BMI Calculator JavaScript

November 25, 2021

I’m very new to JavaScript, but I try to challenge myself a bit with some small projects.
Whenever I get stuck, I Google the problem and try to solve it that way, but this time I can’t figure out why my code isn’t working.

function berekenBMI() {
  var gewicht = Number(document.getElementById("gewicht"); // weight
  var lengte = Number(document.getElementById("lengte"); // height
  var BMI = gewicht / Math.pow(lengte, 2); // the calculation 

  document.getElementById("output").innerHTML = Math.round(BMI * 100) / 100;

  if (BMI < 18.5) document.getElementById("comment").innerHTML = '5';
  if (BMI >= 18.5 && BMI < 25) document.getElementById("comment").innerHTML = '2';
  if (BMI >= 25 && BMI <= 30) document.getElementById("comment").innerHTML = 'geen';
  if (BMI > 30) document.getElementById("comment").innerHTML = 'nooit geen';
}

<h2> BMI Berekenen </h2>

<div class="container">

  <label>Gewicht (KG): </label>
  <input type="text" id="gewicht">

  <label>Lengte (Meters): </label>
  <input type="text" id="lengte">

  <input type="submit" value="Bereken mijn BMI" onclick="berekenBMI()">

</div>

<h3>
  Jouw BMI is: <span id="output"> ? </span>
</h3>

<h4>
  Je mag <span id="comment"> zoveel </span> koekjes!
</h4>

>Solution :

There are 2 problems here. The first one is rather simple, you’re just missing closing parentheses when assigning the gewicht and lengte variables to close out the Number function.

The second issue is a little less obvious to the untrained eye. In your JavaScript, you’re calling document.getElementById() twice, and trying to convert the output to a Number both times. The output of document.getElementById is an Element object. It’s trying to convert the object itself to a number, which returns NaN (Not a Number). To fix this, add .value after each document.getElementById() that you need to get a string value from. The value property contains the string value of the text box. This is true for almost every element that takes input.