How to build a BMI Calculator App using BMI Calculator API?

Today, I will build a BMI calculator app that will take the user’s weight and height and calculate his BMI. So without any further ado, let’s jump in!

Stack

We need to choose a stack first to build this application. I have decided to go with the Jamstack.

I am going to use Next.js for the frontend and TailwindCSS for the styling.

If you don’t know about Next.js, it is a JavaScript framework built on top of React and provides features like server-side rendering, static site generation, etc. Tailwind is a CSS framework that provides utility classes to speed up the development process.

Choosing The API

Let’s find an API that we can use to find the BMI information. Go to RapidAPI Hub and create an account if you haven’t already and then search for “bmi api” in the search section.

You will see different search results related to the BMI API. For this piece, I am using Body Mass Index (BMI) Calculator API.

To use this API, you need to subscribe to it first. You can do this by clicking on Subscribe to Test button.

Subscribe to Body Mass Index (BMI) Calculator API

Once you click the button, you will be redirected to another page where different available subscription packages will be shown. Let’s go with the free one for now.

After all this, you will be redirected back to the original page. Here you will have a key x-rapidapi-key. Save it somewhere. It will be used later in the application.

Building The UI

You can create a Next.js boilerplate with TailwindCSS integrated by running the following command in your terminal. So let’s do that.

sh
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npx create-next-app -e with-tailwindcss bmi-calculator-app

This command is going to take a minute to set everything up. After generating the boilerplate, you will see a folder with the name bmi-calculator-app has been created. Open this folder in your preferred code editor. I will use VSCode for this project.

Project Files

When you open the project in your code editor, you will see the following directories and files in the root directory:

• pages directory: Inside it, you will have files index.js, _app.js, and another directory called - api. You only need to know about the index.js file that is the main entry point in your project.
• public directory: This directory contains icons. You place your static files here to load later in the application.
• node_modules: It’s another directory that contains all the node modules you are using in your application.
• package.json: This file contains the metadata of your project.
• package-lock.json: This file is responsible for tracking the exact version of every installed package.
• postcss.config.js: This file contains PostCSS configurations.
• tailwind.config.js: It contains TailwindCSS configurations.
• readme.md: It’s a markdown file for documentation.

Before we move on to writing the code, open this file, and copy all of its content, then paste it inside the tailwind.config.js file in your project. These are some TailwindCSS configurations I have done specifically for this project. I have added some colors that you do not have by default with TailwindCSS and set some screen sizes.

Now let’s start writing the code. I am going to do it in steps so you can follow along:

→ STEP #1

Open the pages/index.js file and remove all the existing code. After this, copy-paste the following code there:

jsx
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export default function Home() {
	return (
		<div className="flex flex-col items-center">
			<h2 className="font-raleway font-bold text-6xl text-primary pt-20 pb-6 md:text-3xl">
				BMI <span className="text-danger">Calculator</span> App
			</h2>
			<h3 className="text-secondary text-2xl font-raleway font-bold uppercase tracking-wide mb-14 md:text-base md:px-4 md:text-center">
				Calculate your BMI using weight and height
			</h3>
		</div>
	);
}

It will create two headings for you with the text “BMI Calculator App” and “Calculate your BMI using weight and height”. You can change it to anything you prefer.

→ STEP #2

Now let’s create two input fields and a calculate button. The user will be able to type his weight and height in the input boxes and use the calculate button to get the BMI information.

For this, copy the following code and paste it into pages/index.js:

