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Not babashka. Node.js babashka!?

Ad-hoc CLJS scripting on Node.js.

Try it out

Run npx nbb to run nbb on your own machine, or try it in a browser on Replit!

Goals and features

Nbb's main goal is to make it easy to get started with ad hoc CLJS scripting on Node.js.

Additional goals and features are:

  • Fast startup without relying on a custom version of Node.js.
  • Small artifact (current size is around 1.2MB).
  • First class macros.
  • Support building small TUI apps using Reagent.
  • Complement babashka with libraries from the Node.js ecosystem.



Nbb requires Node.js v14 or newer.

How does this tool work?

CLJS code is evaluated through SCI, the same interpreter that powers babashka. Because SCI works with advanced compilation, the bundle size, especially when combined with other dependencies, is smaller than what you get with self-hosted CLJS. That makes startup faster. The trade-off is that execution is less performant and that only a subset of CLJS is available (e.g. no deftype, yet).


Install nbb from NPM:

$ npm install nbb -g

Omit -g for a local install.

Try out an expression:

$ nbb -e '(+ 1 2 3)'

And then install some other NPM libraries to use in the script. E.g. with the following package.json:

  "dependencies": {
    "csv-parse": "^5.3.0",
    "shelljs": "^0.8.5",
    "term-size": "^3.0.2",
    "zx": "^7.1.1"

Create a script which uses the NPM libraries:

(ns example
  (:require ["csv-parse/sync" :as csv]
            ["fs" :as fs]
            ["path" :as path]
            ["shelljs$default" :as sh]
            ["term-size$default" :as term-size]
            ["zx" :refer [$]]
            ["zx$fs" :as zxfs]
            [nbb.core :refer [*file*]]))

(prn (path/resolve "."))

(prn (term-size))

(println (count (str (fs/readFileSync *file*))))

(prn (sh/ls "."))

(prn (csv/parse "foo,bar"))

(prn (zxfs/existsSync *file*))

($ #js ["ls"])

Call the script:

$ nbb script.cljs
#js {:columns 216, :rows 47}
#js ["node_modules" "package-lock.json" "package.json" "script.cljs"]
#js [#js ["foo" "bar"]]
$ ls

What does $default mean?

The :default foo syntax is shadow-cljs only and not supported by vanilla CLJS (and nbb doesn't support it either). The $default syntax is a recent addition to CLJS and should work in shadow-cljs too: this is why nbb supports it too.

See here for more info on that syntax.

Nbb implements :require via dynamic import (import() in JS). This is why you need to add $default to imports when you want to import the default object from a module.


Nbb has first class support for macros: you can define them right inside your .cljs file, like you are used to from JVM Clojure. Consider the plet macro to make working with promises more palatable:

(defmacro plet
  [bindings & body]
  (let [binding-pairs (reverse (partition 2 bindings))
        body (cons 'do body)]
    (reduce (fn [body [sym expr]]
              (let [expr (list '.resolve 'js/Promise expr)]
                (list '.then expr (list 'clojure.core/fn (vector sym)

Using this macro we can make async code look more like sync code. Consider this puppeteer example:

(-> (.launch puppeteer)
      (.then (fn [browser]
               (-> (.newPage browser)
                   (.then (fn [page]
                            (-> (.goto page "")
                                (.then #(.screenshot page #js{:path "screenshot.png"}))
                                (.catch #(js/console.log %))
                                (.then #(.close browser)))))))))

Using plet this becomes:

(plet [browser (.launch puppeteer)
       page (.newPage browser)
       _ (.goto page "")
       _ (-> (.screenshot page #js{:path "screenshot.png"})
             (.catch #(js/console.log %)))]
      (.close browser))

See the puppeteer example for the full code.

Since v0.0.36, nbb includes promesa which is a library to deal with promises. The above plet macro is similar to promesa.core/let.

Startup time

$ time nbb -e '(+ 1 2 3)'
nbb -e '(+ 1 2 3)'   0.17s  user 0.02s system 109% cpu 0.168 total

The baseline startup time for a script is about 170ms seconds on my laptop. When invoked via npx this adds another 300ms or so, so for faster startup, either use a globally installed nbb or use $(npm bin)/nbb script.cljs to bypass npx.


