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nREPL support for ClojureScript REPLs
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Piggieback Travis CI status

nREPL middleware that enables the use of a ClojureScript REPL on top of an nREPL session.


Two reasons:

  • The default ClojureScript REPL (as described in the "quick start" tutorial) assumes that it is running in a teletype environment. This works fine with nREPL tools in that environment (e.g. lein repl in or gnome-terminal, etc), but isn't suitable for development environments that have richer interaction models (including editors like vim [fireplace] and emacs [CIDER] and IDEs like Intellij [Cursive] and Eclipse [Counterclockwise]).
  • Most of the more advanced tool support for Clojure and ClojureScript (code completion, introspection and inspector utilities, refactoring tools, etc) is packaged and delivered as nREPL extensions.

Piggieback provides an alternative ClojureScript REPL entry point (cemerick.piggieback/cljs-repl) that changes an nREPL session into a ClojureScript REPL for eval and load-file operations, while accepting all the same options as cljs.repl/repl. When the ClojureScript REPL is terminated (by sending :cljs/quit for evaluation), the nREPL session is restored to it original state.


These instructions are for Leiningen. Translating them for use in boot should be straightforward.

Piggieback is compatible with Clojure 1.6.0+, and requires ClojureScript 0.0-3165 or later and nREPL 0.2.10 or later.

Modify your project.clj to include the following :dependencies and :repl-options:

:profiles {:dev {:dependencies [[com.cemerick/piggieback "0.2.1"]
                                [org.clojure/tools.nrepl "0.2.10"]]
                 :repl-options {:nrepl-middleware [cemerick.piggieback/wrap-cljs-repl]}}}

The :repl-options bit causes lein repl to automagically mix the Piggieback nREPL middleware into its default stack. (Yes, you need to explicitly declare a local nREPL dependency to use piggieback, due to a Leiningen bug.)

If you're using Leiningen directly, or as the basis for the REPLs in your local development environment (e.g. CIDER, fireplace, counterclockwise, etc), you're done. Skip to starting a ClojureScript REPL.

If you're not starting nREPL through Leiningen (e.g. maybe you're starting up an nREPL server from within an application), you can achieve the same thing by specifying that the wrap-cljs-repl middleware be mixed into nREPL's default handler:

(require '[ :as server]
         '[cemerick.piggieback :as pback])

  :handler (server/default-handler #'pback/wrap-cljs-repl)
  ; ...additional `start-server` options as desired

(Alternatively, you can add wrap-cljs-repl to your application's hand-tweaked nREPL handler. Keep two things in mind when doing so:

  • Piggieback needs to be "above" nREPL's; it doesn't use interruptible-eval's evaluation machinery, but it does reuse its execution queue and thus inherits its interrupt capability.
  • Piggieback depends upon persistent REPL sessions, like those provided by


$ lein repl
user=> (cemerick.piggieback/cljs-repl (cljs.repl.rhino/repl-env))
To quit, type: :cljs/quit
cljs.user=> (defn <3 [a b] (str a " <3 " b "!"))
function cljs$user$_LT_3(a, b) {
    return [cljs.core.str(a), cljs.core.str(" <3 "), cljs.core.str(b), cljs.core.str("!")].join("");
cljs.user=> (<3 "nREPL" "ClojureScript")
"nREPL <3 ClojureScript!"

See how the REPL prompt changed after invoking cemerick.piggieback/cljs-repl? After that point, all expressions sent to the REPL are evaluated within the ClojureScript environment. cemerick.piggieback/cljs-repl's passes along all of its options to cljs.repl/repl, so all of the tutorials and documentation related to it hold (including the ClojureScript Quick Start tutorial).

Important Notes

  1. When using Piggieback to enable a browser REPL: the ClojureScript compiler defaults to putting compilation output in out, which is probably not where your ring app is serving resources from (resources, target/classes/public, etc). Either configure your ring app to serve resources from out, or pass a cljs-repl :output-dir option so that a reasonable correspondence is established.
  2. The load-file nREPL operation will only load the state of files from disk. This is in contrast to "regular" Clojure nREPL operation, where the current state of a file's buffer is loaded without regard to its saved state on disk.

Of course, you can concurrently take advantage of all of nREPL's other facilities, including connecting to the same nREPL server with other clients (so as to easily modify Clojure and ClojureScript code via the same JVM), and interrupting hung ClojureScript invocations:

cljs.user=> (iterate inc 0)
cljs.user=> "Error evaluating:" (iterate inc 0) :as ",,0);\n"
cljs.user=> (<3 "nREPL still" "ClojureScript")
"nREPL still <3 ClojureScript!"

(The ugly ThreadDeath exception will be eliminated eventually.)

Piggieback works well with all known ClojureScript REPL environments, including the Rhino, Nashorn, Node, and browser REPLs.


Nelson Morris was instrumental in the initial development of piggieback.

Need Help?

Send a message to the clojure-tools mailing list, or ping cemerick on freenode irc or twitter if you have questions or would like to contribute patches.


Copyright © 2012-2015 Chas Emerick and other contributors.

Distributed under the Eclipse Public License, the same as Clojure.

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