I am working on a web application I have used JavaScript on it.
I read somewhere that javascript is disable in some browser then Javascript will not function there
My Question is what are the other seneario when JavaScript may not function properly?
(somebody told me slow Internet speed is also a seneraio)
Some browsers, like Lynx, don't support JavaScript at all.
But for all practical purposes you can assume that JavaScript is available for your web application, and for modern general purpose websites it's not unreasonable to require it.
Anyone who turns JavaScript off in their browser cuts themselves off from using pretty much any modern web application; especially those using modern JavaScript frameworks.
For all of the projects I work on, if a customer complains that a site won't work with JavaScript enabled, they are told to either turn it on, or look elsewhere. (Same attitude with antiquated browsers -- update your browser or be gone).
So in my opinion, there are no valid scenarios where JavaScript cannot be counted upon for a modern web application.