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A vanilla phoenix project is structured around modular javascript.
If you don’t want to use a bundler you can always specify:
mix phx.new PATH --no-webpack
Any additional assets are then kept directly under priv/static.
It may be necessary to adjust the endpoint to pick up assets under non-standard directories.
That being said npm is currently the de facto mechanism for publishing JavaScript libraries (essentially making bower redundant) - so there is significant drive towards adopting a bundler that works with npm.
