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The Node.js REPL of My Dreams

29 December 2014

UPDATE: This post now contains obsolete information. Please read the new ClojureScript Quick Start instead

Clojure has always been about embracing existing mature platforms. Being on the JVM means tapping into untold person years poured into battle-tested libraries and access to a vast ecosystem with powerful IDEs, incredible profiling tools, and answers to nearly any question imaginable.

Similarly ClojureScript brings a hefty helping of Clojure's semantics to the most important platform of all. And thanks to some smart folks rethinking best practices, ClojureScript's solid semantics now shine in way that leaves many JavaScript libraries playing catchup and copycat.

But of course these days the Web isn't the only reason to target JavaScript - Node.js is another great platform with attractive properties for ClojureScript.

ClojureScript actually shipped three and a half years ago with some promise of Node.js integration but it remained low priority due to the fact we had a robust technology for writing server side code. But now that the languages bits of ClojureScript aren't evolving so quickly I decided it was a time to go back and make good on this promise.

Prior to the latest commits, ClojureScript shipped with only two REPLs, Rhino and the Browser. These REPLs are unsatisfactory in some simple ways - the Rhino REPL runs ClojureScript more than 100X slower than modern JavaScript engines and the Browser REPL requires you to ... start a browser.

Enter the Node.js REPL!

Why don't you give it a shot? You just need to make sure you have Node.js installed:

git clone
cd clojurescript
npm install source-map-support

Now you're ready to run the REPL:


The first time you launch this REPL it will be a bit slow as we have to compile the standard library. Subsequent launches will be faster. There's still some work to do so expect this to improve over time.

Once you have a REPL up and running try the following session (the Clojure programmers reading this will probably yell Hallelujah!):

Run a simple expression:

ClojureScript:cljs.user> (first [1 2 3])

Get a doc string:

ClojureScript:cljs.user> (doc first)
  Returns the first item in the collection. Calls seq on its
  argument. If coll is nil, returns nil.

Load a library and try it:

ClojureScript:cljs.user> (require '[clojure.string :as string])

ClojureScript:cljs.user> (string/join ", " [1 2 3])
"1, 2, 3"

Time an expression:

ClojureScript:cljs.user> (time (reduce + (range 1000000)))
"Elapsed time: 211 msecs"

Write a bad program and get a ClojureScript stack trace not a JavaScript one:

ClojureScript:cljs.user> (first (js/Date.))
Error: Mon Dec 29 2014 18:16:00 GMT-0500 (EST) is not ISeqable
    at Object.seq (.../cljs/core.cljs:664:20)
    at first (.../cljs/core.cljs:673:16)
    at repl:1:81
    at repl:9:3
    at Socket.<anonymous> ([stdin]:26:80)
    at Socket.emit (events.js:95:17)
    at Socket.<anonymous> (_stream_readable.js:764:14)
    at Socket.emit (events.js:92:17)
    at emitReadable_ (_stream_readable.js:426:10)
    at emitReadable (_stream_readable.js:422:5)

Have fun!