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I am trying to build a calorie calculator using HTML and JS and am currently struggling to show the output on screen (or via console.log). I know I'm doing something very basic quite wrong but can't currently pinpoint what that is.
Here's both my HTML and JS code below:
document.getElementById("bmrForm").addEventListener("submit", calcBMR);
function calcBMR(gender, weightKG, heightCM, age) {
// Calculate BMR
if (gender = 'male') {
let BMR = 10 * weightKG + 6.25 * heightCM - 5 * age + 5;
} else {
let BMR = 10 * weightKG + 6.25 * heightCM - 5 * age - 161;
}
console.log(BMR);
} <body>
<script src="./script.js"></script>
<section>
<form id="bmrForm" onsubmit="calcBMR()">
<input type="text" id="gender" placeholder="Male or female?">
<input type="number" id="weight" placeholder="Weight in KG">
<input type="number" id="height" placeholder="Height in CM">
<input type="number" id="age" placeholder="How old are you?">
<button type="submit" id="submitBtn">Do Magic!</button>
</form>
<p id="output">0</p>
</section>
</body>
310
asked Oct 18 '25 02:10
Several things need to be modified in order to achieve your desired result.
document.getElementById("bmrForm").addEventListener("submit", calcBMR); is not needed because we can pass in a function directly to the onsubmit attribute of the form element.gender, weightKG, heightCM, and age parameters are not automatically passed in to the calcBMR function. The values need to be retrieved from the document.BMR variable needs to be defined above the if/else block because of scoping.return statement needs to be added to the onsubmit attribute so that the form does not submit and refresh the page. Alternatively, if the desired effect is to update the text on the screen, a button element with a click event handler added to it may be a better option that a form with a submit handler.== or === in JavaScript. Therefore, the gender = 'male' part needs to be changed to gender === 'male'.textContent can be changed with document.getElementById("output").textContent = BMR.Below is the code with the changes listed above.
function calcBMR() {
let gender = document.getElementById("gender").value;
let weightKG = document.getElementById("weight").value;
let heightCM = document.getElementById("height").value;
let age = document.getElementById("age").value;
let BMR;
// Calculate BMR
if (gender === 'male') {
BMR = 10 * weightKG + 6.25 * heightCM - 5 * age + 5;
} else {
BMR = 10 * weightKG + 6.25 * heightCM - 5 * age - 161;
}
console.log(BMR);
document.getElementById("output").textContent = BMR;
return false;
}<body>
<script src="./script.js"></script>
<section>
<form id="bmrForm" onsubmit="return calcBMR()">
<input type="text" id="gender" placeholder="Male or female?">
<input type="number" id="weight" placeholder="Weight in KG">
<input type="number" id="height" placeholder="Height in CM">
<input type="number" id="age" placeholder="How old are you?">
<button type="submit" id="submitBtn">Do Magic!</button>
</form>
<p id="output">0</p>
</section>First, you are using a button with a type="submit", which is used to submit form data to a resource that will receive it and process it. In this case, you probably just want a button with type="button" that will only do what you've configured it to do (show the results on the screen).
After making that change, you should populate a pre-existing, but empty element with the result.
But you do have an issue with how and where you are declaring BMR. The let declaration should be outside of the if/then code but inside the function so it has scope throughout the function.
Also, your button's id is incorrect in the event handler setup.
Next, any value that you get from an HTML element will be a string and if you intend to do math with that value, you'll need to convert it to a JavaScript number. There are several ways to do this, but one shorthand way is to prepend the value with a + as you'll see I've done below.
Also, if someone were to type Male into the gender textbox, your code would not process it as a male because your code only checks for male, not Male. By forcing the input to lower case, your code will work (provided they spell male correctly). Preferably, you'd use a set of radio buttons or a drop down list for the user to choose from.
And, in conjunction with that, JavaScript uses = for assigning a value, not comparison. For loose equality (automatic type conversion) use == and for strict equality (no type conversion), use ===.
let out = document.getElementById("output");
let gender = document.getElementById("gender");
let height = document.getElementById("height");
let weight = document.getElementById("weight");
let age = document.getElementById("age");
// If you want to pass arguments to the event handler, you need to wrap the handler call in another function
document.getElementById("submitBtn").addEventListener("click", function(){calcBMR(gender.value.toLowerCase(), +weight.value, +height.value, +age.value)});
function calcBMR(gender, weightKG, heightCM, age) {
let BMR = null; // Declare the variable in the function scope
console.log(gender, weightKG, heightCM, age);
// Calculate BMR
if (gender === 'male') {
BMR = 10 * weightKG + 6.25 * heightCM - 5 * age + 5;
} else {
BMR = 10 * weightKG + 6.25 * heightCM - 5 * age - 161;
}
console.log(BMR);
output.textContent = BMR;
}<body>
<script src="./script.js"></script>
<section>
<form id="bmrForm" onsubmit="calcBMR()">
<input type="text" id="gender" placeholder="Male or female?">
<input type="number" id="weight" placeholder="Weight in KG">
<input type="number" id="height" placeholder="Height in CM">
<input type="number" id="age" placeholder="How old are you?">
<button type="button" id="submitBtn">Do Magic!</button>
</form>
<p id="output">0</p>
</section>
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