<!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>
<form class="form-horizontal" action="/action_page.php">
<div class="form-group">
<label class="control-label col-sm-2" for="email">Email:</label>
<div class="col-sm-10">
<input type="email" class="form-control" id="email" placeholder="Enter email" name="email">
</div>
</div>
<div class="form-group">
<label class="control-label col-sm-2" for="pwd">Password:</label>
<div class="col-sm-10">
<input type="password" class="form-control" id="pwd" placeholder="Enter password" name="pwd">
</div>
</div>
<div class="form-group">
<div class="col-sm-offset-2 col-sm-10">
<div class="checkbox">
<label><input type="checkbox" name="remember"> Remember me</label>
</div>
</div>
</div>
<div class="form-group">
<div class="col-sm-offset-2 col-sm-10">
<button type="submit" class="btn btn-default">Submit</button>
</div>
</div>
</form>
<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; //Gecko + IE
return confirmationMessage; //Gecko + Webkit, Safari, Chrome etc.
});
</script>
</body>
</html>
