This is an small Visual Studio Extension created to be used as a pre-build event in order to be able to have different files for different build configurations.
Imagine we have a project with lots of files, frontend.js, backend.cs, myscript.py, destroy_the_world.cs and any other file you want. Now, we have different needs for different customers or build types. Customer1 has different needs that Customer2, but they share a lot of code too, the only thing that changes is frontend.js.
So, instead of creating a hole new project duplicating all the files just to build and publish with a different frontend.js let's take another approach.
We will create two files called frontend.Customer1.js and frontend.Customer2.js (note this uses the same syntax used in Web.config or App.config files in Visual Studio projects to handle different build configurations using xdt:Transform and xdt:Locator, ie: Web.Release.config)
Now, on frontend.Customer1.js we put all the code for Customer1 and in frontend.Customer2.js all the code from Customer2. In our index.html we will still be adding frontend.js.
Now, create two build configurations named Customer1 and Customer2 and build the project with any of this configurations. Before the build process the extension will replace frontend.js with frontend.Customer1.js or frontend.Customer2.js according to the build configuration. On this way, you end up having a different file for every build configuration in an easy way. It's not a perfect solution but it can very useful specially on Javscript, Python or any other file that can't read settings from the Web.config to manage different configurations. Also, when saving frontend.Customer1.js or frontend.Customer2.js, frontend.js will be automatically updated according to the active build configuration, this enables you to modify .js, .css, .html files and reload the browser without needing to rebuild the project.