ClavaScript

ClavaScript, or clava for friends, is an experimental ClojureScript syntax to JavaScript compiler.

⚠️ This project should be considered experimental and may still undergo breaking changes. It's fine to use it for non-critical projects but don't use it in production yet.

Quickstart

Although it's early days, you're welcome to try out clava and submit issues.

$ mkdir clava-test && cd clava-test
$ npm init -y
$ npm install clavascript@latest

Create a .cljs file, e.g. example.cljs:

(ns example
  (:require ["fs" :as fs]
            ["url" :refer [fileURLToPath]]))

(println (fs/existsSync (fileURLToPath js/import.meta.url)))

(defn foo [{:keys [a b c]}]
  (+ a b c))

(println (foo {:a 1 :b 2 :c 3}))

Then compile and run (run does both):

$ npx clava run example.cljs
true
6

Run npx clava --help to see all command line options.

Why Clava

Clava lets you write CLJS syntax but emits small JS output, while still having parts of the CLJS standard library available (ported to mutable data structures, so with caveats). This may work especially well for projects e.g. that you'd like to deploy on CloudFlare workers, node scripts, Github actions, etc. that need the extra performance, startup time and/or small bundle size.

Differences with ClojureScript

• Clava does not protect you in any way from the pitfalls of JS with regards to truthiness, mutability and equality
• There is no CLJS standard library. The "clavascript/core.js" module has similar JS equivalents
• Keywords are translated into strings
• Maps, sequences and vectors are represented as mutable objects and arrays
• Most functions return arrays and objects, not custom data structures
• Supports async/await:(def x (js/await y)). Async functions must be marked with ^:async: (defn ^:async foo []).
• assoc!, dissoc!, conj!, etc. perform in place mutation on objects
• assoc, dissoc, conj, etc. return a new shallow copy of objects
• println is a synonym for console.log
• pr-str and prn coerce values to a string using JSON.stringify

Seqs

Clava does not implement Clojure seqs. Instead it uses the JavaScript iteration protocols to work with collections. What this means in practice is the following:

• seq takes a collection and returns an Iterable of that collection, or nil if it's empty
• iterable takes a collection and returns an Iterable of that collection, even if it's empty
• seqable? can be used to check if you can call either one

Most collections are iterable already, so seq and iterable will simply return them; an exception are objects created via {:a 1}, where seq and iterable will return the result of Object.entries.

first, rest, map, reduce et al. call iterable on the collection before processing, and functions that typically return seqs instead return an array of the results.

Memory usage

With respect to memory usage:

• Lazy seqs in Clava are built on generators. They do not cache their results, so every time they are consumed, they are re-calculated from scratch.
• Lazy seq function results hold on to their input, so if the input contains resources that should be garbage collected, it is recommended to limit their scope and convert their results to arrays when leaving the scope:

(js/global.gc)

(println (js/process.memoryUsage))

(defn doit []
  (let [x [(-> (new Array 10000000)
               (.fill 0)) :foo :bar]
        ;; Big array `x` is still being held on to by `y`:
        y (rest x)]
    (println (js/process.memoryUsage))
    (vec y)))

(println (doit))

(js/global.gc)
;; Note that big array is garbage collected now:
(println (js/process.memoryUsage))

Run the above program with node --expose-gc ./node_cli mem.cljs

JSX

You can produce JSX syntax using the #jsx tag:

#jsx [:div "Hello"]

produces:

<div>Hello</div>

and outputs the .jsx extension automatically.

You can use Clojure expressions within #jsx expressions:

(let [x 1] #jsx [:div (inc x)])

Note that when using a Clojure expression, you escape the JSX context so when you need to return more JSX, use the #jsx once again:

(let [x 1]
  #jsx [:div
         (if (odd? x)
           #jsx [:span "Odd"]
           #jsx [:span "Even"])])

See an example of an application using JSX here (source).

Async/await

Clava supports async/await:

(defn ^:async foo [] (js/Promise.resolve 10))

(def x (js/await (foo)))

(println x) ;;=> 10

Roadmap

In arbitrary order, these features are planned:

• Macros
• REPL
• Protocols
• Transducers

Core team

The core team consists of:

• Michiel Borkent (@borkdude)
• Will Acton (@lilactown)
• Cora Sutton (@corasaurus-hex)

License

Clava is licensed under the EPL, the same as Clojure core and Scriptjure. See epl-v10.html in the root directory for more information.