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How to check if an alert is present on a page with JavaScript or Selenium

I am making a bot, and this bot is searching constantly but some times when it searches, an alert appears asking the user for access to their location. How do i check if this alert is present or better yet disable it?
So far i have tried to disable stuff through about:config:
var prefs = Components.classes["#mozilla.org/preferences-
service;1"].getService(Components.interfaces.nsIPrefBranch);
prefs.setIntPref("network.cookie.cookieBehavior", 4);
prefs.setBoolPref("privacy.donottrackheader.enabled", true);
prefs.setIntPref("permissions.default.geo", 2);
But alerts still pop up. I also tried making a new firefox profile with pre installed add-ons just to block dialog boxes and pop ups, but it still gives me dialog boxes.
I really am stumped this time. If you know how i can check if a page is alerting something with JS or Selenium python then please tell me cause at this point, that would help me a lot. If you know how to disable javascript alerts then that would be awesome.

You can disable alerts with:
window.alert = () => false
from python that's:
driver.execute_script("window.alert = () => false")

Use :
prefs.setIntPref("permissions.default.desktop-notification", 1);

Assume one is present on the exception of a non located element within a try/catch block, then try to use
driver.switchTo().alert().dismiss();
to proceed with the entended behavior :)

Javascript confirm() still executes action when cancel is clicked. :(

<script type="text/javascript">
confirm("Delete user?.");
window.location.href = "users.php";
</script>
$qqq = mysql_query("DELETE from users WHERE panelistname='$theuser'") or die(mysql_error())
considering the code above, (inside a php file, so no worries with certain syntax errors you might notice) the problem here is that when click cancel on the confirm() dialog box that will show up. the delete action still executes. This question might be considered a double since, yeah, I found some questions relevant to this one but I just can't fixed this one myself.
the one I found codes it something like this:
"if (confirm('Are you...?')) commentDelete(1); return false"
I can't find a way to solve this problem, I don't know which part should I insert the SQL command(delete) in this format. Please someone show me how to do this right. :) thanks!

EDIT: I just saw that Nick Zuber posted a similar answer around 1 minute before I posted mine (actually, while I was writing it :P)
I don't clearly understand what you are trying to do.
You want to show the user a confirm window, and if they click Yes, delete some entry in the database, and if they click No, redirect them to the page 'users.php' ?
If it's what you want to do, then you can't do it like this. You can't use JS conditions with PHP. The PHP code is executed server-side (in the server), whereas the JS code is executed client-side (in the browser). What you would need is to do something like this:
warning: don't use this code, it's unsecure and shouldn't ever be used in a real app, it's just to show you how the whole thing works
(IN USERS.PHP)
if(isset($_GET['delete_dat_user']))
{
$qqq = mysql_query("DELETE from users WHERE panelistname='" . $_GET['delete_dat_user'] . "'") or die(mysql_error());
}
(IN THE WEBPAGE)
if(confirm('u serious u want to delete the user'))
{
window.location = 'users.php?delete_dat_user=theUserName';
}
else
{
nope
}

When your page loads, the PHP on your page will automatically execute, regardless of your JavaScript. Instead, try to prompt the user if they want to delete the account and if they click yes redirect them to a page that has your PHP query.
Also, the confirm function returns a boolean value depending on which option is clicked by the user. Try putting it in an if statement:
if(confirm("Delete user?.")){
window.location.href = "delete_user_page.php";
}else{
// cancel was clicked
}

How to disable browser feature

I am developing a project where user gets a conformation page. I want user not to click back or close tab or reload.
Now either I need to disable the browser features or get back button,tab close event, or reload event to java script so that I could take the needed steps to prevent my data to get lost.
I have used this:
window.onbeforeunload = function()
{
return "Try This";
};
But this get called even when I click a button that redirects the page.

If you just want to have the alert, understanding that the user is ultimately in control and can bypass your alert, then do what you're doing but use a flag that disables it when you're navigating and don't want the alert. E.g.:
var warnWhenLeaving = true;
window.onbeforeunload = function() {
if (warnWhenLeaving) {
return "your message here";
}
};
then in a click handler on the link/button/whatever that moves the user on that you don't want this to pop up on:
warnWhenLeaving = false;
In a comment you asked:
can i know that what user has clicked when alert is generated with this function. That is can i know what user has clicked (leave this page/stay on page)
The answer is: Sort of, but not really; you're almost certainly better off not trying to.
But: If you see your onbeforeunload function run, then you know the user is leaving the page and the browser is likely to show them your message. The browsers I'm familiar with handle the popup like an alert: All JavaScript code on the page is blocked while the popup is there. So if you schedule a callback via setTimeout, you won't get the callback if they leave and you will if they stay:
window.onbeforeunload = function() {
if (warnWhenLeaving) {
setTimeout(function() {
display("You stayed, yay!");
}, 0);
return "No, don't go!";
}
};
Live Example
So in theory, if you get the callback, they stayed; if you see an unload event, they left. (Note that there are very few things you can do in an unload event.)
I've tried that on current Chrome, current Firefox, IE8, and IE11: It works on all of those. Whether it will work in the next release of any of them is anybody's guess. Whether it works reliably on mobile browsers is something you'd have to test, and again could change.