jsx
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export default function Home() {
	return (
		<div className="flex flex-col items-center">
			<h2 className="font-raleway font-bold text-6xl text-primary pt-20 pb-6 md:text-3xl">
				BMI <span className="text-danger">Calculator</span> App
			</h2>
			<h3 className="text-secondary text-2xl font-raleway font-bold uppercase tracking-wide mb-14 md:text-base md:px-4 md:text-center">
				Calculate your BMI using weight and height
			</h3>
			<div className="flex flex-col justify-between items-center w-full md:items-center">
				<form className="flex w-full justify-center md:flex-col md:w-5/6">
					<input
						autoFocus={true}
						className="border-none outline-none bg-primary px-4 py-2 w-1/6 mx-2 rounded-sm font-raleway md:w-full md:mx-0 md:my-4"
						placeholder="Enter your weight (in kgs)..."
					/>
					<input
						className="border-none outline-none bg-primary px-4 py-2 w-1/6 mx-2 rounded-sm font-raleway md:w-full md:mx-0"
						placeholder="Enter your height (in meters)..."
					/>
					<button className="outline-none border border-danger font-bold font-raleway ml-4 px-12 py-2 rounded-sm bg-danger text-lightGrey transition duration-300 hover:bg-bc hover:text-primary md:ml-0 md:mt-4">
						Calculate
					</button>
				</form>
			</div>
		</div>
	);
}

This code is going to create an input field and button. I have also styled them a little bit using TailwindCSS.

→ STEP #3

Let’s create some states to store the weight, height, and BMI information that we will receive from the API. For this, copy-paste the following code in pages/index.js:

jsx
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import {useState} from 'react';
export default function Home() {
	const [weight, setWeight] = useState('80');
	const [height, setHeight] = useState('1.80');
	const [bmi, setBmi] = useState(null);
	return (
		<div className="flex flex-col items-center">
			<h2 className="font-raleway font-bold text-6xl text-primary pt-20 pb-6 md:text-3xl">
				BMI <span className="text-danger">Calculator</span> App
			</h2>
			<h3 className="text-secondary text-2xl font-raleway font-bold uppercase tracking-wide mb-14 md:text-base md:px-4 md:text-center">
				Calculate your BMI using weight and height
			</h3>
			<div className="flex flex-col justify-between items-center w-full md:items-center">
				<form className="flex w-full justify-center md:flex-col md:w-5/6">
					<input
						autoFocus={true}
						className="border-none outline-none bg-primary px-4 py-2 w-1/6 mx-2 rounded-sm font-raleway md:w-full md:mx-0 md:my-4"
						placeholder="Enter your weight (in kgs)..."
						value={weight}
						onChange={e => setWeight(e.target.value)}
					/>
					<input
						className="border-none outline-none bg-primary px-4 py-2 w-1/6 mx-2 rounded-sm font-raleway md:w-full md:mx-0"
						placeholder="Enter your height (in meters)..."
						value={height}
						onChange={e => setHeight(e.target.value)}
					/>
					<button className="outline-none border border-danger font-bold font-raleway ml-4 px-12 py-2 rounded-sm bg-danger text-lightGrey transition duration-300 hover:bg-bc hover:text-primary md:ml-0 md:mt-4">
						Calculate
					</button>
				</form>
			</div>
		</div>
	);
}

You can see that I have added an onChange event handler to set the state value as soon as the user types something in the input box.

→ STEP #4

Let’s integrate the API now. For this, first, create a .env.local file and paste the following in it:

sh
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NEXT_PUBLIC_RAPIDAPI_KEY=YOUR-RAPIDAPI-KEY

You need to replace YOUR-RAPIDAPI-KEY here with the API key you got when you subscribed to the Body Mass Index (BMI) Calculator API. It is the value of x-rapidapi-key that you saved earlier.

Now download and add axios in your project. For this, run the following command in the terminal:

sh
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npm install axios

Now import axios at the top of the pages/index.js.

js
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import axios from 'axios';

→ STEP #5

RapidAPI Hub provides you with code snippets in different languages for integrating the API. I am going to use the (JavaScript) Axios one.

