Template Directives Reference

Template directives are a special kind of HTML attribute available inside of any Astro component template (.astro files), and some can also be used in .mdx files.

Template directives are used to control an element or component’s behavior in some way. A template directive could enable some compiler feature that makes your life easier (like using class:list instead of class). Or, a directive could tell the Astro compiler to do something special with that component (like hydrating with client:load).

This page describes all of the template directives available to you in Astro, and how they work.

Rules

Section titled Rules

For a template directive to be valid, it must:

• Include a colon : in its name, using the form X:Y (ex: client:load).
• Be visible to the compiler (ex: <X {...attr}> would not work if attr contained a directive).

Some template directives, but not all, can take a custom value:

• <X client:load /> (takes no value)
• <X class:list={['some-css-class']} /> (takes an array)

A template directive is never included directly in the final HTML output of a component.

Common Directives

Section titled Common Directives

class:list

Section titled class:list

class:list={...} takes an array of class values and converts them into a class string. This is powered by @lukeed’s popular clsx helper library.

class:list takes an array of several different possible value kinds:

• string: Added to the element class
• Object: All truthy keys are added to the element class
• Array: flattened
• false, null, or undefined: skipped

<!-- This -->
<span class:list={[ 'hello goodbye', { world: true }, [ 'friend' ] ]} />
<!-- Becomes -->
<span class="hello goodbye world friend"></span>

set:html

Section titled set:html

set:html={string} injects an HTML string into an element, similar to setting el.innerHTML.

The value is not automatically escaped by Astro! Be sure that you trust the value, or that you have escaped it manually before passing it to the template. Forgetting to do this will open you up to Cross Site Scripting (XSS) attacks.

---
const rawHTMLString = "Hello <strong>World</strong>"
---
<h1>{rawHTMLString}</h1>
  <!-- Output: <h1>Hello &lt;strong&gt;World&lt;/strong&gt;</h1> -->
<h1 set:html={rawHTMLString} />
  <!-- Output: <h1>Hello <strong>World</strong></h1> -->

You can also use set:html on a <Fragment> to avoid adding an unnecessary wrapper element. This can be especially useful when fetching HTML from a CMS.

---
const cmsContent = await fetchHTMLFromMyCMS();
---
<Fragment set:html={cmsContent}>

set:html={Promise<string>} injects an HTML string into an element that is wrapped in a Promise.

This can be used to inject HTML stored externally, such as in a database.

---
import api from '../db/api.js';
---
<article set:html={api.getArticle(Astro.props.id)}></article>

set:html={Promise<Response>} injects a Response into an element.

This is most helpful when using fetch(). For example, fetching old posts from a previous static-site generator.

<article set:html={fetch('http://example/old-posts/making-soup.html')}></article>

set:html can be used on any tag and does not have to include HTML. For example, use with JSON.stringify() on a <script> tag to add a JSON-LD schema to your page.

<script type="application/ld+json" set:html={JSON.stringify({
  "@context": "https://schema.org/",
  "@type": "Person",
  name: "Houston",
  hasOccupation: {
    "@type": "Occupation",
    name: "Astronaut"
  }
})}/>

set:text

Section titled set:text

set:text={string} injects a text string into an element, similar to setting el.innerText. Unlike set:html, the string value that is passed is automatically escaped by Astro.

This is equivalent to just passing a variable into a template expression directly (ex: <div>{someText}</div>) and therefore this directive is not commonly used.

Client Directives

Section titled Client Directives

These directives control how UI Framework components are hydrated on the page.

By default, a UI Framework component is not hydrated in the client. If no client:* directive is provided, its HTML is rendered onto the page without JavaScript.

A client directive can only be used on a UI framework component that is directly imported into a .astro component. Hydration directives are not supported when using dynamic tags and custom components passed via the components prop.

client:load

Section titled client:load

• Priority: High
• Useful for: Immediately-visible UI elements that need to be interactive as soon as possible.

Load and hydrate the component JavaScript immediately on page load.

<BuyButton client:load />

client:idle

Section titled client:idle

• Priority: Medium
• Useful for: Lower-priority UI elements that don’t need to be immediately interactive.

Load and hydrate the component JavaScript once the page is done with its initial load and the requestIdleCallback event has fired. If you are in a browser that doesn’t support requestIdleCallback, then the document load event is used.

<ShowHideButton client:idle />

timeout

Section titled timeout

Added in: astro@4.15.0

The maximum time to wait, in milliseconds, before hydrating the component, even if the page is not yet done with its initial load.

This allows you to pass a value for the timeout option from the requestIdleCallback() specification. This means you can delay hydration for lower-priority UI elements with more control to ensure your element is interactive within a specified time frame.

<ShowHideButton client:idle={{timeout: 500}} />

client:visible

Section titled client:visible

• Priority: Low
• Useful for: Low-priority UI elements that are either far down the page (“below the fold”) or so resource-intensive to load that you would prefer not to load them at all if the user never saw the element.

Load and hydrate the component JavaScript once the component has entered the user’s viewport. This uses an IntersectionObserver internally to keep track of visibility.

<HeavyImageCarousel client:visible />

client:visible={{rootMargin}}

Section titled client:visible={{rootMargin}}

Added in: astro@4.1.0

Optionally, a value for rootMargin can be passed to the underlying IntersectionObserver. When rootMargin is specified, the component JavaScript will hydrate when a specified margin (in pixels) around the component enters the viewport, rather than the component itself.

