REPL and Evaluation

This reference is intended for people who would like to launch REPLs programmatically from Clojure for some reason. The recommended way to start REPL is documented in the Quick Start.

One of the reasons for creating ClojureScript is that JavaScript reaches. There are many interesting environments in which JavaScript can run. Each of these environments has something unique about it. One of the reasons that Clojure rocks is that it has a REPL which gives developers the most dynamic development experience possible. We would like to support this dynamic development experience in every environment where JavaScript runs. To accomplish this, we have created an abstraction around the JavaScript environment and disconnected the REPL from any particular implementation. This gives the REPL the same reach as JavaScript as well as allowing evaluation environment implementations to be used independently for things like automated testing and cross-environment testing.

Most projects will target a specific environment. These changes will allow developers to have the full benefit of a REPL in their target environment. Currently there are implementations for multiple environments. By implementing one protocol, one may easily support additional environments.

Using the REPL

The basic usage of the REPL is always the same:

  1. require cljs.repl

  2. require the namespace which implements the desired evaluation environment

  3. create a new evaluation environment

  4. start the REPL with the created environment

Using the REPL will also feel the same in each environment; forms are entered, results are printed and side-effects happen where they make the most sense.

Using the browser as an Evaluation Environment

A browser-connected REPL works in much the same way as a normal REPL: forms are read from the console, evaluated and return values are printed. A major and useful difference from normal REPL usage is that all side-effects occur in the browser. You can show alerts, manipulate the dom and interact with a running application.

There is a sample project under samples/repl which shows how to set up a minimal browser-connected REPL. The example below will walk through doing the same thing, step-by-step.

The first step is to create the browser side of the connection. This is done by requiring one file and adding one line of code, as shown below in a file named foo.cljs.

(ns foo
  (:require [clojure.browser.repl :as repl]))
(repl/connect "http://localhost:9000/repl")

The most interesting use case for a browser-connected REPL is to connect it to a project and use the REPL to drive and develop an application while it is running. To accomplish this, add the code above to any namespace in the project.

Next, compile the file in either development mode or with simple optimizations. No advanced optimizations please.

./bin/cljsc foo.cljs > foo.js

Create a host html page named index.html like the one shown below.

    <meta charset="UTF-8">
    <title>Browser-connected REPL</title>
    <div id="content">
      <script type="text/javascript" src="out/goog/base.js"></script>
      <script type="text/javascript" src="foo.js"></script>
      <script type="text/javascript">

There is nothing different about this and what one would do for any other browser-based ClojureScript project.

Start the REPL using the pattern described above, but with the browser as the evaluation environment.

(require '[cljs.repl :as repl])
(require '[cljs.repl.browser :as browser])  ;; require the browser implementation of IJavaScriptEnv
(def env (browser/repl-env)) ;; create a new environment
(repl/repl env) ;; start the REPL

Once the REPL has started, you will see the message "Starting server on port 9000". At this point, open the html page by going to http://localhost:9000 to complete the connection. Once the page is open and the connection is made, the REPL prompt will be displayed.

Port 9000 is the default. Notice that we point the browser to this port in the client code above. To use a different port, pass a :port option when creating a new evaluation environment.

(def env (browser/repl-env :port 8090)) ;; listen on port 8090

Just in case you can’t think of anything interesting to do, here are some ideas.

;; the basics
(+ 1 1)
(:a {:a :b})
(reduce + [1 2 3 4 5])
(defn sum [coll] (reduce + coll))
(sum [2 2 2 2])

;; load a ClojureScript file and use it
(load-file "clojure/string.cljs")
(clojure.string/reverse "ClojureScript")

;; browser specific
(js/alert "I am an evil side-effect")

(ns test.dom (:require [clojure.browser.dom :as dom]))
(dom/append (dom/get-element "content")
            (dom/element "ClojureScript is all up in your DOM."))

;; load and use goog code we haven't used yet
(ns test.crypt (:require [goog.crypt :as c]))
(c/stringToByteArray "ClojureScript")

(load-namespace '

There is currently no require function but ns forms can be used to load, require and alias new namespaces. The functions load-file and load-namespace can be used to load code with any environment and are described in more detail below.

Browser-connected REPL Options

There are currently two options which may be used to configure the browser evaluation environment.

  • :port set the port to listen on - defaults to 9000

  • :working-dir set the working directory for compiling REPL related code - defaults to ".repl"

Loading code

The code above shows examples of three ways to load code into an evaluation environment: load-file, load-namespace and within a ns form. load-file is the most low level method of loading code. It may only be used to load ClojureScript files. It will compile them and evaluate the compiled JavaScript. load-namespace loads any file, ClojureScript or JavaScript, with all of its dependencies, which have not already been loaded, in dependency order. When a namespace is required in an ns form, each required namespace will be loaded using load-namespace.

