CIDER works well with ClojureScript, but not all CIDER features are available in ClojureScript (yet). For instance, the test runner and debugger are currently Clojure-only features.

Unlike the Clojure ecosystem that is dominated by a single REPL, the ClojureScript ecosystem has a number of different choices for REPLs (e.g. browser, node, weasel, figwheel and shadow-cljs). You’ll have to decide which one you want to run and how you want CIDER to interact with it.

In this section we’ll take a look at how ClojureScript support is implemented in CIDER, and in the subsequent sections we’ll discuss how to launch a ClojureScript REPL and how to setup the most popular ClojureScript REPLs.

nREPL and ClojureScript

nREPL doesn’t natively support ClojureScript evaluation, that why an additional middleware is needed. For most REPLs (with the notable exceptions of shadow-cljs, nbb), CIDER relies on the popular Piggieback middleware for its ClojureScript support.

Piggieback works in the following manner:

  • You start a regular Clojure REPL

  • You run some Clojure code in it, that converts it to a ClojureScript REPL

This means that jacking-in is a two-fold process for ClojureScript, compared to Clojure, as now we have the extra REPL "upgrade" step.

On the bright side - this also means that you can host side by side Clojure and ClojureScript REPLs in a single nREPL connection! This opens up all sorts of interesting possibilities that we’ll discuss later on.

`shadow-cljs’s REPL is implemented in a very similar fashion, but its mechanism is provided by its own nREPL middleware - not Piggieback.

Piggieback differences with the Standard ClojureScript REPL

While the Piggieback-powered ClojureScript REPLs behave more or less the same as the standard ClojureScript REPL, there are few subtle differences everyone has to be aware of.

Handling of Multiple Forms

Here’s how the standard ClojureScript behaves with multiple input forms:

(declare is-odd?)
(defn is-even? [n] (if (= n 0) true (is-odd? (dec n))))
(defn is-odd? [n] (if (= n 0) false (is-even? (dec n))))
cljs.user> (is-even? 4)

And here’s how a Piggieback-powered REPL behaves:

(declare is-odd?)
(defn is-even? [n] (if (= n 0) true (is-odd? (dec n))))
(defn is-odd? [n] (if (= n 0) false (is-even? (dec n))))
cljs.user> (is-even? 4)
Compile Warning   <cljs repl>   line:1  column:2

  Use of undeclared Var cljs.user/is-even?

  1  (is-even? 4)

#object[TypeError TypeError: Cannot read property 'call' of undefined]

This difference comes from a performance optimization in Piggieback, which avoids creating an different REPLs for each ClojureScript form it evaluates.

You can learn more about this difference here.

Dealing with Dependencies

CIDER doesn’t handle automatically ClojureScript REPL dependencies when you’re doing cider-jack-in-cljs. You’ll have to configure those manually yourselves as documented in the subsequent sections of this manual.

Actually, CIDER will handle automatically the most important dependency - namely Piggieback. The problem with the other dependencies, however, is that you might need to install some external tools (e.g. node, shadow-cljs) and that ClojureScript development tools like Figwheel and shadow-cljs also require some setup to be useful.

CIDER will try to help you identify missing requirements by running a check, right before attempting to upgrade a Clojure REPL to a ClojureScript REPL. The nature of this check differs for the different REPL types:

  • For a node REPL we check whether the node binary is on your exec-path (Emacs’s version of PATH)

  • For tools like figwheel we check whether they are available on the classpath (by trying to require some of their namespaces)

We’ll discuss those checks further in the upcoming sections.


CIDER currently doesn’t support self-hosted ClojureScript implementations (e.g. Lumo). The reason for this is that there’s no self-hosted version of nREPL (implemented in ClojureScript) available today.

Another unsupported REPL is Rhino. Supporting in it Piggieback required a lot of ugly hacks and eventually it was decided we were better off without Rhino. Given the abundance of better solutions today, I doubt anyone’s going to miss Rhino anyways.

Additionally, as noted earlier on this page - many of CIDER’s advanced features are currently not available for ClojureScript.


In the next section we’ll show you how to start a ClojureScript REPL with CIDER.