A Tip Calculator is a calculator that calculates a tip based on the percentage of the total bill.

Let's build one now.

## Step 1 - HTML:

We create a form in order to enter the preferred amount:

<!doctype html>
<html lang="en">
<title>Tip Calculator</title>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

<!-- Bootstrap CSS -->

<body class="bg-dark">
<div class="container">
<div class="row">
<div class="col-md-6 mx-auto">
<div class="card card-body text-center mt-5">
<form id="tip-form">
<div class="form-group">
<div class="input-group">
<input type="number" class="form-control" id="billTotal" placeholder="Total Bill">
</div>
</div>
<div class="form-group tipPersent">
<div class="input-group">
<label for="">Tip:</label>

<input type="range" class="form-control" id="tipInput" value="0">
</div>
</div>

</form>
<hr>

<!-- RESULTS -->
<div id="results" class="pt-4">
<h5>Results</h5>
<div class="form-group">
<div class="input-group">
<input type="number" class="form-control" id="tipAmount" disabled>
</div>
</div>

<div class="form-group">
<div class="input-group">
<input type="number" class="form-control" id="totalBillWithTip" disabled>
</div>
</div>

</div>
</div>
</div>
</div>
</div>

<!-- Optional JavaScript -->
<!-- jQuery first, then Popper.js, then Bootstrap JS -->
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js" integrity="sha384-vFJXuSJphROIrBnz7yo7oB41mKfc8JzQZiCq4NCceLEaO4IHwicKwpJf9c9IpFgh" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js" integrity="sha384-alpBpkh1PFOepccYVYDB4do5UnbKysX5WZXm3XxPqe5iKTfUKjNkCk9SaVuEZflJ" crossorigin="anonymous"></script>

</body>
</html>

## Step 2 - CSS:

You design the style however you want. You can also use CSS to hide the results and show them through JavaScript after the user fills in the form:

#results {
display:none;
}

## Step 3: JavaScript:

We add an onchange event. The onchange event occurs when the user interacts with the form.

This action will execute a function that computes the final bill amount based on the percentage tip, then returns the results.

document.querySelector('#tip-form').onchange = function(){

var bill = Number(document.getElementById('billTotal').value);
var tip = document.getElementById('tipInput').value;
document.getElementById('tipOutput').innerHTML = `\${tip}%`;
var tipValue = bill * (tip/100)
var finalBill = bill + tipValue
console.log(finalBill)
var tipAmount = document.querySelector('#tipAmount')
var totalBillWithTip = document.querySelector('#totalBillWithTip')

tipAmount.value = tipValue.toFixed(2);
totalBillWithTip.value =finalBill.toFixed(2);

//Show Results

document.getElementById('results').style.display='block'
}

You can see a working example and its code on Codepen.io.