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Swank backend for Node.JS and in-browser JavaScript
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swank-js provides SLIME REPL and other development tools for in-browser JavaScript and Node.JS. It consists of SWANK backend and accompanying SLIME contrib. Socket.IO is used to communicate with wide range of web browsers.


From my experience an ability to use REPL for JavaScript debugging and interactive development is very helpful when developing Web applications. Previously I was using a heavily patched MozRepl version that was adapted for in-browser JavaScript. Primary downsides of that approach were extreme instability of communication between Emacs and the browser, the lack of cross-browser support and the lack of good RPC between Emacs and JS that can be used to develop some handy extensions.

I knew there exists slime-proxy project that provides such functionality for Parenscript. The problem is that most of us including myself can't use Lisp all the time and a lot of code needs to be developed using languages like plain JavaScript (as opposed to Parenscript), Python and so on. My first thought was to adapt slime-proxy for use with plain JS, but then I decided to roll my own SWANK backend using Node.JS. I wanted to find out what this buzz around Node.JS is about and perhaps steal an idea or two from there for use in my Lisp projects. Another reason was availability of Socket.IO and an example of tiny http server proxy.

Some people may prefer Firebug or built-in browser development tools to Emacs-based development, but for example in case of mobile browsers you don't have much choice. At some point I did try swank-js with an colleague's iPhone and it worked, which is not too unexpected given that Socket.IO has good cross-browser support.


As of now swank-js provides REPL with an ability to work with multiple browser connections, supports dynamic updates of JavaScript code using C-c C-c / C-M-x, provides debug output function and an ability to reload web pages in the browser or refresh their CSS using Emacs commands.

Many aspects of full-fledged SWANK backends aren't implemented yet, there's no debugger/completion/autodoc and so on, but as I plan to use swank-js a lot in future there's a good chance many of these features will be eventually added.


  1. Install Node.JS, npm and then Socket.IO:

    npm install
  2. Get recent SLIME from its CVS or the git mirror.
  3. Make sure you have latest js2-mode. Add it to your .emacs:

    (add-to-list 'load-path "/path/to/js2-mode/directory")
    (autoload 'js2-mode "js2-mode" nil t)
    (add-to-list 'auto-mode-alist '("\\.js$" . js2-mode))
  4. Create symbolic link to slime-js.el in the contrib subdirectory of SLIME project.
  5. In your .emacs, add the following lines (you may use other key for slime-js-reload; also, if you're already using SLIME, just add slime-js to the list of contribs, otherwise adjust the load-path item):

    (add-to-list 'load-path "/path/to/slime/installation")
    (require 'slime)
    (slime-setup '(slime-repl slime-js))
    (global-set-key [f5] 'slime-js-reload)
    (add-hook 'js2-mode-hook
              (lambda ()
                (slime-js-minor-mode 1)))
  6. If you're using CSS mode, you may want to add the following lines too:

    (add-hook 'css-mode-hook
              (lambda ()
                (define-key css-mode-map "\M-\C-x" 'slime-js-refresh-css)))


Start swank-js with the following command in the project directory:

node swank.js

Make SLIME connect to the backend using M-x slime-connect and specifying localhost and port 4005. You will see REPL buffer with the following prompt:


This means that you're currently talking to Node.JS. You may play around with it by running some JavaScript expressions.

If you get warning about SLIME version mismatch, you may make it disappear until the next SLIME upgrade by typing ,js-slime-version at the REPL and entering your SLIME version (e.g. 2010-11-13).

Point your web browser to


You will see the following message appear in the REPL (browser name and version may differ):

Remote attached: (browser) Firefox3.6:

This means that the browser is now connected. Several browsers can connect simultaneously and you can switch between them and Node.JS REPL using ,select-remote REPL shortcut. To use it, press ',' (comma) and type select-remote (completion is supported). You will see "Remote:" prompt. Press TAB to see completions. Select your browser in the list by typing its name or clicking on the completion. The following message will appear:

Remote selected: (browser) Firefox3.6:

After that, you can interactively evaluate expressions in your browser. To go back to Node.JS repl, switch back to node.js/direct remote.

