AEM HTL Style Guide

A style guide for the HTML Template Language (HTL), formerly known as Sightly, the HTML templating system from Adobe Experience Manager (AEM).

1. HTML

1.1 Avoid inline JavaScript or CSS.

In order to encourage keeping a clean separation of concerns, HTL has by design some limitations for inline JavaScript or CSS. First, because HTL doesn't parse JavaScript or CSS, and therefore cannot automatically define the corresponding escaping, all expressions written there must provide an explicit context option. Then, because the HTML grammar ignores elements located inside a <script> or <style> elements, no block statement can be used within them.

Therefore JavaScript and CSS code should instead be placed into corresponding .js and .css files. Data attributes are the easiest way to communicate values to JavaScript, and class names are the best way to trigger specific styles.

<!--/* Bad */-->
<section data-sly-use.teaser="com.example.TeaserComponent" class="teaser">
    <h2 class="teaser__title">${teaser.title}</h2>
    <script>
        var teaserConfig = {
            skin: "${teaser.skin @ context='scriptString'}",
            animationSpeed: ${teaser.animationSpeed @ context='number'}
        };
    </script>
    <style>
        .teaser__title {
            font-size: ${teaser.titleFontSize @ context='styleToken'}
        }
    </style>
</section>

<!--/* Good */-->
<section data-sly-use.teaser="com.example.TeaserComponent" data-teaser-config="${teaser.jsonConfig}" class="teaser">
    <h2 class="teaser__title teaser__title--font-${teaser.titleFontClass}">${teaser.title}</h2>
</section>

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2. Comments

2.1 Use HTL comments.

Normal HTML comments get rendered to the final markup. To keep the DOM clean, always use HTL comments over normal HTML comments.
```



```

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3. Expression language

3.1 Set a display context only if necessary

In most cases you can leave out the display context, because it is determined automatically.
```

<a href="${teaser.link @ context = 'uri'}"></a>


<a href="${teaser.link}"></a>
```
3.2 Use the safest possible display context.

From the following list of contexts, always choose the one closest to the top that fits your needs:
number: For whole numbers (in HTML, JS or CSS)
uri: For links and paths (in HTML, JS or CSS, applied by default for src and href attributes)
elementName: For HTML element names (applied by default by data-sly-element)
attributeName: For HTML attribute names (applied by default by data-sly-attribute for attribute names)
scriptToken: For JavaScript identifiers and keywords
styleToken: For CSS identifiers and keywords
scriptString: For text within JavaScript strings
styleString: For text within CSS strings
attribute: For HTML attribute values (applied by default for attribute values)
text: For HTML text content (applied by default for any content)
html: For HTML markup (it filters out all elements and attributes that could be dangerous)
unsafe: Unescaped and unfiltered direct output
```

<section data-sly-use.teaser="com.example.TeaserComponent" class="teaser">
    <h4 onclick="${teaser.clickHandler @ context='unsafe'}">${teaser.title}</h4>
    <div style="color: ${teaser.color @ context='unsafe'};">
        ${teaser.htmlContent @ context='unsafe'}
    </div>
</section>


<section data-sly-use.teaser="com.example.TeaserComponent" class="teaser">
    <h4 onclick="${teaser.clickHandler @ context='scriptToken'}">${teaser.title}</h4>
    <div style="color: ${teaser.color @ context='styleToken'};">
        ${teaser.htmlContent @ context='html'}
    </div>
</section>
```

3.3 Avoid writing unnecessary expressions for literals.

It might sound obvious, but an expression with just a string literal inside equals just that string literal.

<!--/* Bad */-->
<sly data-sly-use.clientlib="${'/libs/granite/sightly/templates/clientlib.html'}">
    ...
</sly>

<!--/* Good */-->
<sly data-sly-use.clientlib="/libs/granite/sightly/templates/clientlib.html">
    ...
</sly>

3.4 Avoid using the ternary operator unnecessarily.

Take advantage of the logical || operator to simplify your code.

<!--/* Bad */-->
<div class="${cssClass ? cssClass : 'my-class'}"></div>

<!--/* Good */-->
<div class="${cssClass || 'my-class'}"></div>

3.5 Use the native URI manipulation capabilities of HTL.

Rolling out a custom URI builder is error prone and hardcoding URL's is even worse. Use HTL URI Manipulation instead, in particular, the extension option.
```

<a href="${component.link}.html"></a>


<a href="${component.link @ extension = 'html'}"></a>
```
3.6 Drop Method Prefixes When Accessing Properties from Java Getter Functions.

