What is i.js? It is is a browser-based tool that brings the style of computation pioneered by IPython Notebook to those of us who want to use JavaScript.

If you have never seen IPython Notebook before, then you will unlikely find this explanation satisfactory. It seems to me that the easiest way to show what I mean is to quote the original IPython Notebook's documentation:

The [IPython] Notebook [...] is [..] providing a web-based application suitable for capturing the whole computation process: developing, documenting, and executing code, as well as communicating the results. The IPython notebook combines two components:

A web application: a browser-based tool for interactive authoring of documents which combine explanatory text, mathematics, computations and their rich media output.

Notebook documents: a representation of all content visible in the web application, including inputs and outputs of the computations, explanatory text, mathematics, images, and rich media representations of objects.

As of now i.js has the following features:

• Syntax highlight and Code auto-completion
• Basic management of i.js documents (from this point on I will call them 'scrapbooks'): create, delete, rename.
• REPL/server decoupling. Each scrapbook has it's own JavaScript REPL attached. That means that when you use different scrapbooks they do not interfere. It also means that scrapbook evaluation environment is separated from the actual node.js server, so no matter what happens in a REPL session the core i.js server should be up and running.
• Full execution flow control. A scrapbook is essentially a collections of small JavaScript code snippets ('cells') that can be evaluated and edited independently. You have the full control of which cells should be executed and in what order. You can also edit and re-evaluate cells as you see fit, that allows one to run iterative experiments with a super-short feedback cycle.

The following are my reasons for developing i.js:

• Language. As the name implies, IPython Notebook speaks python. Although browser consoles and JS REPL are decent alternatives, I was looking for the IPython Notebook style of iterative computation and ability to run repetitive experiments.
• Matplotlib. IPython Notebook is tightly integrated with matplotlib, which is a great library if you are preparing an article for publication. However I want to render more interactive charts in d3.js. While it seems possible to hack your way through to get d3.js graphics in IPython - it feel convoluted.
• Great model. IPython Notebook is a unique tool that suggests a beautiful paradigm for research and experimentation. It should be made available to the widest audience possible.
• Fun Last but not the least: full-stack JS applications are fun.

As always you do all your data processing on the server. D3 will be executed on the server as well. Once this is done you will receive an SVG on the client and i.js will render it for you.

Here I will use an adopted Mike Bostock's example:

• First of all include d3 library

%init_d3

This is essentially a syntactic sugar for including a node module into the i.js context. Instead of the magic command above one could inject any module dependency like this:

var d3 = require(__modules_dir + 'd3');

• Now get some data

var data = [4, 8, 15, 16, 23, 42];

• Next you will need to create an empty container for your chart:

var container = d3Container();

• From that point on you can do your normal D3 coding, for example you can style your container the way you want:

container.append("style").text(
  ".chart div {\
  	background-color: steelblue; \
  	text-align: right; \
  	padding: 3px; \
  	margin: 1px; \
  	color: white; \
  }");

• Now you can render some bars:

var chart = container.append("div").attr("class", "chart");
var width = function(d) { return d * 10 + "px"; };
var text = function(d) { return d; };
var svg = chart.selectAll("div").data(data).enter().append("div").style("width", width).text(text);

• Your chart is generated on server, and now all you need is just to render the result in your scrapbook, use the special character % for this (it should be the first character in the cell):

%inline
container.render()

See prepackaged d3 example scrapbooks for more details.

i.js is built on top of:

1. node.js + express + jade
2. Bootstrap as you can immediately see from the screenshot above
3. REPL for evaluating scrapbooks and getting code auto-completion hints
4. CodeMirror for code syntax-highlight and auto-completion UI
5. JQuery for browser scripting
6. d3.js for charts rendering.

1. Install node from http://nodejs.org if you have not done it yet
2. Clone i.js from git: git clone https://github.com/mksenzov/i.js
3. Go to the cloned repo: cd i.js
4. Install all i.js dependencies: npm install
5. Start i.js server: node app.js
6. Now you can open i.js in the browser: http://localhost:3000
7. [Optional] To run tests: node_modules/jasmine-node/bin/jasmine-node --verbose spec

Shortcuts

• Use Shift+Enter to evaluate the current cell
• Use Ctrl+Space to auto-complete the code
• Use Ctrl+S (Meta+S) to save the current i.js scrapbook

**Special commands and built-in definitions **

Built-in variables

• __base_dir - is a pre-defined variable pointing to i.js base directory.
• __modules_dir - is a pre-defined variable pointing to directory with i.js module dependencies..

Use any of the following within i.js cells:

• %inline the result of cell evaluation should be rendered as HTML.
• %clear clear the context.