ClojureScript browser REPL using WebSockets
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Clojars Project

Weasel uses WebSockets to communicate between a ClojureScript REPL, which is typically hosted on nREPL using piggieback, and an environment which can execute compiled ClojureScript, which can be a web browser or any JavaScript environment that supports the WebSocket APIs.


  • A WebSocket transport is simple and avoids some of the thornier bugs caused by the CrossPageChannel transport, which is used in the standard ClojureScript browser REPL and Austin. (see: cemerick/austin#17, cemerick/austin#49, cemerick/austin#49)
  • WebSocket APIs are available in unusual JavaScript environments like JavaScriptCore, QML, WinJS, browser extensions, Mac OS X Dashboard widgets, and so on, where use of CrossPageChannel may be unfeasible due to restrictions on or unavailability of <iframe> elements. Weasel allows the ClojureScript developer to still enjoy the benefits of REPL driven development in these exotic domains.


Weasel is intended to be used with Chas Emerick's piggieback nREPL middleware. Once you've set that up, add Weasel as a dependency to project.clj:

[weasel "0.7.0" :exclusions [org.clojure/clojurescript]]

Start up lein repl and piggieback the Weasel REPL environment onto the nREPL session, optionally specifying a port (defaults to 9001) and an address to bind to (defaults to "").

user> (require 'weasel.repl.websocket)
user> (cemerick.piggieback/cljs-repl
        (weasel.repl.websocket/repl-env :ip "" :port 9001))
<< started Weasel server on ws:// >>
<< waiting for client to connect ...

Weasel will block the REPL, waiting for a client to connect.

In your project's ClojureScript source, require the Weasel client namespace and connect to the REPL.

(ns my.cljs.core
  (:require [weasel.repl :as repl]))

(when-not (repl/alive?)
  (repl/connect "ws://localhost:9001"))

You may optionally specify the following:

:verbose ; boolean, defaults to true
:print ; :repl to print only to the repl,
       ; :console to print only to the console
       ; #{:repl :console} to print to both
       ; or any variadic function to handle printing differently.
       ; defaults to :repl
:on-open, :on-error, :on-close ; fns for handling websocket lifecycle events.
                               ; default for all is nil

Connecting with options:

(repl/connect "ws://localhost:9001"
   :verbose true
   :print #{:repl :console}
   :on-error #(print "Error! " %))

Load the script in your WebSocket-enabled environment (probably a page in a web browser) and start evaluating ClojureScript at the REPL:

cljs.user> (= (js/Number. 34) (js/Number. 34))
cljs.user> (do (js/alert "Hello world!") 42)

Note that unless a client is connected to the WebSocket channel, evaluation will fail:

cljs.user> (+ 5 10) No client connected to Websocket

Only a single client can be connected to the REPL at once. Attempting to connect to an occupied REPL server will throw an exception in the client.


An example project is included in the weasel-example subdirectory of this project.

Need help?

If you have any feedback or issues to report, feel free to open an issue on GitHub.

A weasel "piggiebacking" on a woodpecker

A little treat for reading the whole README!