handlebars

What is handlebars?

Handlebars is a popular templating engine for JavaScript. It allows you to create templates with dynamic content that can be rendered with different contexts. It is commonly used to generate HTML for web pages, but can also be used for other types of text output.

What are handlebars's main functionalities?

Simple Templating

Handlebars allows you to iterate over an array and generate HTML for each item. In this example, 'people' is an array that is iterated over, and for each item, a paragraph element is created with the content of the item.

{{#each people}}<p>{{this}}</p>{{/each}}

Conditional Statements

You can use conditional statements in your templates to render different HTML based on the context. Here, if 'isAdmin' is true, a button is displayed; otherwise, a paragraph is shown.

{{#if isAdmin}}<button>Admin</button>{{else}}<p>Not an admin</p>{{/if}}

Custom Helpers

Handlebars allows you to define custom helpers that you can use in your templates. In this example, a 'loud' helper is created that converts a string to uppercase.

Handlebars.registerHelper('loud', function (aString) { return aString.toUpperCase(); });

Built-in Helpers

Handlebars provides built-in helpers like 'with' which you can use to change the context within a block. This example shows how to use the 'with' helper to access properties of an object without repeating the object name.

{{#with person}}<p>{{firstName}} {{lastName}}</p>{{/with}}

Partials

Partials are reusable template fragments in Handlebars. You can define a partial and then include it in other templates. This code shows how to include a partial named 'userMessage'.

{{> userMessage}}

Other packages similar to handlebars

mustache

Mustache is a logic-less template syntax that can be used for HTML, config files, source code, etc. It's similar to Handlebars but with fewer features and a focus on simplicity.

pug

Pug, formerly known as Jade, is a high-performance template engine heavily influenced by Haml and implemented with JavaScript for Node.js and browsers. It offers a more terse syntax and compiles to HTML.

ejs

EJS, or Embedded JavaScript templates, is a simple templating language that lets you generate HTML markup with plain JavaScript. It is more straightforward than Handlebars, embedding JavaScript code directly in the template.

README

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 or the bower repository and add it to your web pages (you should usually use the most recent version).

Alternatively, if you prefer having the latest version of handlebars from the 'master' branch, passing builds of the 'master' branch are automatically published to S3. You may download the latest passing master build by grabbing a handlebars-latest.js file from the builds page. When the build is published, it is also available as a handlebars-gitSHA.js file on the builds page if you need a version to refer to others. handlebars-runtime.js builds are also available.

Note: The S3 builds page is provided as a convenience for the community, but you should not use it for hosting Handlebars in production.

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 new Handlebars.SafeString("<a href='" + Handlebars.Utils.escapeExpression(this.url) + "'>" + Handlebars.Utils.escapeExpression(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:

{{#with 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>

Compatibility

There are a few Mustache behaviors that Handlebars does not implement.

• Handlebars deviates from Mustache slightly in that it does not perform recursive lookup by default. The compile time compat flag must be set to enable this functionality. Users should note that there is a performance cost for enabling this flag. The exact cost varies by template, but it's recommended that performance sensitive operations should avoid this mode and instead opt for explicit path references.
• The optional Mustache-style lambdas are not supported. Instead Handlebars provides it's own lambda resolution that follows the behaviors of helpers.
• Alternative delimeters are not supported.

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]
  -c, --commonjs       Exports CommonJS style, path to Handlebars module                      [string]
  -h, --handlebarPath  Path to handlebar.js (only valid for amd-style)                        [string]
  -n, --namespace      Template namespace                                                     [string]
  -p, --partial        Compiling a partial template                                           [boolean]
  -d, --data           Include data when compiling                                            [boolean]
  -e, --extension      Template extension.                                                    [string]
  -b, --bom            Removes the BOM (Byte Order Mark) from the beginning of the templates. [boolean]

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 Handlebars.template method and the resulting object may be used 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.
• Implementations that do not use @data variables can improve performance of iteration centric templates by specifying {data: false} in the compiler options.

Supported Environments

Handlebars has been designed to work in any ECMAScript 3 environment. This includes

• Node.js
• Chrome
• Firefox
• Safari 5+
• Opera 11+
• IE 6+

Older versions and other runtimes are likely to work but have not been formally tested. The compiler requires JSON.stringify to be implemented natively or via a polyfill. If using the precompiler this is not necessary.

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, with many performance tests being 5 to 7 times faster than the Mustache equivalent.

Upgrading

See release-notes.md for upgrade notes.

Known Issues

See FAQ.md for known issues and common pitfalls.

Handlebars in the Wild

• Assemble, by @jonschlinkert and @doowb, is a static site generator that uses Handlebars.js as its template engine.
• CoSchedule An editorial calendar for WordPress that uses Handlebars.js
• Ember.js makes Handlebars.js the primary way to structure your views, also with automatic data binding support.
• Ghost Just a blogging platform.
• handlebars_assets: A Rails Asset Pipeline gem from Les Hill (@leshill).
• handlebars-helpers is an extensive library with 100+ handlebars helpers.
• hbs: An Express.js view engine adapter for Handlebars.js, from Don Park.
• koa-hbs: koa generator based renderer for Handlebars.js.
• jblotus created http://tryhandlebarsjs.com for anyone who would like to try out Handlebars.js in their browser.
• jQuery plugin: allows you to use Handlebars.js with jQuery.
• Lumbar provides easy module-based template management for handlebars projects.
• 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.
• YUI implements a port of handlebars
• Swag by @elving is a growing collection of helpers for handlebars.js. Give your handlebars.js templates some swag son!
• DOMBars is a DOM-based templating engine built on the Handlebars parser and runtime

External Resources

• Gist about Synchronous and asynchronous loading of external handlebars templates

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

License

Handlebars.js is released under the MIT license.