how to get value of webhtmleditor using javascript in client side
You can use the Client-Side Object Model listed in the documentation to see the methods and properties that are available to help you achieve what you want.
The first topic below will explain how to get a reference to the WebHtmlEditor client-side. The second topic talks about what properties and methods are available once you have a reference tot he WebHtmlEditor. The getText method sounds like what you are wanting (as it says it returns the text in HTML format).
http://help.infragistics.com/Help/NetAdvantage/NET/2008.2/CLR2.0/html/WebHtmlEditor_Utility_Functions_CSOM.html
http://help.infragistics.com/Help/NetAdvantage/NET/2008.2/CLR2.0/html/WebHtmlEditor_Object_CSOM.html
Hopefully this helps.