When following the JavaBeans naming conventions (as you should) to name your getter methods, you can access the properties with their property name directly. You should access properties this way for consistency and readability.
```

<p>${component.getTitle}</p>
<a href="${item.link}" data-sly-unwrap="${item.isActive}">...</a>


<p>${component.title}</p>
<a href="${item.link}" data-sly-unwrap="${item.active}">...</a>
```

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4. Block statements

4.1 Use the sly tag name for all elements that are not part of the markup.

HTML elements with the tag name sly are automatically getting unwrapped and will not be part of the final markup.

<!--/* Bad */-->
<div data-sly-include="content.html" data-sly-unwrap></div>

<!--/* Good */-->
<sly data-sly-include="content.html"></sly>

<!--/* Bad */-->
<div data-sly-resource="${item @ selectors='event'}" data-sly-unwrap></div>

<!--/* Good */-->
<sly data-sly-resource="${item @ selectors = 'event'}"></sly>

<!--/* Bad */-->
<div data-sly-test="${event.hasDate}" data-sly-unwrap>
    ...
</div>

<!--/* Good */-->
<sly data-sly-test="${event.hasDate}">
    ...
</sly>

IMPORTANT - The sly element will not automatically unwrap itself if you use HTL 1.0 (AEM 6.0). In that case, you still have to add the data-sly-unwrap attribute.

<!--/* Bad - HTL 1.0 */-->
<sly data-sly-include="content.html"></sly>
 
<!--/* Good - HTL 1.0 */-->
<sly data-sly-include="content.html" data-sly-unwrap></sly>

4.2 Try to place data-sly-use statements only on top-level elements.

Since data-sly-use identifiers are always global (https://docs.adobe.com/docs/en/htl/docs/use-api/java.html#Local%20identifier), these attributes should only be placed in the top-level element. That way one can easily see name clashes and also it prevents initializing the same object twice.
```

<section class="teaser">
   <h3 data-sly-use.teaser="com.example.TeaserComponent">${teaser.title}</h3>
</section>


<section data-sly-use.teaser="com.example.TeaserComponent" class="teaser">
   <h3>${teaser.title}</h3>
</section>
```

4.3 Use meaningful identifier names.

This will enhance the readability of your HTL scripts and and makes it easier for others to understand.

<!--/* Bad */-->
<sly data-sly-use.comp="com.example.TeaserComponent">
    ...
</sly>
 
<!--/* Good */-->
<sly data-sly-use.teaser="com.example.TeaserComponent">
    ...
</sly>

4.4 Use lowerCamelCase for identifier names.

Using lowerCamelCase (You start by making the first word lowercase. Then, you capitalize the first letter of each word that follows i.e.: "sampleIdentifierName") will help to increase the readability of your identifiers. Notice though that HTL will internally only use (and log) full lowercase identifiers. Also dashes are not allowed for identifiers.
```

<sly data-sly-use.mediagallery="com.example.MediaGallery">
    ...
</sly>


<sly data-sly-use.mediaGallery="com.example.MediaGallery">
    ...
</sly>
```

4.5 Re-use expressions with identifiers

If a test block statement is used multiple times, define an identifer and re-use it this way instead. This will allow the htl compiler to cache the expression result and will also make your code easier to read and understand.

<!--/* Bad */-->
<section data-sly-test="${!teaser.empty}" class="teaser">
    ...
</section>
 
<div data-sly-test="${teaser.empty}" class="cq-placeholder"></div>
 
<!--/* Good */-->
<section data-sly-test.hasContent="${!teaser.empty}" class="teaser">
    ...
</section>
 
<div data-sly-test="${!hasContent}" class="cq-placeholder"></div>

Similarly, if a generic expression is used multiple times, define an identifer with data-sly-set and re-use it, for the same reasons stated above.

<!--/* Bad */-->
<div data-sly-unwrap="${!(wcmmode.edit || wcmmode.preview) || resource.hasChildren}">
    <sly
        data-sly-test="${resource.hasChildren}"
        ...>
    </sly>
    <sly
        data-sly-test="${(wcmmode.edit || wcmmode.preview) && !resource.hasChildren}"
        .... >
    </sly>
</div>
 
<!--/* Good */-->
<div
     data-sly-set.editMode="${(wcmmode.edit || wcmmode.preview)}"
     data-sly-set.hasChildren="${resource.hasChildren}"
     data-sly-unwrap="${!editMode || hasChildren}">
    <sly
        data-sly-test="${hasChildren}"
        ...>
    </sly>
    <sly
        data-sly-test="${editMode && !hasChildren}"
        ...>
    </sly>
</div>

