Handlebars.js
=============

Handlebars.js is an extension to the [Mustache templating language](http://mustache.github.com/) created by Chris Wanstrath. Handlebars.js and Mustache are both logicless templating languages that keep the view and the code separated like we all know they should be.

Installing
----------
Installing Handlebars is easy. Simply [download the package from GitHub](https://github.com/downloads/wycats/handlebars.js/handlebars.js) and add it to your web pages. You can also get a [debug version](https://github.com/downloads/wycats/handlebars.js/handlebars.debug.js) which helps newer browsers print out useful stack traces.

Usage
-----
In general, the syntax of Handlebars.js templates is a superset of Mustache templates. For basic syntax, check out the [Mustache manpage](http://mustache.github.com/mustache.5.html).

Once you have a template, use the Handlebars.compile method to compile the template into a function. The generated function takes a context argument, which will be used to render the template.

    var source = "<p>Hello, my name is {{name}}. I am from {{hometown}}. I have " + 
        "{{kids/length}} kids:</p>" +
        "<ul>{{#kids}}<li>{{name}} is {{age}}</li>{{/kids}}</ul>";
    var template = Handlebars.compile(source);

    var data = { "name": "Alan", "hometown": "Somewhere, TX",
                  "kids": [{"name": "Jimmy", "age": "12"}, {"name": "Sally", "age": "4"}]};
    var result = template(data);

    // Would render:
    // <p>Hello, my name is Alan. I am from Somewhere, TX. I have 2 kids:</p>
    // <ul>
    //   <li>Jimmy is 12</li>
    //   <li>Sally is 4</li>
    // </ul>


Registering Helpers
-------------------

You can register helpers that Handlebars will use when evaluating your
template. Here's an example, which assumes that your objects have a URL
embedded in them, as well as the text for a link:

    Handlebars.registerHelper('link_to', function(context) {
      return "<a href='" + context.url + "'>" + context.body + "</a>";
    });

    var context = { posts: [{url: "/hello-world", body: "Hello World!"}] };
    var source = "<ul>{{#posts}}<li>{{{link_to this}}}</li></ul>"

    var template = Handlebars.compile(source);
    template(context);

    // Would render:
    //
    // <ul>
    //   <li><a href='/hello-world'>Hello World!</a></li>
    // </ul>


Escaping
--------

By default, the `{{expression}}` syntax will escape its contents. This
helps to protect you against accidental XSS problems caused by malicious
data passed from the server as JSON.

To explicitly *not* escape the contents, use the triple-mustache
(`{{{}}}`). You have seen this used in the above example.


Differences Between Handlebars.js and Mustache
----------------------------------------------
Handlebars.js adds a couple of additional features to make writing templates easier and also changes a tiny detail of how partials work.

### Paths

Handlebars.js supports an extended expression syntax that we call paths. Paths are made up of typical expressions and / characters. Expressions allow you to not only display data from the current context, but to display data from contexts that are descendents and ancestors of the current context.

To display data from descendent contexts, use the `/` character. So, for example, if your data were structured like:

    var data = {"person": { "name": "Alan" }, company: {"name": "Rad, Inc." } };

you could display the person's name from the top-level context with the following expression:

    {{person/name}}

Similarly, if already traversed into the person object you could still display the company's name with an expression like `{{../company/name}}`, so:

    {{#person}}{{name}} - {{../company/name}}{{/person}}

would render:

    Alan - Rad, Inc.

### Strings

When calling a helper, you can pass paths or Strings as parameters. For
instance:

    Handlebars.registerHelper('link_to', function(title, context) {
      return "<a href='/posts" + context.id + "'>" + title + "</a>"
    });

    var context = { posts: [{url: "/hello-world", body: "Hello World!"}] };
    var source = '<ul>{{#posts}}<li>{{{link_to "Post" this}}}</li></ul>'

    var template = Handlebars.compile(source);
    template(context);

    // Would render:
    //
    // <ul>
    //   <li><a href='/hello-world'>Post!</a></li>
    // </ul>

When you pass a String as a parameter to a helper, the literal String
gets passed to the helper function.


### Block Helpers

Handlebars.js also adds the ability to define block helpers. Block helpers are functions that can be called from anywhere in the template. Here's an example:

    var source = "<ul>{{#people}}<li>{{{#link}}}{{name}}{{/link}}</li>{{/people}}</ul>";
    Handlebars.registerHelper('link', function(context, fn) {
      return '<a href="/people/' + this.__get__("id") + '">' + fn(this) + '</a>';
    });
    var template = Handlebars.compile(source);

    var data = { "people": [
        { "name": "Alan", "id": 1 },
        { "name": "Yehuda", "id": 2 }
      ]};
    template(data);

    // Should render:
    // <ul>
    //   <li><a href="/people/1">Alan</a></li>
    //   <li><a href="/people/2">Yehuda</a></li>
    // </ul>

Whenever the block helper is called it is given two parameters, the argument that is passed to the helper, or the current context if no argument is passed and the compiled contents of the block. Inside of the block helper the value of `this` is the current context, wrapped to include a method named `__get__` that helps translate paths into values within the helpers.

### Partials

You can register additional templates as partials, which will be used by
Handlebars when it encounters a partial (`{{> partialName}}`). Partials
can either be String templates or compiled template functions. Here's an
example:

    var source = "<ul>{{#people}}<li>{{> link}}</li>{{/people}}</ul>";

    Handlebars.registerPartial('link', '<a href="/people/{{id}}">{{name}}</a>')
    var template = Handlebars.compile(source);

    var data = { "people": [
        { "name": "Alan", "id": 1 },
        { "name": "Yehuda", "id": 2 }
      ]};

    template(data);

    // Should render:
    // <ul>
    //   <li><a href="/people/1">Alan</a></li>
    //   <li><a href="/people/2">Yehuda</a></li>
    // </ul>

Precompiling Templates
----------------------

TODO in the rewrite. This will use RubyRacer and not node.

Performance
-----------

In a rough performance test, precompiled Handlebars.js templates (in the original version of Handlebars.js) rendered in about half the time of Mustache templates. It would be a shame if it were any other way, since they were precompiled, but the difference in architecture does have some big performance advantages. Justin Marney, a.k.a. [gotascii](http://github.com/gotascii), confirmed that with an [independent test](http://sorescode.com/2010/09/12/benchmarks.html). The rewritten Handlebars (current version) is faster than the old version, and we will have some benchmarks in the near future.


Building
--------

To build handlebars, just run `rake release`, and you will get two files
in the `dist` directory. The debug version comes with stack trace
annotations for webkit browsers, but will slightly increase startup
time.


Known Issues
------------
* Handlebars.js can be cryptic when there's an error while rendering.

Handlebars Contrib
------------------
Alan Johnson, a.k.a. commondream, keeps a repository of useful helpers in the [handlebars-contrib](http://github.com/commondream/handlebars-contrib) repository. Feel free to copy those, or add some of your own.

Handlebars in the Wild
-----------------
* Don Park wrote an Express.js view engine adapter for Handlebars.js called [hbs](http://github.com/donpark/hbs)
* [sammy.js](http://github.com/quirkey/sammy) by Aaron Quint, a.k.a. quirkey, supports Handlebars.js as one of its template plugins.

Helping Out
-----------
If you notice any problems, please report them to the GitHub issue tracker at [http://github.com/wycats/handlebars.js/issues](http://github.com/wycats/handlebars.js/issues). Feel free to contact commondream or wycats through GitHub with any other questions or feature requests. To submit changes fork the project and send a pull request.

License
-------
Handlebars.js is released under the MIT license.