Handlebars.js

Handlebars.js is an extension to the Mustache templating language 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 and add it to your web pages (you should usually use the most recent version).

Usage

In general, the syntax of Handlebars.js templates is a superset of Mustache templates. For basic syntax, check out the Mustache manpage.

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, confirmed that with an independent test. 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 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
• sammy.js 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. 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.