Javascript on leaving page

How to control browser confirmation dialog on leaving page in Javascript?

When a user leaves a web page, the browser may display a confirmation dialog asking the user if they are sure they want to leave the page. In some cases, developers may want to control this dialog and either display it or prevent it from being shown. In JavaScript, there are several methods to control the browser’s confirmation dialog when leaving a page.

Method 1: Event listener for beforeunload

How to Control Browser Confirmation Dialog on Leaving Page with Event Listener for beforeunload in Javascript

To control the browser confirmation dialog on leaving page, you can use the beforeunload event listener in Javascript. This event is triggered when the user is about to leave the page, either by closing the tab/window or by navigating to a different URL.

Here is an example code snippet that demonstrates how to use the beforeunload event listener to show a confirmation dialog to the user:

window.addEventListener('beforeunload', function(event)  // Cancel the event as long as the user hasn't confirmed event.preventDefault(); // Prompt the user with a confirmation dialog event.returnValue = ''; >);

In the above code, we are adding a beforeunload event listener to the window object. When the event is triggered, we are preventing the default behavior by calling event.preventDefault() . This effectively cancels the event, preventing the user from leaving the page.

Next, we are prompting the user with a confirmation dialog by setting event.returnValue to an empty string. This will display a default confirmation message to the user, asking them if they really want to leave the page.

You can customize the confirmation message by setting event.returnValue to a string value. For example:

window.addEventListener('beforeunload', function(event)  // Cancel the event as long as the user hasn't confirmed event.preventDefault(); // Prompt the user with a custom confirmation message event.returnValue = 'Are you sure you want to leave this page?'; >);

In the above code, we are setting event.returnValue to a custom message, asking the user if they really want to leave the page.

It’s worth noting that some browsers may not allow you to customize the confirmation message. In these cases, the default message will be displayed instead.

In summary, to control the browser confirmation dialog on leaving page with event listener for beforeunload in Javascript, you can add a beforeunload event listener to the window object and customize the confirmation message by setting event.returnValue to a string value.

Method 2: Return a string from a beforeunload event handler

To control the browser confirmation dialog on leaving a page using the beforeunload event handler in JavaScript, you can return a string from the handler function. This string will be displayed in the confirmation dialog, allowing the user to choose whether to stay on the page or leave.

Here’s an example code snippet that demonstrates this approach:

window.addEventListener('beforeunload', function (event)  // Cancel the event as otherwise the browser will show the confirmation dialog event.preventDefault(); // Chrome requires returnValue to be set event.returnValue = ''; // Return a string to display in the confirmation dialog return 'Are you sure you want to leave?'; >);

In this example, we’re using the addEventListener method to attach a beforeunload event handler to the window object. When the user tries to leave the page, the handler function will be called.

Inside the handler function, we’re preventing the default behavior of the event using event.preventDefault() . This is necessary to prevent the browser from showing the confirmation dialog automatically.

We’re also setting the returnValue property of the event to an empty string, which is required by some browsers (such as Chrome).

Finally, we’re returning a string from the handler function, which will be displayed in the confirmation dialog. In this case, we’re asking the user if they’re sure they want to leave the page.

You can customize the string to display whatever message you want in the confirmation dialog.

That’s it! With this approach, you can control the browser confirmation dialog on leaving a page using the beforeunload event handler in JavaScript.

Method 3: Prevent the default behavior of the beforeunload event

To prevent the default behavior of the beforeunload event and control the browser confirmation dialog on leaving the page, you can use the following code:

window.addEventListener('beforeunload', function (event)  // Cancel the event event.preventDefault(); // Chrome requires returnValue to be set event.returnValue = ''; >);

This code adds an event listener to the beforeunload event of the window object. When the event is triggered, it prevents the default behavior of the event, which is to show the browser confirmation dialog. Then it sets the returnValue property of the event object to an empty string, which is required by some browsers (e.g. Chrome) to show the confirmation dialog.

You can also add a custom message to the confirmation dialog by setting the returnValue property to a string message:

window.addEventListener('beforeunload', function (event)  // Cancel the event event.preventDefault(); // Set the custom message event.returnValue = 'Are you sure you want to leave?'; >);

This code sets the returnValue property to the string message «Are you sure you want to leave?», which will be displayed in the confirmation dialog.

If you want to conditionally show the confirmation dialog based on some condition, you can use an if statement inside the event listener:

window.addEventListener('beforeunload', function (event)  // Check some condition if (someCondition)  // Cancel the event event.preventDefault(); // Set the custom message event.returnValue = 'Are you sure you want to leave?'; > >);

This code checks the someCondition variable and only shows the confirmation dialog if the condition is true.