Fetching data using (JavaScript) Axios

Now create a file with the name bmi.js inside the pages/api directory. It is going to create a REST API endpoint for you. The endpoint point will look like this:

sh
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http://localhost:3000/api/bmi

Now copy the following code in this file:

js
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import axios from 'axios';
export default function handler(req, res) {
	if (req.method === 'GET') {
		const weight = req.query.weight || 0;
		const height = req.query.height || 0;
		const options = {
			method: 'GET',
			url: 'https://body-mass-index-bmi-calculator.p.rapidapi.com/metric',
			params: {weight, height},
			headers: {
				'X-RapidAPI-Host':
					'body-mass-index-bmi-calculator.p.rapidapi.com',
				'X-RapidAPI-Key': process.env.NEXT_PUBLIC_RAPIDAPI_KEY
			}
		};
		axios
			.request(options)
			.then(function (response) {
				res.status(200).json(response.data);
			})
			.catch(function (error) {
				console.error(error);
			});
	} else {
		res.status(400);
	}
}

This code makes an API call to the Body Mass Index (BMI) Calculator API on the server and returns the results to the user. It executes when the user makes an API call to the bmi endpoint I mentioned above.

Once you are done, copy the following code in the pages/index.js file:

jsx
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import {useState} from 'react';
import axios from 'axios';
export default function Home() {
	const [weight, setWeight] = useState('80');
	const [height, setHeight] = useState('1.80');
	const [btnText, setBtnText] = useState('Calculate');
	const [bmi, setBmi] = useState(null);
	/**
	 *
	 *
	 * Fetch BMI
	 */
	const fetchBMI = async e => {
		e.preventDefault();
		try {
			setBtnText('Calculating...');
			const response = await axios.get(`/api/bmi`, {
				params: {
					weight,
					height
				}
			});
			setBmi(response.data.bmi);
		} catch (err) {
			console.log(err);
		}
		setBtnText('Calculate');
	};
	return (
		<div className="flex flex-col items-center">
			<h2 className="font-raleway font-bold text-6xl text-primary pt-20 pb-6 md:text-3xl">
				BMI <span className="text-danger">Calculator</span> App
			</h2>
			<h3 className="text-secondary text-2xl font-raleway font-bold uppercase tracking-wide mb-14 md:text-base md:px-4 md:text-center">
				Calculate your BMI using weight and height
			</h3>
			<div className="flex flex-col justify-between items-center w-full md:items-center">
				<form className="flex w-full justify-center md:flex-col md:w-5/6">
					<input
						autoFocus={true}
						className="border-none outline-none bg-primary px-4 py-2 w-1/6 mx-2 rounded-sm font-raleway md:w-full md:mx-0 md:my-4"
						placeholder="Enter your weight (in kgs)..."
						value={weight}
						onChange={e => setWeight(e.target.value)}
					/>
					<input
						className="border-none outline-none bg-primary px-4 py-2 w-1/6 mx-2 rounded-sm font-raleway md:w-full md:mx-0"
						placeholder="Enter your height (in meters)..."
						value={height}
						onChange={e => setHeight(e.target.value)}
					/>
					<button
						className="outline-none border border-danger font-bold font-raleway ml-4 px-12 py-2 rounded-sm bg-danger text-lightGrey transition duration-300 hover:bg-bc hover:text-primary md:ml-0 md:mt-4"
						onClick={fetchBMI}
					>
						{btnText}
					</button>
				</form>
				{bmi && (
					<div className="border border-secondary text-secondary mt-16 md:w-4/5">
						<p className="px-4 py-4 tracking-wide leading-8">{`Your Body Mass Index: ${bmi}`}</p>
					</div>
				)}
			</div>
			<div className="flex flex-col mt-10 justify-end h-36 md:h-24">
				<p className="block mb-10 text-center text-primary text-xs">
					Made by RapidAPI DevRel Team -{' '}
					<a
						className="hover:text-danger"
						href="https://github.com/RapidAPI/DevRel-Examples-External"
					>
						See more examples like this
					</a>
				</p>
			</div>
		</div>
	);
}

You can see that I have created a function, fetchBMI, to request the API. I have also created a UI that is conditionally rendering on the screen based on the bmi state variable value.

Wrap Up

That’s it. We have successfully built a BMI Calculator App using Body Mass Index (BMI) Calculator API. You can find the source code of this web app here.

In the end, it will look something like this:

BMI Calculator App built with Next.js and Body Mass Index (BMI) Calculator API