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ClojureScript on Node.js

The purpose of this template is just to gather together as much information together as possible about running ClojureScript effectively under Node.js, and to simplify the process of creating a new Node-hosted ClojureScript project.

All of this information is current as of Sept. 26th, 2014, although it's likely to change rapidly due to the rather bleeding-edge nature of both ClojureScript and Node.js, not to mention other related technologies such as core.async, and to some extent Clojure itself.

Once you get the development process dialled, writing Node.js code in ClojureScript is not just easy, it's fun and FAST: No more frustrating JVM startup times.


Create a new project:

lein new node-cljs zanglegax

then cd into your new project and run

lein npm install
lein cljsbuild once

Now you should just be able to run the project directly using Node:

node run.js

And that's all there is to it.


The basic structure of the template here is:

├── project.clj
├── resources
│   └── config
│       └── development.js
├── run.js
├── test
│   └── foo.cljs
└── src
    ├── cljs
    │   └── node_cljs
    │       ├── config.cljs
    │       ├── core.cljs
    │       ├── log.cljs
    │       └── web.cljs
    └── js
        └── some_node_module.js

There are cljs/ and js/ directories in the src/ directory - this is because you will most probably have files of both types in your project. This is slightly problematic because the cljs compiler will spit out .js files in the :output-dir specified in the project.clj file. When those files are run from node.js, they're in a different location, so it's tricky for node.js to find your .js files. The kludgey way to do it now is to know where your .cljs file will end up, transformed into Javascript, and load your .js relative to that using

(def foo-module (apply (js* "require") ["../../src/js/some_node_module"]))

Note that most documentation that you read about loading existing node modules from cljs will seem to indicate that you can first do something like

(ns zip.zap
  (:require [cljs.nodejs :as node]))

(def foo-module (node/require "../../src/js/some_node_module"))

but it won't work. This form is fine to load things from node_modules directories, but it will most certainly not work for loading .js files in your own project. The problem seems to lie with the fact that the .js file that actually ultimately performs the "require" is somewhere deep in the cljs lib, and its relative path to your module is mysterious and unknown.

Node.js dependencies

This project.clj file uses the lein-npm plugin to maintain node.js dependencies (rather than maintaining a separate package.json file) but there is absolutely no requirement to do that.

But to run the example project, first you'll need to do

lein npm install

to install its node dependencies.

Dependency Injection and Configuration

Developing with node.js can always be a bit trying in terms of modules and dependencies. How can code in module X, that needs to access a MySQL DB, a redis queue, and a logger, somehow be sure that all those have been loaded and initialised correctly from the config. To assist with this, I've ported Stuart Sierra's "component" library from Clojure, which makes this stuff much, much easier. Take it from me - the ease with which you can set up your different modules and dependencies between them is a massive win.

Development process

Much older documentation about using cljsbuild under node.js often claims that you can't use

:optimization :none

when you're using

:target :nodejs

in your project.clj. However, this changed fairly recently, and now it is very possible. This completely changes your development flow, because now

  • it's FAST
  • it produces vaguely readable JS

The only trick (of course there's a trick) is that you need to use the run.js file to start your application, because when you have

:optimization :none

the cljs compiler doesn't generate the right code to load all the required dependencies.

So, bearing all that in mind, in general you can just run

lein cljsbuild auto

in one terminal, and it will pick up all changes to your cljs files and quickly rebuild them (very, very quickly). In another window, you can then just do

node run.js

and your app should fire up. If you run the default code, it should produce:

;; Loading some_node_module
2014-09-20 22:55:05.792 - ;; Starting ConfigComponent
2014-09-20 22:55:05.796 - ;; Starting WebComponent
2014-09-20 22:55:05.812 - ;; Starting NodeComponent
2014-09-20 22:55:10.819 - ;; Stopping NodeComponent
2014-09-20 22:55:10.819 - ;; Stopping WebComponent
2014-09-20 22:55:10.819 - ;; Stopping ConfigComponent

If you dig into the code, you'll see some examples of how to use:

* Stuart Sierra's Components and the Lifecycle protocol
* Interop between your own JS and .cljs code
* Interop between your .cljs code and Node modules installed using npm
* How to make tests and how to run them
* How to load in some config and use it to initialise modules
  (this is fairly cheesy right now, but will improve greatly in the near future)


  • Wrap some basic node.js functionality with ClojureScript
  • Fill in some basic things missing from cljs.core, such as slurp


Leiningen template for a (working and useful) ClojureScript project that runs on Node.js



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