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In Javascript environments where a Firebug like tool is not available, repl-server can be used to get a REPL. It requires two things from the Javascript environment:

  • An HTTP client like XMLHttpRequest
  • A JSON parser/serializer

It should work with any modern (web or mobile) browser, and it also works with Titanium Mobile and Phonegap applications. It can also work with any other Javascript environment, provided the above two requirements are met.

repl-server has two components:

  • A CL based (web) server, which also provides the REPL.
  • A javascript file (repl-client.js) which needs to be included in the application

Features include:

  • Syntax checker, thanks to parse-js
  • Colored output on ANSI terminals
  • Special variables like _ for quick access to previously returned values
  • Load javascript code from local files using the //load command

The features section below explains some of these in more detail. There's also a screenshot:

repl-server screenshot


Check out the source code from github:

git clone git://


Common Lisp

Setting up a Common Lisp environment for repl-server requires three things:

  1. A CL compiler. e.g. SBCL (Linux) or Clozure CL (OS X or Windows).

  2. Quicklisp is required to download Common Lisp libraries.

  3. Finally, there are the libraries which repl-server requires. Quicklisp will download them automatically the first time you load repl-server.

Getting Started

Running the server

The quick and easy way to run the repl-server is to load example-launch.lisp in your Lisp runtime. Change to the repl-server directory and run the following command:

$ sbcl --load example-launch.lisp

To load Clozure CL, run:

$ ccl --load example-launch.lisp

For the equivalent of --load in another CL runtime, check its manual.

The first time you do this, Quicklisp will ask your permission to download various Common Lisp libraries which are required to run repl-server. If everything goes well, you should see a REPL> prompt on your terminal.

Here's what you should do in case you didn't use example-launch.lisp.

In the repl-server directory, fire up your Common Lisp runtime:

$ sbcl

Once you have the CL REPL, load repl-server with this command:

(ql:quickload :repl-server)

The first time you do this, Quicklisp will ask your permission to download various Common Lisp libraries which are required to run repl-server.

Once the required libraries are downloaded, and the repl-server source code is loaded, you start the HTTP server:


By default, the server listens on port 8000. To make it listen on another port, you can pass the port number:

(repl-server:start-server 9000)

Now, start the javascript REPL:


Installing the javascript client

Copy src/repl-client.js into your web/javascript application. After it is loaded, you can start the ReplClient by passing it the URL of your REPL server:'http://localhost:8000');

To stop communicating with the REPL server, call stop:



Special variables

Inspired by the Common Lisp variable * and friends, repl-server defines a few variables which provide similar functionality in Javascript. Specifically, the following variables contain:

  • _: the value returned by the last expression evaluated at the REPL

  • __: the value of the second last expression evaluated at the REPL

  • ___: the value of the third last expression evaluated at the REPL

  • _$: the last expression evaluated at the REPL (returned as a string)

  • _$$: the second last expression evaluated at the REPL

  • _$$$: the third last expression evaluated at the REPL

For example, if the last three forms and their results at the repl look like this:

REPL> x = "foo";

REPL> y = "bar";

REPL> x + y;

Then the values returned by the following forms (entered one after the other) will look like this:

REPL> [_, __, ___];

REPL> [_$, _$$, _$$$];
["[_, __, ___];","x + y;","y = \"bar\";"]


Commands are special forms (which you type at the REPL) which are treated differently from Javascript forms. A command starts with //. This is followed immediatedly by the command name. A command can have one or more arguments. Arguments are delimited by whitespace. They can also be delimited by double quotes in case you want to pass an argument containing whitespace(s).

//command arg1 arg2 ...

Currently, the following commands are defined:

  • //quit: Quit the current REPL

  • //version: Returns the server version. Note: if the version of repl-client.js differs from that of the server, repl-server gives a warning when a client is connected.

  • //load: Takes one argument -- the path to a .js file on the local machine. This contents of the entire file are eval'd.

  • //pwd: Returns the present working directory. This gives the default directory for the //load command.

  • //cd: Takes one argument which should be a directory. This will updated your pwd.


On ANSI terminals, repl-server can use colors to differentiate between different display elements. You can define colors and styles for the following:

  • The REPL prompt (:repl)
  • The value returned by evaluating a form (:result)
  • Informational messages (:info)
  • Warnings (:warn)
  • Errors (:error)

You can define these colors using the Lisp macro repl-server:defcolor. Evaluate (repl-server:define-color type ...) before starting the REPL. Some examples:

(repl-server:defcolor :repl :style :bright)
(repl-server:defcolor :result :fg :white)
(repl-server:defcolor :info :bg :cyan)
(repl-server:defcolor :warn :fg :yellow)
(repl-server:defcolor :error :style :bright :fg :red)

In the above forms, :fg stands for foreground and :bg stands for background. The available colors are:

  • :BLACK
  • :RED
  • :GREEN
  • :YELLO
  • :BLUE
  • :VIOLE
  • :CYAN
  • :WHITE

The available styles are:

  • :DIM


A REPL for javascript enviroments






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