Speeding up ClojureScript compilation

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If you are using ClojureScript (and if not, why not? :-)) then I *strongly* recommend you enable parallel build.

I mistakenly thought this meant that it would run separate builds in parallel, but no, it is far cleverer and more useful than that, it builds a single build in parallel.

Also, to be crystal clear, this isn’t going to do anything to your runtime performance, only the build time._

Awesome sauce - speed me up!

It is as simple as adding it to your compiler options in your cljsbuild config:

:cljsbuild {:test-commands
	      ;; Test command for running the unit tests in "test-cljs" (see below).
	      ;; $ lein cljsbuild test
	      ;; NOTE: this does not call health/bootstrap as that causes errors, hence cljs-dev and cljs-prod
	      {"unit" ["phantomjs"
		       "phantom/unit-test.js"
		       "resources/private/html/unit-test.html"]}
	      :builds
	      {:test {:source-paths ["src/clj" "src/clj-prod" "src/cljc" "src/cljs" "src/cljs-prod"]
		      :compiler     {:parallel-build true
				     :output-to      "resources/private/js/unit-test.js"
				     :output-dir     "resources/private/js/out"
				     :optimizations  :whitespace
				     :pretty-print   true}}
	       :dev  {:source-paths ["src/clj" "src/clj-prod" "src/cljc" "src/cljs" "src/cljs-prod"]
		      :figwheel     {:on-jsload      "health.bootstrap/run-render"
				     :websocket-host :js-client-host}
		      :compiler     {:main                 health.core
				     :parallel-build       true
				     :output-to            "resources/public/compiled/js/app.js"
				     :output-dir           "resources/public/compiled/js/out"
				     :asset-path           "compiled/js/out"
				     :source-map-timestamp true}}
	       :prod {:source-paths ["src/clj" "src/clj-prod" "src/cljc" "src/cljs" "src/cljs-prod"]
		      :compiler     {:main           health.core
				     :parallel-build true
				     :output-to      "resources/public/compiled/js/app.js"
				     :optimizations  :advanced
				     ;; for debugging
				     :pretty-print   true
				     :pseudo-names   true
				     ;; for debugging
				     }}}}

I only added it to the :dev build but I guess you could add it to each build if you wanted to_

Anecdotally (meaning I can’t be bothered to time it properly) it sped up my build by about 300% - yep, that much. Restarting the REPL in the lovely CIDER is no longer an excuse to go and grab some coffee.

Have a great day!