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Sunday, December 06, 2009

mjst - Micro JavaScript Template Engine

This is the content I've written so far in the official mjst repository. I'd love to know your thoughts about this little experiment, cheers!


mjst is a lightweight, about 1Kb minified and gzipped, and performances focused template engine with standards support. There are dozens JavaScript? Template Engines solutions out there but as far as I know this is the only one based on validation for both client and server side programming languages.


There is one single function to call:

// via text (e.g. Ajax or innerHTML from a template node)
var transformed = mjst('<?js var hello = "mjst!"; ?><js-hello/>');

// classic template node example
<script id="mytpl" type="text/html">
var hello = "mjst!";
myEl.innerHTML = mjst(document.getElementById("mytpl"));

/ via XML (e.g. Ajax or a created document)
// this is a configuration object as second argument example as well
.innerHTML = mjst(
new DOMParser().parseFromString(
'<root><js-hello/><br class="${myBrClass}" /></root>',
), {
:"Hi there!",

// Ajax example with an XML template as response
      for(var i = 0; i < collection.length; ++i) {
      <li class="li-${i}">Item No: <js-i/> Name: <?js print(collection[i]); ?></li>

// transformation ...
.innerHTML = mjst(xhr.responseXML, {collection:["a", "b", "c"]});

It is possible to write whatever we want inside an mjst template block via print(1, 2, 3, N, func(), whatever) function, the only one injected in the execution scope.

JavaScript? block VS Attributes

To make mjst templates XML compatible I decided to remove JavaScript? block inside attributes. It is still possible to access JavaScript? variables via ${myVar.propName} as showed, as example, in the br node class.

PHP And JavaScript? Template Example

<?php // mjst example
function mjst($name, $value){
'<?js var ', $name, '=', json_encode($value), ' ?>';
header('Content-Type: text/xml');
$a = array(
    'some database result' => array(1, 2, 3),
    'some other value' => 'Hi There'
<?xml version="1.0"?>
<?php mjst('result', $a); ?>
for(var k in result)
print(result[k], "<br />")

<script type="text/javascript" src="mjst.js"></script>
<script type="text/
= function(){
with(this.XMLHttpRequest ?
new XMLHttpRequest :
new ActiveXObject("Microsoft.MSXML")
("get", "mjst.php", true);
= function(){
if(readyState == 4)
.body.innerHTML = mjst(responseXML);

Why Standards

Apparently every other JavaScript? template engine is implementing a manual, char by char, or RegExp? based, parser, over a syntax that in PHP world has basically defined deprecated ages ago: the classic <%=stuff%> (<?=stuff?> in PHP). The first reason to avoid this kind of syntax is ambiguity. Which program language should consider that block of code? PHP, ASP, JavaScript? ... who else? Moreover, to make things as clear and simple as possible, avoiding ambiguity, PHP community decided to solve the classic <?xml?> problem, caused by short code block, via an explicit <?php ?> which is the suggested, default one. This is also standards safe, in the meaning that a nodeType 7 is defined as a PROCESSING_INSTRUCTION_NODE In few words mjst is compatible with both valid XML and XHTML, which means that its templates can be created via standard DOM API being sure the template will be valid as well. At the same time, using official engines to validate and parse templates blocks mjst could be considered more reliable than other solutions, more robust, and at the same time fast, thanks to core functionality. The good part, at least for me, is that delegating templates parsing operations to the browser core, is more probable that the error is inside a non valid template block, rather than inside this tiny library source code: a welcome side-effect about standards and core validation.

Why Fast

Every transformation could create one or two pre-compiled functions reused every time we would like to transform an already parsed template block. The reason mjst creates up to two functions is that we can arbitrary send a second argument as configuration object. Since this procedure requires the usage of the with statement, avoiding outer scope name conflicts during the execution, and since we do not necessary need to send this second configuration object, the runtime compiled function will consider both cases, only when encountered, making the with free execution 3 to 10 times faster. In other words, if we transform the same template block always without the second argument, the function that will contain the with statement will never be created and vice versa.

Why Robust

The XML core parser and transformer is also another reason to consider mjst both fast and reliable. As example, probably the most famous JavaScript micro templating, from John Resig, is surely lightweight, but it could suffer some problem if the template block is not perfect. For instance, try to use a string like s = "<%" for whatever reason, and you'll be able to break the John micro suggestion, but this is not only about John code. Another well known product is the EJS library, adopted in many different environments and definitively a good piece of code. EJS offers somethng more than mjst but it is still based on manual parsing over a syntax that XML does not like that much. This means that we could have hard life to generate EJS templates via server or directly in our page while mjst delegates template validity to the markup itself.

<?xml version="1.0" ?> 
<script id="mytpl" type="text/html">
<!-- msjt accepts valid XML (as XHTML as well) or HTML5 -->
var hello = "mjst is here!";
<js-hello />

Why ... Generally Speaking

I am a PHP developer, before being a JavaScript? one, and I have always complained about the open close everythere PHP style. This technique does not split presentation and business logic layers and it is generally "slower than a single echo". I have started to appreciate the couple XML and XSL-T years ago but XSL-T sometimes is just a pain in the ass and via server is difficult to cache XSL-T instances. As example, the New York Time had to create their own core library to speed up transformations over common XSL files allowing fast serialization. Brilliant, but still this ad hoc library does not scale for the whole World Wide Web scenario. Different hosts, different PHP versions, but at the end of the day, what could happen if we delegate the transformation to the client side via core JavaScript? features? Nothing bad, actually it's almost a year I am transforming complex layout into html in the fastest way ever even in IE6 and only thanks to XSL-T but, as I have said, the XSL syntax could not sound familiar at all, specially for programmers that would like to have much more power while they generate a layout. This is basically the reason PHP had such great success over these years, so why don't try to emulate XSL bringing there directly JavaScript? rather than XSL syntax, and in a PHP familiar style? This is all about this experiment. It delegates layout render into the client side, the best place imho to do this. It avoids useless server side stress, it could be embed in the page thanks to the fake script with an unknown type, and as side effect, it could speed up client server interactions re-using templates blocks whenever we need and requiring only data, which nowadays, it is just all we need to make Web users happy. So, as pure final evil plan, we could create a view able to produce mjst templates directly via CouchDB resolving some greedy map reduce operation in the db moving partial logic in the client via simple row data sets or generating clear layout directly via Server Side JavaScript? so that all we need is a language, rather than 3 different technologies ... does it sound insane? ;-)


Oleg Slobodskoi said...

Hi Andrea,

just a small mistake in your code - you have to define window argument in the outside closure :)

Interesting idea !!!

Andrea Giammarchi said...

lol ... fixed, cheers, it's in the repo I need to test latest changes but ... well, you know where to put it :D

Unknown said...

Nice ideas Andrea, looking forward to diving into the source (esp. for those gems about XML handling in JS). Have you considered making this CommonJS compliant so frameworks such as JuiceJS/Node.js/Persevere can use it?

On another note, just curious to know why you haven't hopped on the github bandwagon like everyone else - much prefer it as a VC system (and workflow) - there's substance to the hype!

Keep up the great work, you always have very interesting ideas that challenge the status quo!

Andrea Giammarchi said...

I am testing Aptana Studio and Subversion was the quickest/simplest to handle. No idea yet how to use the Github plugin but as soon as I'll find out I'll probably move into Github.
Nice idea about CommonJS, I'll have a look ASAP and probably make mjst compatible since ideally I would like to make it usable in every kind of JS environment, server or client.