Gmail-style Exit Message

I realize this is likely a duplicate, but I've been googling/SOing for a day now and I can't find a satisfactory answer. If there is an answer already on SO, please send me there.
I have a client that insists on having an exit message popup confirming they want to exit the site, just like Gmail does. (I've already tried arguing against it. He is immovable, so no comments about how that is bad practice please.)
I've found this code:
<script>
window.onbeforeunload = function() {
return 'Are you sure you want to exit?';
}
<script>
But it runs no matter what I do - reloading the page, clicking on the nav, etc.
I just want the message to show up when the user closes the tab/browser. I suspect it's something simple I'm missing but I'm not a Javascript expert.
Any help would be greatly appreciated.
Thanks
EDIT
Here's what is working pretty good. Thanks to all!
var isLeavingSite = true;
//This would be called on each link/button click that navigates
$('a, input[type="submit"]').click(function(){
isLeavingSite = false;
});
window.onbeforeunload = function() {
if(isLeavingSite)
return 'Are you sure you want to exit?';
}

Though it could be a fair amount of work (depending on how your site is written), you could do something like this (pseudo-code):
var isLeavingSite = true;
//This would be called on each link/button click that navigates
function GlobalLinkHandler()
{
isLeavingSite = false;
}
window.onbeforeunload = function() {
if(isLeavingSite)
return 'Are you sure you want to exit?';
}
If you're using jQuery, you can use the code below to flip the isLeavingSite flag:
$('a, input[type="submit"]').click(function(){ isLeavingSite = false; });

What'll have to do is make use a variable that you set if any link is clicked on the site, then inside the onbeforeunload event check if that variable is set meaning they clicked a link or not set meaning they're closing the tab.
You can also use that variable to simple set the href of the link; that will allow you to then check what link they clicked on inside the onbeforeunload event and allow you to check if they're clicking on a link to go to another page on your site or clicking on an external link to another site.

If your using jQuery try this Confirm before exit

How to block users from closing a window in Javascript?

Is it possible to block users from closing the window using the exit button [X]? I am actually providing a close button in the page for the users to close the window.Basically what I'm trying to do is to force the users to fill the form and submit it. I don't want them to close the window till they have submitted it.
I really appreciate your comments, I'm not thinking of hosting on any commercial website. Its an internal thing, we are actually getting all the staff to participate in this survey we have designed....
I know its not the right way but I was wondering if there was a solution to the problem we have got here...

Take a look at onBeforeUnload.
It wont force someone to stay but it will prompt them asking them whether they really want to leave, which is probably the best cross browser solution you can manage. (Similar to this site if you attempt to leave mid-answer.)
<script language="JavaScript">
window.onbeforeunload = confirmExit;
function confirmExit() {
return "You have attempted to leave this page. Are you sure?";
}
</script>
Edit: Most browsers no longer allow a custom message for onbeforeunload.
See this bug report from the 18th of February, 2016.
onbeforeunload dialogs are used for two things on the Modern Web:
Preventing users from inadvertently losing data.
Scamming users.
In an attempt to restrict their use for the latter while not stopping the former, we are going to not display the string provided by the webpage. Instead, we are going to use a generic string.
Firefox already does this[...]

If you don't want to display popup for all event you can add conditions like
window.onbeforeunload = confirmExit;
function confirmExit() {
if (isAnyTaskInProgress) {
return "Some task is in progress. Are you sure, you want to close?";
}
}
This works fine for me

What will you do when a user hits ALT + F4 or closes it from Task Manager
Why don't you keep track if they did not complete it in a cookie or the DB and when they visit next time just bring the same screen back...:BTW..you haven't finished filling this form out..."
Of course if you were around before the dotcom bust you would remember porn storms, where if you closed 1 window 15 others would open..so yes there is code that will detect a window closing but if you hit ALT + F4 twice it will close the child and the parent (if it was a popup)

This will pop a dialog asking the user if he really wants to close or stay, with a message.
var message = "You have not filled out the form.";
window.onbeforeunload = function(event) {
var e = e || window.event;
if (e) {
e.returnValue = message;
}
return message;
};
You can then unset it before the form gets submitted or something else with
window.onbeforeunload = null;
Keep in mind that this is extremely annoying. If you are trying to force your users to fill out a form that they don't want to fill out, then you will fail: they will find a way to close the window and never come back to your mean website.

How about that?
function internalHandler(e) {
e.preventDefault(); // required in some browsers
e.returnValue = ""; // required in some browsers
return "Custom message to show to the user"; // only works in old browsers
}
if (window.addEventListener) {
window.addEventListener('beforeunload', internalHandler, true);
} else if (window.attachEvent) {
window.attachEvent('onbeforeunload', internalHandler);
}

If your sending out an internal survey that requires 100% participation from your company's employees, then a better route would be to just have the form keep track of the responders ID/Username/email etc. Every few days or so just send a nice little email reminder to those in your organization to complete the survey...you could probably even automate this.

It's poor practice to force the user to do something they don't necessarily want to do. You can't ever really prevent them from closing the browser.
You can achieve a similar effect, though, by making a div on your current web page to layer over top the rest of your controls so your form is the only thing accessible.

Well you can use the window.onclose event and return false in the event handler.
function closedWin() {
confirm("close ?");
return false; /* which will not allow to close the window */
}
if(window.addEventListener) {
window.addEventListener("close", closedWin, false);
}
window.onclose = closedWin;
Code was taken from this site.
In the other hand, if they force the closing (by using task manager or something in those lines) you cannot do anything about it.