There are two way to check.first is you can directly check whether key contain A or not in binding like this:-. but i think this is not the better way so instead of this you can use ko.computed to check.
Check for operating system information in Windows 7. 1  Click the Start button , enter Computer in the search box, right-click Computer, and then click Properties. 2  Look under Windows edition for the version and edition of Windows that your PC is running.
Type about in the search box on your taskbar, and then select About your PC. Look under PC for Edition to find out which version and edition of Windows that your PC is running. Look under PC for Version to find out which version of Windows 10 your PC is running. Look under PC for System type to see if you're running a 32-bit or 64-bit version of Windows. The Start screen can also help you know which operating system you're using. Here's what it might look like in Windows 8.1 and Windows RT 8.1.