<!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>