amp-mustache

0.2 0.1

<script async custom-template="amp-mustache" src="https://cdn.ampproject.org/v0/amp-mustache-0.2.js"></script>

Version notes

Syntax

Mustache is a logic-less template syntax. See Mustache specification for more details. Some of the core Mustache tags are:

• {{variable}}: A variable tag. It outputs the HTML-escaped value of a variable.
• {{#section}} {{/section}}: A section tag. It can test the existence of a variable and iterate over it if it's an array.
• {{^section}} {{/section}}: An inverted tag. It can test the non-existence of a variable.
• {{{unescaped}}}: Unescaped HTML. It's restricted in the markup it may output (see "Restrictions" below).

Usage

The amp-mustache template has to be defined and used according to the AMP Template Spec.

First, the amp-mustache has to be declared/loaded like this:

<script
  async
  custom-template="amp-mustache"
  src="https://cdn.ampproject.org/v0/amp-mustache-0.2.js"
></script>

Then, the Mustache templates can be defined either in a script or template tag like this:

<!-- Using template tag. -->
<template type="amp-mustache">
  Hello {{world}}!
</template>

or

<script type="text/plain" template="amp-mustache">
  Hello {{world}}!
</script>

Use template tag wherever possible, as AMP validation provides useful dev-x hints. Use the script template for edge cases and issues with templating in the context of tables. See the "Tables" section further below.

How templates are discovered, when they are rendered, how data is provided is all decided by the target AMP element that uses this template to render its content (for example, in an amp-list, amp-form, etc.).

Restrictions

Validation

Like all AMP templates, amp-mustache templates are required to be well-formed DOM fragments. This means that among other things, you can't use amp-mustache to:

• Calculate tag name. E.g. <{{tagName}}> is not allowed.
• Calculate attribute name. E.g. <div {{attrName}}=something> is not allowed.

The output of "triple-mustache" is sanitized to only allow the following tags:

a, amp-img, article, aside, b, blockquote, br, caption, code, col, colgroup, dd, del, details, div, dl, dt, em, figcaption, figure, footer, h1, h2, h3, header, hr, i, ins, li, main, mark, nav, ol, p, pre, q, s, section, small, span, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, time, tr, u, ul.

Sanitization

Mustache output is sanitized for security reasons and to maintain AMP validity. This may result in certain elements and attributes being silently removed.

Pitfalls

Nested templates

Per AMP Validation, <template> elements must not be children of other <template> elements. This can happen when nesting two components that use templates, e.g. amp-list and amp-form.

To workaround this, <template> elements can also be referenced by id via the template attribute on the component. For example:

<amp-list id="myList" src="https://foo.com/list.json">
  <template type="amp-mustache">
    <div>{{title}}</div>
  </template>
</amp-list>

Can also be represented as:

<!-- Externalize templates to avoid nesting. -->
<template type="amp-mustache" id="myTemplate">
  <div>{{title}}</div>
</template>

<amp-list id="myList" src="https://foo.com/list.json" template="myTemplate">
</amp-list>

Tables

Since AMP template strings must be specified in <template> elements, this can cause unexpected behavior due to browser parsing. For example, <table> elements can cause foster parenting of text. In the following example:

<template type="amp-mustache">
  <table>
    <tr>
{{#foo}}      <td></td>
{{/foo}}    </tr>
  </table>
</template>

The browser will foster parent the text nodes {{#foo}} and {{/foo}}:

{{#foo}} {{/foo}}<table>
  <tr>
    <td></td>
  </tr>
</table>

Workarounds include wrapping Mustache sections in HTML comments (e.g. <!-- {{#bar}} -->), using non-table elements like <div> instead or using a <script type="text/plain"> tag to define your templates.

<script type="text/plain" template="amp-mustache">
  <table>
    <tr>
{{#foo}}<td></td>{{/foo}}
    </tr>
  </table>
</script>

Quote escaping

When using amp-mustache to calculate attribute values, quote escaping can be an issue. For example:

<template type="amp-mustache">
  <!-- A double-quote (") in foo will cause malformed HTML. -->
  <amp-img alt="{{foo}}" src="example.jpg" width="100" height="100"></amp-img>

  <!-- A single-quote (') or double-quote (") in bar will cause an AMP runtime parse error. -->
  <button on="tap:AMP.setState({foo: '{{bar}}'})">Click me</button>
</template>

Using HTML character codes in the {{foo}} or {{bar}} variables won't work since Mustache will HTML escape & characters (e.g. " -> "). One workaround is to use facsimile characters e.g. ′ (′) and ″ (″).

HTML entities

HTML entities are not preserved in <template> elements.

This can be an issue if you want to server-side render a <template> containing user-generated text, since user-generated text containing {{, }}, {{{, }}} will be treated as a Mustache section. E.g. replacing {{ with HTML entities {{ won't work because they aren't preserved when the browser parses the <template>.

Workarounds include replacing strings like {{ with different characters or stripping them outright from user-generated content.

Validation

See amp-mustache rules in the AMP validator specification.