These functions are available in every evaluation environment.

Auto-loaded user code

When a REPL starts, it automatically loads any user.cljs or user.cljc file present on your classpath. This is an ideal location to place code that is useful for development time.

The file may optionally contain an ns form in order to load required namespaces or to establish the namespace for any def forms that appear in the file.

If no namespace is specified, cljs.user is assumed.


If you would like to work on this code then the following notes about implementation will be helpful.


  • No additional dependencies

  • Should work now in all browsers

  • Security is a non-goal, this is for development and testing

The IJavaScriptEnv Protocol

To create a new environment, implement the IJavaScriptEnv protocol.

(defprotocol IJavaScriptEnv
  (-setup [this opts])
  (-evaluate [this filename line js])
  (-load [this ns url])
  (-tear-down [this]))

setup and tear-down do any work which is required to create and destroy the JavaScript evaluation environment. These functions will have side-effects and will return nil.

evaluate takes a file name, line number and a JavaScript string and evaluates the string returning a map with the keys :status and :value. The value of status may be :success, :error or :exception. :value will be the return value or an error message. In the case of an exception, there may be a :stacktrace key containing the stack trace.

The load function takes a list of namespaces which are provided by a JavaScript file and the URL for the file and will load JavaScript from the given URL into the environment. The implementation is not responsible for ensuring that each namespace is loaded once and only once, as this is managed by the infrastructure.

Browser as Evaluation Environment

To create the browser-connected REPL and meet the goals described above, we use long-polling and Google’s CrossPageChannel. Long-polling allows us to treat the browser as the server and CrossPageChannel helps us get around the same-origin policy.

The model for a browser-connected REPL is that the REPL is the client and the browser is the server which evaluates JavaScript code. How do we implement this without resorting to WebSockets? If we think of the connection as a series of messages being passed between the browser and the REPL, and we ignore the first message sent from the browser, then we have what we need. When the browser initially connects, the REPL will hold that connection until is has something to send for evaluation. Once the next form is read and compiled, it will be sent to the browser using that saved connection. The browser will evaluate it and send the result with a new connection. And the cycle repeats…​

Browsers enforce a same-origin policy for JavaScript code. This means that the JavaScript which is evaluated in a page can come from only one origin domain. This is a problem for the browser-connected REPL because FireFox and Chrome both view opening a file from the file system and connecting to localhost:9000 as different domains. It may also be a valid use case to want to connect to an application served from a totally different domain, which would be prohibited in all browsers.

Fortunately, Google has also run into this problem and has created something called a CrossPageChannel. Without going into the details, this allows an iframe served from one domain (the REPL) to communicate with the parent page which was served from another domain (the application server). This is accomplished in a way that is supported by all modern browsers.

Code loading in the Browser

Google Closure has a technique for loading dependencies. It uses a dependency file to create a dependency graph and to map namespaces to files. The ClojureScript build function creates this kind of dependency file when compiling a project in development mode. Google Closure makes the assumption that everything that needs to known about dependencies will be known when the application starts. This assumption is not valid when using a REPL and leads to two limitations.

The first limitation is that all dependencies need to be included in these files before the application starts. We cannot add new dependencies later for new ClojureScript or JavaScript namespaces that we would like to use.

Another limitation is that Google’s method of loading dependencies assumes that all dependencies will be loaded when the application starts. The implementation of goog.writeScriptTag_ uses document.write to add new script tags to a page. This works when it is used during the initial page load but if used after the page is loaded, it will remove the document’s content. This means that even if the dependency file contains the dependency that we would like to load, it cannot be loaded. This can be fixed. See for an example.

The ClojureScript REPL already has a load-file function which can be used to load a single ClojureScript file. This function does not account for dependencies and cannot be used to load third-party JavaScript files.

This suggests that we need one unified way to load things which will work for anything that we may want to load. The load-namespace function was created for this purpose. It uses the build system to calculate all dependencies for the given namespace. This includes anything that we can currently build into a project: ClojureScript files, JavaScript files as well as third-party ClojureScript and JavaScript. Each dependency is then passed to the -load function in dependency order. The -load function is responsible to determining if the namespace has already been loaded and, if it has not, evaluating the JavaScript.

When the REPL compiles a namespace form, it will check for required namespaces and call load-namespace on each of them.

Note: conveying the :libs option to the REPL so that it can find third-party JavaScript libraries has not yet been implemented.