In summary, to control the browser confirmation dialog on leaving the page in JavaScript, you can add an event listener to the beforeunload event, prevent the default behavior of the event, and set the returnValue property of the event object to control the message displayed in the confirmation dialog.

Method 4: Use window.onbeforeunload

You can use the window.onbeforeunload event to control the browser confirmation dialog that appears when the user tries to leave the page. Here’s how you can use it:

function confirmExit()  return "Are you sure you want to leave this page?"; >

window.onbeforeunload = confirmExit;

window.onbeforeunload = null;

Here’s a complete example that demonstrates how to use window.onbeforeunload :

function confirmExit()  return "Are you sure you want to leave this page?"; > window.onbeforeunload = confirmExit; // Remove the event listener // window.onbeforeunload = null;

Note that the confirmation dialog message may not be customizable in all browsers. Also, be careful not to abuse this feature as it can be annoying for users if overused.

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Top 3 ways to easily detect when a user leaves a page using JavaScript

This article was originally published on webinuse.com There are times when we need to detect when a user leaves a web page or clicks on a link. Sometimes we use this data to change something on the website, sometimes to send analytics and sometimes we just need to know this because of something else. In this article, we are going to cover the top 3 easiest ways how we can detect when a user leaves the page. Unfortunately, each and every one of these ways has some flaws, but also it has its application to some situations.

1. Detect exit before page start unloading

As per MDN: The beforeunload event is fired when the window, the document, and its resources are about to be unloaded. The document is still visible and the event is still cancelable at this point. This means that when we click on a link, or we want to close the tab, or browser, or anything that would remove our current link from the browser, one moment before the browser starts doing that action will fire beforeunload event. Let’s say we clicked some link. One moment before the browser starts opening this new link, it will fire the event. This event can be used to send some data to the back end, or to change something in our localStorage, or whatever we need to do.

const beforeUnloadListener = (event) =>  //Send something to back end fetch('uri') >; window.addEventListener("beforeunload", beforeUnloadListener); 

We need to keep in mind though that this event is not reliably fired. There are some issues with it. It does not always detect when a user leaves a web page, especially on mobile devices. For more info on issues and other data, we can visit MDN.

2. Detect when a user leaves a web page using pagehide event

Similar to beforeunload event, pagehide is fired when a user is leaving the current page, e.g. clicking the back button. But, according to MDN, same as beforeunload the pagehide event is not always reliably fired.

const pageHideListener = (event) =>  //Send something to back end fetch('uri') >; window.addEventListener("pagehide", pageHideListener); 

3. visiblitychange event

The most reliable way to detect when a user leaves a web page is to use visiblitychange event. This event will fire to even the slightest changes like changing tabs, minimizing the browser, switching from browser to another app on mobile, etc. More info about this event can be found on MDN. This is an excellent way to send some analytics to our back end and is shown in the example below.

document.onvisibilitychange = function()  if (document.visibilityState === 'hidden')  navigator.sendBeacon('/log', analyticsData); > >; 

NOTE: This event is excellent for sending analytics, and similar, data to the back end, but if we want to send information if a user still has opened a certain page we should use either the first two events or we should ping user every few seconds. If you have any questions or anything you can find me on my Twitter, or you can read some of my other articles like Everything we need to know about CSS Borders.

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How To Create A Simple Alert For An onbeforeunload Event (Closing Browser or Navigating Away From Page) [updated]

edit: credit to Alex Lohr for suggesting to put the code inside a useEffect 🙂 Here is a useful little snippet for if you have a page like a checkout or a survey and you want to warn people about leaving the page before they’ve completed it.

 window.onbeforeunload = (event) => < if (whatever your condition is here) < return true; >>; // inside a useEffect: useEffect(() => < if (conditionState) < const listener = () =>true window.addEventListener('beforeunload', listener) return () => window.removeEventListener('beforeunload', listener) > >, [conditionState]) // the return here removes the eventListener, until next time the useEffect runs again 

The alert gives the user the option to confirm or cancel their navigation. Alerts should never stop someone from leaving a page or closing their browser though; that’s a big no-no. An onbeforeunload event occurs when the document (the current page of your site/application) is about to be unloaded (closed/exited). A example of this would be someone closing the browser window or clicking a link to a different page. Most browsers will show a default message like «changes you’ve made will not be saved». You can’t edit this message, but it does the job as a warning. Due to this, you don’t need to return any specific content or message in the function, just return true in order for it to work. This function will be called when an onbeforeunload event occurs. If the condition inside is met, then the function will return true and the alert will be shown. So all you need to do is apply a condition to decide when the alert will be shown. For example, if using this in a React application, then the condition could be based on a certain state. There are other ways of achieving this sort of behaviour, such as using Prompt in react-router, but if you’re just looking for a simple solution then I hope this helps! If you have anything to add, please leave a comment.

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