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.
We're looking for more maintainers for the project. If you're interested in helping out please ping @bbatsov.
- A WebSocket transport is simple and avoids some of the thornier bugs
caused by the
CrossPageChanneltransport, which is used in the standard ClojureScript browser REPL and Austin. (see: cemerick/austin#17, cemerick/austin#47, cemerick/austin#49) - WebSocket APIs are available far beyond the browser - Node, Deno, Bun, web and service workers, browser extensions, React Native, and more exotic embeddings like QML - so Weasel can give you a REPL driven workflow in environments where the iframe-based standard browser REPL simply can't run.
Weasel is intended to be used with nREPL's
piggieback middleware. Once you've set that up,
add Weasel as a dependency to project.clj:
[weasel "0.7.1" :exclusions [org.clojure/clojurescript]]Or, if you're using deps.edn:
{:deps {weasel/weasel {:mvn/version "0.7.1"}}}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 "127.0.0.1").
user> (require 'weasel.repl.websocket)
nil
user> (cider.piggieback/cljs-repl
(weasel.repl.websocket/repl-env :ip "0.0.0.0" :port 9001))
<< started Weasel server on ws://127.0.0.1:9001 >>
<< 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. :on-open and :on-close
; fire once per connection (not on every
; reconnect); :on-error receives the native
; WebSocket event.
:reconnect? ; boolean, whether to reconnect automatically when the
; connection drops. defaults to true
:reconnect-delay ; initial reconnect backoff in ms, defaults to 1000.
; the delay doubles after each failed attempt
:max-reconnect-delay ; ceiling for the backoff in ms, defaults to 30000
:heartbeat-interval ; ms between keepalive pings used to detect a silently
; dead connection. 0 disables it, which is the default.
; a few missed pongs are tolerated before the link is
; considered dead, and a server that never answers pings
; is left undisturbedThe connection survives page reloads and server restarts on its own, retrying
with the backoff until it gets back in. Call (repl/disconnect) to close it for
good and stop the reconnection attempts.
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))
false
cljs.user> (do (js/alert "Hello world!") 42)
42Because the client talks over the platform's native WebSocket, it isn't tied
to the browser. Any runtime with a global WebSocket works, including Node 22+,
Deno, Bun, and web/service workers. Require weasel.repl from code compiled for
that target and call repl/connect exactly as you would in a browser:
(ns my.app
(:require [weasel.repl :as repl]))
(repl/connect "ws://localhost:9001")The only browser-specific piece is Closure's script-tag code loading used to
pull in namespaces first required at the REPL; it's skipped automatically when
there is no document. Evaluating forms, printing, reconnection and the
heartbeat all behave the same everywhere.
Note that unless a client is connected to the WebSocket channel, evaluation will fail:
cljs.user> (+ 5 10)
java.io.IOException: No client connected to Websocket
nilMore than one client may be connected at once. Evaluations are sent to the most recently connected client, so a newly connected client takes over the REPL; the others stay connected and their printed output still reaches the REPL. This pairs naturally with auto-reconnect - a client that drops and comes back simply becomes the active one again.
The REPL server validates the Origin header of every WebSocket handshake.
WebSocket connections aren't subject to the same-origin policy, so without this
any page a developer happens to have open could connect to the server and take
over the REPL. By default only origins on the local machine (localhost,
127.0.0.1, [::1], on any port, over http or https) are accepted, which covers
the usual "app served from localhost" setup. Non-browser clients (Node, Deno,
Bun) send no Origin header and are always allowed.
If you serve your app from somewhere else - a LAN IP for testing on a phone, a
custom dev domain - widen the allowlist with :allowed-origins:
(weasel.repl.websocket/repl-env
:ip "0.0.0.0" :port 9001
:allowed-origins ["http://192.168.1.5:8080"]):allowed-origins accepts a single origin string, a collection of exact origin
strings, a one-argument predicate that receives the origin (which may be nil),
or :all to turn the check off entirely.
Two things worth knowing about the built-in policies. A page opened straight from
disk over file:// sends Origin: null and is rejected - serve it over
http://localhost instead, or pass :all. And a client that sends no Origin
header at all (every non-browser runtime) is always accepted, since browsers are
the only thing the same-origin rule constrains; pass your own predicate if you
need to restrict header-less clients too.
Weasel is an ordinary piggieback ClojureScript REPL environment, so any nREPL
client can drive it: start an nREPL server with piggieback on the classpath,
connect your editor to it, and evaluate the repl-env form to upgrade the
session into a Weasel REPL.
If you use the Clojure CLI, an alias like this gives you that nREPL server:
;; deps.edn
{:aliases
{:cljs-repl
{:extra-deps {nrepl/nrepl {:mvn/version "1.3.1"}
cider/piggieback {:mvn/version "0.6.0"}
weasel/weasel {:mvn/version "0.7.1"}
org.clojure/clojurescript {:mvn/version "1.12.134"}}
:main-opts ["-m" "nrepl.cmdline"
"--middleware" "[cider.piggieback/wrap-cljs-repl]"]}}}Run clj -M:cljs-repl, connect your editor to the port it prints, then start the
Weasel REPL from the session:
(require 'weasel.repl.websocket)
(cider.piggieback/cljs-repl (weasel.repl.websocket/repl-env :port 9001))CIDER (Emacs) ships a built-in Weasel REPL type, so you can skip the manual
form: run M-x cider-jack-in-cljs (or cider-connect-cljs) and pick weasel
when prompted for the ClojureScript REPL type.
Calva (VS Code) has no Weasel preset, but the generic path works: use
"Connect to a Running REPL in your Project", point it at the nREPL port from the
alias above, and evaluate the cljs-repl form in the REPL window to drop into
the Weasel REPL.
An example project is included in the weasel-example subdirectory of
this project.
If you have any feedback or issues to report, feel free to open an issue on GitHub.
A little treat for reading the whole README!
This is free and unencumbered software released into the public domain.
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