4.6 Use identifiers instead of the default “item” variable for list block statements.

<!--/* Bad */-->
<ul data-sly-list="${tagList.tags}" class="tagList">
    <li class="tagList__tag">
        <a class="tagList__button" href="${item.url}">${item.title}</a>
    </li>
</ul>
 
<!--/* Good */-->
<ul data-sly-list.tag="${tagList.tags}" class="tagList">
    <li class="tagList__tag">
        <a class="tagList__button" href="${tag.url}">${tag.title}</a>
    </li>
</ul>

4.7 Place block statements before the regular HTML attributes.

The reason for that is that regular HTML attributes might use HTL variables which have been declared in the same element via data-sly-use. One should always declare things before using them. Also HTL block elements might influence if the element appears at all (via data-sly-test) or multiple times (via data-sly-repeat) and therefore are just too important to put them at the end of the attribute list. Further details in issue 25.
```

<p class="teaser__text" data-sly-test="${teaser.text}"></p>
     

<p data-sly-test="${teaser.text}" class="teaser__text"></p>
```

4.8 Use existing HTML elements for your block statements if possible.

<!--/* Bad */-->
<sly data-sly-test="${!teaser.active}">
    <section class="teaser">
        …
    </section>
</sly>
 
<!--/* Good */-->
<section data-sly-test="${!teaser.active}" class="teaser">
    …
</section>

4.9 Avoid the element, attribute and text block statements.

It's a lot cleaner and explicit to write your HTL scripts without these block statements.

<!--/* Bad */-->
<div data-sly-element="${headlineElement}">${event.year}</div>
<a data-sly-attribute.href="${event.link}" href="#"></a>
<p data-sly-text="${event.year}" class="event__year"></p>
 
<!--/* Good */-->
<h2>${event.year}</h2>
<a href="${event.link}"></a>
<p class="event__year">${event.year}</p>

4.10 Define your templates in a separate file.

It's cleaner to create separate files for your template markup, so your HTL scripts will not get cluttered.

<!--/* Bad */-->
<sly data-sly-use.teaser="com.example.TeaserComponent">
  <sly data-sly-template.teaserSmall="${@ title, text}">
    <h2>${title}</h2>
    <p>${text}</p>
  </sly>
  
  <sly data-sly-call="${teaserSmall @ title=teaser.title, text=teaser.text}"></sly>
</sly>

<!--/* Good - Separate template file: "teaser-templates.html" */-->
<sly data-sly-template.teaserSmall="${@ teaserModel}">
  <h2>${teaserModel.title}</h2>
  <p>${teaserModel.text}</p>
</sly>

<!--/* Good - HTL script */-->
<sly data-sly-use.teaser="com.example.TeaserComponent" data-sly-use.teaserTemplates="teaser-templates.html">
  <sly data-sly-call="${teaserTemplates.teaserSmall @ teaserModel=teaser}"></sly>
</sly>

4.11 Avoid using data-sly-test to set arbitrary variable bindings

Instead of binding a variable with data-sly-test, use the purposefully defined data-sly-set. This avoids unintentionally hiding elements if the result of the expression evaluates to false (see HTL expressions evaluating to false ) and/or stopping the evaluation of further block statements; This is specially difficult to debug when various data-sly-test statements affect the same element.

<!--/* Instead of */-->
<sly data-sly-test="${person.firstName && person.lastName && person.image}" data-sly-test.fullName="${person.firstName} ${person.lastName}">
<h1>${fullName}</h1>
<img src=${person.image}" alt="${fullName}"/>

<!--/* Use */-->
<sly data-sly-test="${person.firstName && person.lastName && person.image}" data-sly-set.fullName="${person.firstName} ${person.lastName}">
<h1>${fullName}</h1>
<img src=${person.image}" alt="${fullName}"/>

4.12 Avoid unnecessary <sly> tags.

It's cleaner and easier to understand your intentions if you add your block statements in the relevant elements directly instead of wrapping them with an sly tag.
```

<sly data-sly-test.title="${component.title}">
  <h1>${title}</h1>
</sly>


<h1 data-sly-test.title="${component.title}">${title}</h1>
```
4.13 Use an explicit </sly> end tag to close <sly> tags.

Because sly is neither a void nor a foreign element (See html5 start tags), it must be explicitly closed with an end tag </sly>. Using a self-closing tag is not allowed.
```

<sly data-sly-include="content.html"/>


<sly data-sly-include="content.html"></sly>
```

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License

The MIT License (MIT)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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