Develop Reference

JS, How can we open the desktop app from browser, like Zoom or Webex does

We all use Zoom or Webex to attend the meetings, then there is a popup that allows you to open the desktop app or you can continue on the web app. I want to implement a similar kind of user experience in my web application. But unable to understand how did they(zoom and Webex) did it.
I am not looking for a foolproof solution I just need to know the best approach to achieve it.
Update:
I am not doing it exactly like Zoom or Webex does. I have a button in my application so I am doing it onClick event.

In order to do this, you must register a protocol on your OS (ie. you can use protocol lib to register a specific one in an Electron app), this would allow your app to be called with simple URIs like myappprotocol://myappaction?myappparams

They make you install a client, that has caused many controversies since this is a gate for eventual security breaches
https://medium.com/bugbountywriteup/zoom-zero-day-4-million-webcams-maybe-an-rce-just-get-them-to-visit-your-website-ac75c83f4ef5

Check if app is installed from webpage

I have an iOS and Android app and I'm building a corresponding website. I would like that the webpage, if opened using a mobile device, opens the app or its corresponding app store page (without using Facebook app links).
On the app side everything is working, including the url schema.
Does someone know how to implement this procedure, without external services, using HTML and JS?
Thanks in advance for your help.

To be honest, this is kind of a pain to implement on your own. There is no easy way to handle everything without a ton of nasty edge cases, most notably the 'Cannot Open Page" error users will see if they don't have your app installed. Until iOS 9, a reasonable basic implementation was putting a JavaScript redirect like this into a dedicated redirect page on your site:
setTimeout(function() {
window.location = "https://yourdomain.com";
}, 25);
// If "yourapp://" is registered, the user will see a dialog
// asking if they want to open your app. If they agree, your
// app will launch immediately and the timer won't fire.
// If not installed, you'll get an ugly "Cannot Open Page"
// dialogue and your fallback page will open when the timer expires.
window.location = "yourapp://";
Unfortunately this would still show a 'Cannot Open Page' error, but until recently it was possible to get around this in a reasonably user-friendly way by using a more nuanced version of this script. Sadly, Apple intentionally broke that with the iOS 9.2 update, so custom URL schemes are actually pretty much useless for deep linking now, unless you are certain the app is already installed on that device.
Apple is obviously trying to push the newer Universal Links standard as much as possible. Universal Links lets you use a normal http:// URL to a page on your website (the page could be a simple redirection to your desired fallback webpage without the custom URL trigger that causes the 'Cannot Open Page' error), which is intercepted by your phone and sent directly into your app if installed.
This is quite a lot to handle, so the best option might be a free service like Branch.io (full disclosure: I work with the team) to take care of all the technical aspects. You can find examples of apps using the Branch service here.

Check if app is existing on device

I need to check if app is installed on iPhone or android device, using javascript code only. As i am working on html website.
I have used
window.location="appname://";
but it makes me to open an app.
Thanks in advance.

As stated in the comments there are security and privacy reasons surrounding why you are not able to do what you are asking.
However, #CommonsWare, has already given a workaround to this problem, in a similar question, that has already been asked:
Fortunately, this is not possible, for obvious privacy reasons.
The closest that you can do is in the application, have an activity
that has an for some URL structure, and have a link in
the mobile Web site to a matching URL.
If the user clicks the link and the app is installed, the activity
will be a chooser option for the user.
If the user clicks the link and the app is not installed, or they
choose to stick with their Web browser from the chooser, whatever Web
page exists at that URL will be displayed (E.g., instructions of how
to download the app).

How to detect that User has added webapp to the homescreen on mobile device?

I am working on a webapp. I want to count the number of Users that added my webapp to home screen.
suppose my website is.
http://www.example.com
I have added meta tags that allow Mobile users to add it to home screen. I want a count of users that downloaded my webapp. If User has downloaded even if he has not opened that then also I should get notification that User has added it to home screen.
So I need any event that should work when an Webapp is added to home screen.
My webapp is in HTML5

First of all, officially it's not possible, as per the official FAQ:
How can I detect if the app is running as an installed app?
You can’t, directly.
which is again re-iterated
Best practices
Do not prompt the user to add your app to the homescreen. There is no way to detect if the app is running installed or not.
Source: https://developer.chrome.com/multidevice/android/installtohomescreen
You could do some manual checking of the screen sizes though on page load, as chrome can't go fullscreen by hand, but do not rely on this (though for statistical purposes it might be interesting).
Another clarification which might be useful, the application is not downloaded when it's added to the homescreen. All that happens is that a 'link' will be created to chrome with certain parameters. By design little information is disclosed to developers regarding this process to prevent companies from forcing users to 'install' webapps before they would function.

As of 2018, according to https://developers.google.com/web/fundamentals/app-install-banners/:
To determine if the app was successfully added to the user's home screen after they accepted the prompt, you can listen for the appinstalled event.
window.addEventListener('appinstalled', (evt) => {
app.logEvent('a2hs', 'installed');
});

Website needs button for Blackberry users to add event to their calendar

I am showing a list of events in a calendar-like display on our website, and would like visitors to be able to click a link/button to add a certain date to their own computer/PDA/phone calendar.
For Outlook and iCal I return an ICS file to the user. For Google Calendar there is a simple URL which does it. I would like to cater for Blackberry users too, but am unsure what format they need, and whether their browser supports such actions.
Googling for answers has confused me as to whether Blackberrys can accept ICS files or need VCS? I have never touched a Blackberry either, so am generally very ignorant of all aspects!
Feel free to shout about Android to me too.
Thank you.

Seems like you are searching for blackberry.launch.newAppointment().
FTA:
The newAppointment() method creates a new appointment in the calendar application. The user must save the appointment before it is saved to the calendar.
Syntax
blackberry.launch.newAppointment({ summary, location, allDay, start, end, freeBusy, note })
And regarding user interaction, Just a small clarification (FTA):
The blackberry.launch object provides methods that lets you access core and add-on BlackBerry® applications through JavaScript®. Depending on the application, you can either launch the application or open the application and create a new item, such as a calendar entry or a task, and prepopulate any of the entry fields. Although the methods of the blackberry.launch object can open and create new items for an application, changes are not made to the application without confirmation from the user. For example, if you use the blackberry.launch object to make a phone call to a specific phone number, the user is prompted to confirm whether the user wants to proceed with the call before the number is dialed. Similarly, if you create a new item for any application, the user must save the item before it is added to that application.
Edit: it seems there's a workaround for integrating .ics on blackberry.

You don't need to have those devices to develop for them.
Being so oriented toward mobiles, I think you should definitely get SDK's / official simulators of these devices, and test first hand what works and how.
Blackberry:
http://us.blackberry.com/developers/resources/simulators.jsp
Android:
http://developer.android.com/sdk/index.html

As far as I can tell, BlackBerry just doesn't support an .ics file the way other platforms do. I'm using the BB Storm 9559 (Verizon) emulator to test mobile web pages, and have tried all the .ics options (including the METHOD:REQUEST workaround) to no avail. But the good news is, as mentioned above, a simple client-side JavaScript can be used to create BlackBerry calendar appointments by clicking a link on the mobile web page.
#Rodrigo Hahn's answer above is correct, although you should check the BlackBerry developer docs for a more complete explanation of the syntax. Check here for the official explanation of the blackberry.launch.newAppointment() method:
http://docs.blackberry.com/.../blackberry_launch_971534_11.jsp
I've implemented this code myself and it works great on my 9550 emulator. See a working example at http://medcomp.mobi/tradeshows
Hopefully this helps someone!
Cheers.