Optional Self hosting

This page documents advice for people wanting to build ClojureScript projects that want or need to leverage cljs.js/eval-str and other self-hosted functionality.

WARNING: If you are building a web application where the final size of the deployed artifact matters significantly you should not be leveraging any of the functionality outlined on this page.

Production Builds

You can leverage :optimization :simple with builds that include the cljs.js namespace. Other recommended settings:

:pretty-print false
:optimize-constants true
:static-fns true

:optimize-constants is significant for code size, all keys and symbols in your source will be compiled into a single lookup table.

By default for ease of use cljs.js/empty-state will dump the analysis cache for cljs.core directly into the cljs.js namespace. This doubles the size of the final file. You can disable this with setting :dump-core false in your compiler build options.

This means you will need to load the analysis for core yourself. It's easy to dump the core analysis cache. In your ClojureScript project load a Clojure REPL and do something like the following:

(require '[clojure.java.io :as io]
         '[cognitect.transit :as transit])
(import [java.io ByteArrayOutputStream])

(def out-path
  "../../assets/js/cljs/core.cljs.cache.aot.json")
(def out (ByteArrayOutputStream. 1000000))
(def writer (transit/writer out :json))

(def cache
  (read-string
    (slurp (io/resource "cljs/core.cljs.cache.aot.edn"))))

(transit/write writer cache)

(spit (io/file out-path) (.toString out))

Then in your ClojureScript source you can load this cache - for example that might look something like this:

(def st (cljs.js/empty-state))

;; path to Transit encoded analysis cache
(def cache-url "/assets/js/cljs/core.cljs.cache.aot.json")

(defn main []
  (http/get cache-url
    (fn [json]
      (let [rdr   (transit/reader :json)
            cache (transit/read rdr json)]
        (cljs.js/load-analysis-cache! st 'cljs.core cache)
        ;; ...
        ))))

This simple optimization eliminates about 2.7mb of generated JavaScript.