JavaScript Micro-Templating

I’ve had a little utility that I’ve been kicking around for some time now that I’ve found to be quite useful in my JavaScript application-building endeavors. It’s a super-simple templating function that is fast, caches quickly, and is easy to use. I have a couple tricks that I use to make it real fun to mess with.

Here’s the source code to the templating function (a more-refined version of this code will be in my upcoming book Secrets of the JavaScript Ninja):

// Simple JavaScript Templating
// John Resig - - MIT Licensed
  var cache = {};
  this.tmpl = function tmpl(str, data){
    // Figure out if we're getting a template, or if we need to
    // load the template - and be sure to cache the result.
    var fn = !/\W/.test(str) ?
      cache[str] = cache[str] ||
        tmpl(document.getElementById(str).innerHTML) :
      // Generate a reusable function that will serve as a template
      // generator (and which will be cached).
      new Function("obj",
        "var p=[],print=function(){p.push.apply(p,arguments);};" +
        // Introduce the data as local variables using with(){}
        "with(obj){p.push('" +
        // Convert the template into pure JavaScript
          .replace(/[\r\t\n]/g, " ")
          .replace(/((^|%>)[^\t]*)'/g, "$1\r")
          .replace(/\t=(.*?)%>/g, "',$1,'")
      + "');}return p.join('');");
    // Provide some basic currying to the user
    return data ? fn( data ) : fn;

You would use it against templates written like this (it doesn’t have to be in this particular manner – but it’s a style that I enjoy):

<script type="text/html" id="item_tmpl">
  <div id="<%=id%>" class="<%=(i % 2 == 1 ? " even" : "")%>">
    <div class="grid_1 alpha right">
      <img class="righted" src="<%=profile_image_url%>"/>
    <div class="grid_6 omega contents">
      <p><b><a href="/<%=from_user%>"><%=from_user%></a>:</b> <%=text%></p>

You can also inline script:

<script type="text/html" id="user_tmpl">
  <% for ( var i = 0; i < users.length; i++ ) { %>
    <li><a href="<%=users&#91;i&#93;.url%>"><%=users&#91;i&#93;.name%></a></li>
  <% } %>

Quick tip: Embedding scripts in your page that have a unknown content-type (such is the case here – the browser doesn’t know how to execute a text/html script) are simply ignored by the browser – and by search engines and screenreaders. It’s a perfect cloaking device for sneaking templates into your page. I like to use this technique for quick-and-dirty cases where I just need a little template or two on the page and want something light and fast.

and you would use it from script like so:

var results = document.getElementById("results");
results.innerHTML = tmpl("item_tmpl", dataObject);

You could pre-compile the results for later use. If you call the templating function with only an ID (or a template code) then it’ll return a pre-compiled function that you can execute later:

var show_user = tmpl(“item_tmpl”), html = “”;
for ( var i = 0; i < users.length; i++ ) { html += show_user( users[i] ); }[/js] The biggest falling-down of the method, at this point, is the parsing/conversion code - it could probably use a little love. It does use one technique that I enjoy, though: If you're searching and replacing through a string with a static search and a static replace it's faster to perform the action with .split(“match”).join(“replace”) – which seems counter-intuitive but it manages to work that way in most modern browsers. (There are changes going in place to grossly improve the performance of .replace(/match/g, "replace") in the next version of Firefox – so the previous statement won’t be the case for long.)

Feel free to have fun with it – I’d be very curious to see what mutations occur with the script. Since it’s so simple it seems like there’s a lot that can still be done with it.

Posted: July 16th, 2008

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