During my quest on upping my JavaScript game I began updating projects to use ES6. I don't recall having an issue pushing other projects to heroku after updating, but for some reason this time I did.
Before the error happened, possibly coincidential, I was on a seperate branch and ran git push origin master after which I assumed nothing had actually happened since I pushed master and not the branch I was on. I merged in the branch anyway and when I tried to push from master I couldn't.
The error I was getting at the time had something to do with assets not compiling so I looked up the issue and found a few that mentioned rake assets:precompile may work, it did.
However, I realized it was creating files and such in public/assets I didn't have in my project/repo before.
Again, on next push of edits I couldn't push normally, but this time the error was of that below.
rake aborted!
Uglifier::Error: Unexpected token: operator (>). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true).
Solution: change config.assets.js_compressor = :uglifier to config.assets.js_compressor = Uglifier.new(harmony: true) in production.rb file in config/environments/
Before I came to that solution, I believe, I ran rake assets:clean or rake assets:clobber (Rails 4+) to remove the files that had shown up from the inital error/solution of rake assets:precompile.