<HeavyImageCarousel client:visible={{rootMargin: "200px"}} />

Specifying a rootMargin value can reduce layout shifts (CLS), allow more time for a component to hydrate on slower internet connections, and make components interactive sooner, enhancing the stability and responsiveness of the page.

client:media

Section titled client:media

• Priority: Low
• Useful for: Sidebar toggles, or other elements that might only be visible on certain screen sizes.

client:media={string} loads and hydrates the component JavaScript once a certain CSS media query is met.

Note

If the component is already hidden and shown by a media query in your CSS, then it can be easier to just use client:visible and not pass that same media query into the directive.

<SidebarToggle client:media="(max-width: 50em)" />

client:only

Section titled client:only

client:only={string} skips HTML server-rendering, and renders only on the client. It acts similarly to client:load in that it loads, renders, and hydrates the component immediately on page load.

You must pass the component’s correct framework as a value! Because Astro doesn’t run the component during your build / on the server, Astro doesn’t know what framework your component uses unless you tell it explicitly.

<SomeReactComponent client:only="react" />
<SomePreactComponent client:only="preact" />
<SomeSvelteComponent client:only="svelte" />
<SomeVueComponent client:only="vue" />
<SomeSolidComponent client:only="solid-js" />
<SomeLitComponent client:only="lit" />

Display loading content

Section titled Display loading content

For components that render only on the client, it is also possible to display fallback content while they are loading. Use slot="fallback" on any child element to create content that will be displayed only until your client component is available:

<ClientComponent client:only="vue">
  <div slot="fallback">Loading</div>
</ClientComponent>

Custom Client Directives

Section titled Custom Client Directives

Since Astro 2.6.0, integrations can also add custom client:* directives to change how and when components should be hydrated.

Visit the addClientDirective API page to learn more about creating a custom client directive.

Script & Style Directives

Section titled Script & Style Directives

These directives can only be used on HTML <script> and <style> tags, to control how your client-side JavaScript and CSS are handled on the page.

is:global

Section titled is:global

By default, Astro automatically scopes <style> CSS rules to the component. You can opt-out of this behavior with the is:global directive.

is:global makes the contents of a <style> tag apply globally on the page when the component is included. This disables Astro’s CSS scoping system. This is equivalent to wrapping all of the selectors within a <style> tag with :global().

You can combine <style> and <style is:global> together in the same component, to create some global style rules while still scoping most of your component CSS.

See the Styling & CSS page for more details about how global styles work.

<style is:global>
  body a { color: red; }
</style>

is:inline

Section titled is:inline

By default, Astro will process, optimize, and bundle any <script> and <style> tags that it sees on the page. You can opt-out of this behavior with the is:inline directive.

is:inline tells Astro to leave the <script> or <style> tag as-is in the final output HTML. The contents will not be processed, optimized, or bundled. This limits some Astro features, like importing an npm package or using a compile-to-CSS language like Sass.

The is:inline directive means that <style> and <script> tags:

• Will not be bundled into an external file. This means that attributes like defer which control the loading of an external file will have no effect.
• Will not be deduplicated—the element will appear as many times as it is rendered.
• Will not have its import/@import/url() references resolved relative to the .astro file.
• Will be rendered in the final output HTML exactly where it is authored.
• Styles will be global and not scoped to the component.

Caution

The is:inline directive is implied whenever any attribute other than src is used on a <script> or <style> tag. The one exception is using the define:vars directive on the <style> tag, which does not automatically imply is:inline.

<style is:inline>
  /* inline: relative & npm package imports are not supported. */
  @import '/assets/some-public-styles.css';
  span { color: green; }
</style>

<script is:inline>
  /* inline: relative & npm package imports are not supported. */
  console.log('I am inlined right here in the final output HTML.');
</script>

See how client-side scripts work in Astro components.

define:vars

Section titled define:vars

define:vars={...} can pass server-side variables from your component frontmatter into the client <script> or <style> tags. Any JSON-serializable frontmatter variable is supported, including props passed to your component through Astro.props. Values are serialized with JSON.stringify().

---
const foregroundColor = "rgb(221 243 228)";
const backgroundColor = "rgb(24 121 78)";
const message = "Astro is awesome!";
---
<style define:vars={{ textColor: foregroundColor, backgroundColor }}>
  h1 {
    background-color: var(--backgroundColor);
    color: var(--textColor);
  }
</style>

<script define:vars={{ message }}>
  alert(message);
</script>

Caution

Using define:vars on a <script> tag implies the is:inline directive, which means your scripts won’t be bundled and will be inlined directly into the HTML.

This is because when Astro bundles a script, it includes and runs the script once even if you include the component containing the script multiple times on one page. define:vars requires a script to rerun with each set of values, so Astro creates an inline script instead.

For scripts, try passing variables to scripts manually instead.

Advanced Directives

Section titled Advanced Directives

is:raw

Section titled is:raw

is:raw instructs the Astro compiler to treat any children of that element as text. This means that all special Astro templating syntax will be ignored inside of this component.

For example, if you had a custom Katex component that converted some text to HTML, you could have users do this:

---
import Katex from '../components/Katex.astro';
---
<Katex is:raw>Some conflicting {syntax} here</Katex>