Twitter About Home

Rich Javascript MVC user interfaces with jMVC

Update: jMVC is now integrated with ASP.NET – Check out details and demos here

Published Oct 4, 2007

So we can build clean, responsive user interfaces with the MVC design pattern using client-side XSLT. Nice. Now let’s imagine something much better.

What if we could take plain Javascript objects as our model (perhaps transferred to/from the server in JSON form), and translate them into a HTML user-interface using a clean, fast, and familiar templating language? A language as familiar as… Javascript itself?

Perhaps we could even set up a simple framework that would let us route events in the UI (such as button presses) into our own Javascript handler functions, and then after we update the model, it automatically refreshes the UI…

Introducing jMVC

It’s really simple. It’s a templating engine written in less than 100 lines of Javascript (excluding comments). jMVC provides a templating language which is actually Javascript, so you don’t have to learn any new syntax.

It uses ASP-style <% code %> syntax, like this:

Here are some items from an array:
   <% for(var i = 0; i < model.items.length; i++) { %>
      <li><%= model.items[i].name %></li>
   <% } %>

There are only three syntactical constructs (plus everything you can normally do in Javascript):

  • Code blocks, which you’ll use mostly for looping, or if/else constructs, like this:
<% if(model.cart.count > 0) { %>
    You have some stuff in your cart.
<% } else { %>
    You have nothing in your cart.
<% } %>
  • Evaluations, used to insert a value from your model into the UI, like this:
Hello <%= %>, the time is <%= new Date() %>.
  • Closures (these are the fancy ones), used to attach Javascript code to events in the dynamically-generated user interface, like this:
<input type="button"
       onclick="<%* function(currentItem) { deleteItem(currentItem) } %>" />

Simple, eh?

Notice the last one – closures – because this is the only bit that isn’t completely obvious. While code blocks and evaluations run at template-processing time, the closures run later, for example when the user clicks a button or link. The magic <%</span> function() { ..code..* } %> syntax (notice the asterisk) means, “Here’s a block of code, don’t run it now, just register it to this event handler”.

They are closures because you can reference any variable from your model that’s in scope at the time. This is really helpful when your UI contains lists with buttons next to each item, as you can pass in the relevant item as a parameter to the event handler.

After your closure runs (during which you’ll probably update the model), the UI is automatically refreshed – that’s one less thing to think about.

Online demos

Just to prove it works, here are two demos. They will launch in a new window. They’re deliberately kept simple to demonstrate the method, so don’t expect fancy graphics.

Simple list-editing demo – download the source code to see how easy this is.

Recursive controls demo – this replicates the demo given for the XSLT method, but shows how jMVC is much more streamlined. Source code

Integrating with ASP.NET

… or another server-side language is pretty easy, because as you will see from the second demo, the JSON model can be kept in a hidden INPUT field or TEXTAREA so it gets posted back to the server. I haven’t explained this in more detail but it is simple, and I will do so if needed.



You can download the full source code version here (~5kb).

There’s also a minified version (i.e. comments & blanks removed) here (~2kb).

At the moment there isn’t any documentation beyond this blog post, the source code, and the demos. It’s under 100 lines of code long, you can probably work it out. Nonetheless, if there is interest, I will write more documentation/tutorials.


You’re free to use and include this in commercial and non-commercial projects, including making your own modifications. You only have to retain the original copyright notice and link in the source code.

Note: it uses (and includes) the json.js library, placed into the public domain by


This is not a completely new idea. Although jMVC provides some unique benefits, there are loads of related projects and frameworks, each with their own advantages. Most are more developed than jMVC.

I like jMVC because it’s so small and simple. The main other benefits, compared to most Javascript templating systems, are:

  • no new, proprietary flow syntax to learn. It’s just Javascript.
  • an incredibly easy event handling mechanism, so you only need to think in MVC terms and the plumbing is done for you

Here are some alternatives you might like to check out:

  • TrimPath is one of the oldest. Unfortunately it has its own nonstandard syntax, but the website explains it well.
  • Helma looks feature-rich, e.g. the concept of sub-templates. Proprietary syntax again though.
  • ZParse might be the most sophisticated one, a bit complex perhaps, but a really nice website and demos.
  • ESTE is perhaps the most similar to jMVC. It also uses real Javascript syntax and keeps the whole business simple. Is this still actively maintained?

Check your XSS filters (Cross-site scripting)

In the last couple of days I’ve tested the effectiveness of XSS filters in two different commercial forum applications, both advertised as being able to filter out malicious scripts. Neither were effectively protected against this:

Published Sep 27, 2007