Build web application before or after deployment?

I am not familiar with both Azure and Heroku so I can't give any ideas about those specific deployment options.

I am using (4 dedicated servers with 2 of them solely for serving static files), the option to build the bundled and minified javascript files (for front-end) and add all those files to the main repository has several advantages

• You only need to run it once (either on your dev machine or on staging server, whatever way you want). This is particularly helpful when you have to run multiple static servers since you don't have to run the build command on each server. One might argue that they can use something like Glusterfs to synchronise files from one static server to all other servers and the build process only needs to be run once. However, it is a whole different story when it comes to this kind of setup
• It makes your deployment process simple, just pull new code and restart the server(s) if necessary (assuming that you have some mechanism to increase the static file version so that all your clients will receive the latest version)
• Avoid unnecessary dependencies on your production servers. This might sound weird for some people but I just don't want to install any extra libraries on my production servers unless they are absolutely necessary. With the build process run locally on my dev machine, my production servers only have what they need to run the production code and nothing else

However this approach also has some disadvantages:

• When more than one developer in your team (accidentally) run the build process and commit the code, then you will have a crazy list of conflicts. However, it can be solved by simply running the build process again after you merge all the changes from other guys. This is more about the workflow
• Your repository will be bigger. I personally don't think this is a big issue considering few extra MB of my bundled and minified files. If your front-end javascript is big enough for this to be an issue then it is another story