FIREFOX-3.6> document.body.nodeName
FIREFOX-3.6> alert("test!")

When working with browser, you may use F5 to reload the page. swank-js connection with browser is lost in this case, but to solve this problem you may use ,sticky-select-remote instead of ,select-remote. This way swank-js will remember your selection and automagically attach to the browser whenever it connects. If you press F5 after using ,sticky-select-remote, you will see that browser briefly disconnects but then connects again:

Remote detached: (browser) Firefox3.6:
Remote selected (auto): (direct) node.js
Remote attached: (browser) Firefox3.6:
Remote selected (auto): (browser) Firefox3.6:

The sticky remote selection is saved in the config file, ~/.swankjsrc, so you don't need to do ,sticky-select-remote after restarting the browser.

Now, let's try to make it work with an actual site. swank-js acts as a proxy between your browser and the site so it can inject necessary script tags into HTML pages and avoid cross-domain HTTP request problems. Let's point it to reddit. Type ,target-url and then (www. part is important, otherwise it will redirect to skipping the proxy). Point your browser to http://localhost:8009, you'll see reddit frontpage load. If you didn't do ,select-remote or ,sticky-select-remote yet do it now and select your browser from the list of remotes. Now you can execute JavaScript in the context of reddit:

FIREFOX-3.6> $(".sitetable a.title").map(function(n) { return (n + 1) + ". " + $(this).text(); }).get().join("\n")
1. Wikileaks currently under a mass DDOS attack
2. Munich University - Jealous

Let's make a function from it. Create a file test.js somewhere and make sure it uses js2-mode (if it doesn't, switch it to js2-mode using M-x js2-mode). Type the following there:

function listRedditTitles () {
  $(".sitetable a.title").map(
    function (n) {
      SwankJS.output((n + 1) + ". " + $(this).text() + "\n");

Note SwankJS.output() function being used there. It allows you to send debug print to SLIME REPL.

Move the point somewhere into the middle of the listRedditTitles() function and press C-M-x. Now you may try it out in the REPL:

FIREFOX-3.6> listRedditTitles()
1. Wikileaks currently under a mass DDOS attack
2. Munich University - Jealous

You may edit the function definition and update it using C-M-x any number of times.

Now let's try some CSS hacking. Create a directory named zzz and start a Web server in it from your command prompt:

$ mkdir zzz && cd zzz && python -mSimpleHTTPServer

Create a file named a.css there and make sure it uses css-mode (like with js2-mode, you can force it with M-x css-mode). Add some CSS rules to this file:

body {
    background: green;

Now let's add the stylesheet to the reddit page:

FIREFOX-3.6> $('head').append('<link rel="stylesheet" href="http://localhost:8000/a.css" type="text/css" />');
[object Object]

You will see some parts of the page become green. Now, change green to blue in the CSS file and press C-M-x (it will save the file automatically):

body {
    background: blue;

You will see the page become blue, maybe after some flickering as all CSS used on the page is reloaded. This way you may update CSS in an AJAX application without reloading the page, which is often rather handy. Unlike editing CSS in Firebug in case when you're editing CSS of your own application changes will not disappear upon page reload (with reddit page you'll have to readd the stylesheet).


I've noticed that flashsocket Socket.IO transport does exhibit some instability. You may want to try other transports by changing the socketio cookie, e.g.:

document.cookie = "socketio=xhr-polling"

Be careful not to lose connection to the browser though especially in case of REPL-less browser like IE6/7 or you'll have to type something like

javascript:void(document.cookie = "socketio=flashsocket")

in the address bar.

In case of IE, increasing the maximum number of HTTP connections may help with non-Flash transports, although I didn't try it yet. To do it add DWORD value MaxConnectionsPer1_0Server to the following registry key:

HKEY\_CURRENT\_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings


swank-js is distributed under X11-style license. See LICENSE file.

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