cljs-build configuration tip: prefer maps over a vector

If you look at the lein cljs-build sample project, you'll see that it states: ; The :builds option should be set to a sequence of maps. Each ; map will be treated as a separate, independent, ClojureScript ; compiler configuration. If you look at the example right below that description, you'll see that it is actually set as a map of configuration profiles with the configuration name as the key. So what gives? Both options work, since the convert-builds-map function on cljs-build ends up converting :builds map to a vector if it instead receives a map. Should you just save this step and do it as a vector yourself? Turns out there is an advantage to having the :builds as a straight map with the profile as the key, than a vector of maps. Profiles on lein are merged. If globally you declare your :builds as a vector of maps, instead of a map, then when you try to alter just one value on a profile you end up with one more build configuration. If instead :builds is a map indexed by the build id, then lein will just merge both configuration values. To give an example, suppose you have the following configuration. :cljsbuild {:builds {:app {:source-paths ["src/cljs"] :compiler {:other :options-here} }}} Later, on your :profiles section you could change the :source-paths, to do: :test {:cljsbuild {:builds {:app {:source-paths ["src/cljs" "src/test-cljs/"] }}}} This would work just fine, with the :source-paths property being merged into the base :cljsbuild configuration.