NPM dependencies

All NPM libraries loaded by a script are resolved relative to that script. When using the Reagent module, React is resolved in the same way as any other NPM library.

Clojure dependencies

To load dependencies from the Clojure ecosystem, you can create an nbb.edn:

{:deps {com.github.seancorfield/honeysql {:mvn/version "2.2.868"}}}

Similar to node_modules, nbb will unpack these dependencies in an .nbb directory and will load them from there.


To load .cljs files from local paths or dependencies, you can use the --classpath argument. The current dir is added to the classpath automatically. So if there is a file foo/bar.cljs relative to your current dir, then you can load it via (:require [ :as fb]). Note that nbb uses the same naming conventions for namespaces and directories as other Clojure tools: foo-bar in the namespace name becomes foo_bar in the directory name.

Current file

The name of the file that is currently being executed is available via nbb.core/*file* or on the metadata of vars:

(ns foo
  (:require [nbb.core :refer [*file*]]))

(prn *file*) ;; "/private/tmp/foo.cljs"

(defn f [])
(prn (:file (meta #'f))) ;; "/private/tmp/foo.cljs"


Nbb includes reagent.core which will be lazily loaded when required. You can use this together with ink to create a TUI application:

$ npm install ink


(ns ink-demo
  (:require ["ink" :refer [render Text]]
            [reagent.core :as r]))

(defonce state (r/atom 0))

(doseq [n (range 1 11)]
  (js/setTimeout #(swap! state inc) (* n 500)))

(defn hello []
  [:> Text {:color "green"} "Hello, world! " @state])

(render (r/as-element [hello]))

Working with promises


Working with callbacks and promises can become tedious. Since nbb v0.0.36 the promesa.core namespace is included with the let and do! macros. An example:

(ns prom
  (:require [promesa.core :as p]))

(defn sleep [ms]
   (fn [resolve _]
     (js/setTimeout resolve ms))))

(defn do-stuff
   (println "Doing stuff which takes a while")
   (sleep 1000)

(p/let [a (do-stuff)
        b (inc a)
        c (do-stuff)
        d (+ b c)]
  (prn d))
$ nbb prom.cljs
Doing stuff which takes a while
Doing stuff which takes a while

Also see API docs.


In the REPL it can be convenient to bind the resolved value of promises to a var. You can do that like this:

(defmacro defp [binding expr]
  `(-> ~expr (.then (fn [val]
                     (def ~binding val)))))

(defp browser (.launch puppeteer #js {:headless false}))
(defp page (.newPage browser))
(.goto page "")


Since nbb v0.1.0 cljs-bean is available.

See the example for an example.


Since nbb v0.0.75 applied-science/js-interop is available:

(ns example
  (:require [applied-science.js-interop :as j]))

(def o (j/lit {:a 1 :b 2 :c {:d 1}}))

(prn (j/select-keys o [:a :b])) ;; #js {:a 1, :b 2}
(prn (j/get-in o [:c :d])) ;; 1

Most of this library is supported in nbb, except the following:

  • destructuring using :syms
  • property access using .-x notation. In nbb, you must use keywords.

See the example of what is currently supported.

Reader conditionals

Nbb supports the following reader conditional features: :org.babashka/nbb and :cljs in that order of priority:

#?(:org.babashka/nbb 1 :cljs 2) ;;=> 1
#?(:cljs 2 :org.babashka/nbb 1) ;;=> 2

Main function

It is possible to use the -main function as the software (script) start point when using the m parameter of nbb passing your software namespace.

(ns example)

(defn -main
  [& args]
  (prn "print in -main"))


nbb -m example


See doc/testing.


Console REPL

To start a console REPL, simply run nbb.

