how to read the web.config file values in office addins office.js

Solution 1:

Generally speaking, this is not something that is Office Add-in specific. You would you would use the same approach as with a regular website. For example, see Reading web.config from JavaScript.

In terms of the manifest start URL, you can point it at any web-browsable page, it doesn't have to end with .html. So you can use an aspx or MVC view page. Alternatively, you can have an html/js/css page that makes an AJAX web service call to your own web API that returns you the settings. If I may recommend, see my blog post for how to create a web service for an Office Add-in using the ASP.NET Web API.

Hope this helps!

~ Michael Zlatkovsky, developer on Office Extensibility team, MSFT