How to display warning before leaving the web page with unsaved changes using JavaScript the following code will help you make before leaving the web page with unsaved changes.
#index.php
<!Doctype html>
<html>
<head>
<title>Warning before leave the page Javascript</title>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
</head>
<body>
<div class="container">
<div class="row">
<div class="col-md-4"></div>
<div class="col-md-4" style="margin-top: 20px;">
<form class="form-horizontal" action="thepage_where_to_save.php" method="post">
<div class="form-group">
<label class="control-label" for="email">Email:</label>
<input type="email" class="form-control" id="email" placeholder="Enter email" name="email">
</div>
<div class="form-group">
<label class="control-label" for="pwd">Mobile:</label><br>
<input type="text" class="form-control" id="pwd" placeholder="Enter password" name="pwd">
</div>
<div class="form-group">
<button type="submit" class="btn btn-default">Save</button>
</div>
</form>
</div>
<div class="col-md-4"></div>
</div>
</div>
<script>
window.addEventListener("beforeunload", function(e) {
var confirmationMessage = 'It looks like you have been editing something. ' + 'If you leave before saving, your changes will be lost.';
(e || window.event).returnValue = confirmationMessage;
});
</script>
</body>
</html>Run the following code and see the results