Socket REPL

To start a socket server REPL, run:

$ nbb socket-repl :port 1337


Nbb exposes the nbb.repl namespace to programmatically start a REPL. See API for more info. An example:

(ns example
  (:require [nbb.repl :as repl]
            [promesa.core :as p]))

(defn available-in-repl [] :yolo)

 ;; type (available-in-repl) in the REPL and it will return :yolo
 (println "The end"))

The repl function returns a promise. The promesa.core/do! macro waits for the REPL to finish and after that "The end" is printed:

$ nbb example.cljs
example=> (available-in-repl)
example=> The end

To launch a REPL from a Node.js script, you can use loadString or loadFile:

import { loadString } from 'nbb'
await loadString(`
(require '[nbb.repl :refer [repl]])
console.log('The end!')
$ node repl.mjs
user=> (+ 1 2 3)
user=> The end!


The nREPL server probably still has rough edges. Please report issues here.

An nREPL server can be started with:

$ nbb nrepl-server :port 1337

After that you can connect using an nREPL client:

$ lein repl :connect 1337

and evaluate expressions.

Running nREPL in Docker container is supported with the optional :host argument.

$ nbb nrepl-server :port 1337 :host


In Calva connect to the REPL with:

  • Connect to a Running REPL Server not in Project > ClojureScript nREPL server


Use cider-jack-in-cljs as usual to start the nbb nREPL server from within an nbb project

or start an nREPL server from the command line with

$ nbb nrepl-server

and use cider-connect-cljs with a ClojureScript REPL type of nbb to connect to it.

CIDER prior to v1.6.0, needs the following workaround.

See also this article by Benjamin Scherdtner.

Vim Iced

See this tweet.


You can programmatically start and stop an nREPL server through:

(require '[nbb.nrepl-server :as nrepl])
(nrepl/start-server! {:port 1337})

In a JavaScript project you can do the above through:

import { loadString } from 'nbb'

globalThis.inspectMyProcess = () => {
  return {version: process.version};

await loadString(`

(require '[nbb.nrepl-server :as nrepl])
(nrepl/start-server! {:port 1337})


If you calling this from a CommonJS module, you can use dynamic import:

async function nREPL() {
  const { loadString } = await import('nbb');
  await loadString(`
  (require '[nbb.nrepl-server :as nrepl])
  (nrepl/start-server! {:port 1337})


And then you can connect with an nREPL client:

$ node scratch.mjs &
nREPL server started on port 1337 on host - nrepl://

$ lein repl :connect 1337
Connecting to nREPL at
user=> js/process.argv
#js ["/Users/borkdude/.nvm/versions/node/v17.8.0/bin/node" "/Users/borkdude/dev/nbb/scratch.mjs"]
user=> (js/inspectMyProcess)
#js {:version "v17.8.0"}

Projects using nbb

The following projects are using nbb or are supporting it as a development platform:



See API documentation with a list of built-in Clojure libraries.

Calling nbb from JavaScript

You can load nbb from JavaScript. Exposed functions are loadFile, loadString, addClassPath, getClassPath and printErrorReport.

An example:


(ns example)

(defn foo [] "Hello")

;; this JS object is the return value of loadFile:
#js {:foo foo}


import { loadFile } from 'nbb'

// destructure JS object returned from .cljs file:
const { foo } = await loadFile('example.cljs')

// execute the foo function

Printing errors

Here's an example of how to print errors from the JS API:

import { loadString, printErrorReport } from 'nbb'

try {
  await loadString(`(assoc :foo :bar)`)
catch (e) {




Migrating to shadow-cljs

See this gist on how to convert an nbb script or project to shadow-cljs.

Publishing an nbb project to npm

See Publishing an nbb project to npm.

Creating a standalone executable with caxa

See Creating a standalone executable with caxa.

Nbb on AWS Lambda

See Nbb on AWS Lambda.

Nbb on Google Cloud Functions

See Nbb on Google Cloud Functions.


Nbb on

See Deploying an nbb app to



  • babashka >= 0.4.0
  • Clojure CLI >=
  • Node.js 16.5.0 (lower version may work, but this is the one I used to build)

To build:

  • Clone and cd into this repo
  • bb release

Run bb tasks for more project-related tasks.


  • Original babashka logo by Nikita Prokopov. Node.js modifications by MnRa.


Copyright © 2021-2022 Michiel Borkent

Distributed under the EPL License. See LICENSE.