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.

Checkout the official Handlebars docs site at http://www.handlebarsjs.com.

Installing

Installing Handlebars is easy. Simply download the package from the official site 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() {
  return "<a href='" + this.url + "'>" + this.body + "</a>";
});

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

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

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

Helpers take precedence over fields defined on the context. To access a field that is masked by a helper, a path reference may be used. In the example above a field named link_to on the context object would be referenced using:

{{./link_to}}

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 descendants and ancestors of the current context.

To display data from descendant 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}}

You can backtrack using ../. For example, if you've 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, options) {
  return "<a href='/posts" + this.url + "'>" + title + "!</a>"
});

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

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

// Would render:
//
// <ul>
//   <li><a href='/posts/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(options) {
  return '<a href="/people/' + this.id + '">' + options.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 one or more parameters, any arguments that are passed in the helper in the call and an options object containing the fn function which executes the block's child. The block's current context may be accessed through this.

Block helpers have the same syntax as mustache sections but should not be confused with one another. Sections are akin to an implicit each or with statement depending on the input data and helpers are explicit pieces of code that are free to implement whatever behavior they like. The mustache spec defines the exact behavior of sections. In the case of name conflicts, helpers are given priority.

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>

Comments

You can add comments to your templates with the following syntax:

{{! This is a comment }}

You can also use real html comments if you want them to end up in the output.

<div>
    {{! This comment will not end up in the output }}
    <!-- This comment will show up in the output -->
</div>

Precompiling Templates

Handlebars allows templates to be precompiled and included as javascript code rather than the handlebars template allowing for faster startup time.

Installation

The precompiler script may be installed via npm using the npm install -g handlebars command.

Usage

Precompile handlebar templates.
Usage: handlebars template...

Options:
  -a, --amd        Create an AMD format function (allows loading with RequireJS)         [boolean]
  -f, --output     Output File                                                           [string]
  -k, --known      Known helpers                                                         [string]
  -o, --knownOnly  Known helpers only                                                    [boolean]
  -m, --min        Minimize output                                                       [boolean]
  -s, --simple     Output template function only.                                        [boolean]
  -r, --root       Template root. Base value that will be stripped from template names.  [string]

If using the precompiler's normal mode, the resulting templates will be stored to the Handlebars.templates object using the relative template name sans the extension. These templates may be executed in the same manner as templates.

If using the simple mode the precompiler will generate a single javascript method. To execute this method it must be passed to the using the Handlebars.template method and the resulting object may be as normal.

Optimizations

• Rather than using the full handlebars.js library, implementations that do not need to compile templates at runtime may include handlebars.runtime.js whose min+gzip size is approximately 1k.
• If a helper is known to exist in the target environment they may be defined using the --known name argument may be used to optimize accesses to these helpers for size and speed.
• When all helpers are known in advance the --knownOnly argument may be used to optimize all block helper references.

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.

Upgrading

When upgrading from the Handlebars 0.9 series, be aware that the signature for passing custom helpers or partials to templates has changed.

Instead of:

template(context, helpers, partials, [data])

Use:

template(context, {helpers: helpers, partials: partials, data: data})

Known Issues

• Handlebars.js can be cryptic when there's an error while rendering.
• Using a variable, helper, or partial named class causes errors in IE browsers. (Instead, use className)

Handlebars in the Wild

• jblotus created http://tryhandlebarsjs.com for anyone who would like to try out Handlebars.js in their browser.
• 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.
• SproutCore uses Handlebars.js as its main templating engine, extending it with automatic data binding support.
• Ember.js makes Handlebars.js the primary way to structure your views, also with automatic data binding support.
• Les Hill (@leshill) wrote a Rails Asset Pipeline gem named handlebars_assets.
• Gist about Synchronous and asynchronous loading of external handlebars templates
• Lumbar provides easy module-based template management for handlebars projects.
• YUI implements a port of handlebars

Have a project using Handlebars? Send us a pull request!

Helping Out

To build Handlebars.js you'll need a few things installed.

• Node.js
• Ruby
• therubyracer, for running tests - gem install therubyracer
• rspec, for running tests - gem install rspec

There's a Gemfile in the repo, so you can run bundle to install rspec and therubyracer if you've got bundler installed.

To build Handlebars.js from scratch, you'll want to run rake compile in the root of the project. That will build Handlebars and output the results to the dist/ folder. To run tests, run rake test. You can also run our set of benchmarks with rake bench. Node tests can be run with npm test or rake npm_test. The default rake target will compile and run both test suites.

Some environments, notably Windows, have issues running therubyracer. Under these envrionments the rake compile and npm test should be sufficient to test most handlebars functionality.

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.