person = '';
let message = `Hey, \${person || "you"}, welcome back!`
``````

for example this one checks if `person` has a value or not, an empty string is falsey (behaves like `false` in a boolean context), the OR operator gives back `you`, making the sentence `Hey, you, welcome back!`.
``````let person = "rexon";
This will give `Hey, rexon, welcome back!` because `person` has a truthy value.
There is also `??` which specifically gives back the second value only if the first is `null` or `undefined`, not for all falsey values.