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Astro Syntax

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If you know HTML, you already know enough to write your first Astro component.

Astro component syntax is a superset of HTML. The syntax was designed to feel familiar to anyone with experience writing HTML or JSX, and adds support for including components and JavaScript expressions.

JSX-like Expressions

You can define local JavaScript variables inside of the frontmatter component script between the two code fences (---) of an Astro component. You can then inject these variables into the component’s HTML template using JSX-like expressions!

Variables

Local variables can be added into the HTML using the curly braces syntax:

src/components/Variables.astro
---
const name = "Astro";
---
<div>
<h1>Hello {name}!</h1> <!-- Outputs <h1>Hello Astro!</h1> -->
</div>

Dynamic Attributes

Local variables can be used in curly braces to pass attribute values to both HTML elements and components:

src/components/DynamicAttributes.astro
---
const name = "Astro";
---
<h1 class={name}>Attribute expressions are supported</h1>
<MyComponent templateLiteralNameAttribute={`MyNameIs${name}`} />

Dynamic HTML

Local variables can be used in JSX-like functions to produce dynamically-generated HTML elements:

src/components/DynamicHtml.astro
---
const items = ["Dog", "Cat", "Platypus"];
---
<ul>
{items.map((item) => (
<li>{item}</li>
))}
</ul>

Astro can conditionally display HTML using JSX logical operators and ternary expressions.

src/components/ConditionalHtml.astro
---
const visible = true;
---
{visible && <p>Show me!</p>}
{visible ? <p>Show me!</p> : <p>Else show me!</p>}

Dynamic Tags

You can also use dynamic tags by setting a variable to an HTML tag name or a component import:

src/components/DynamicTags.astro
---
import MyComponent from "./MyComponent.astro";
const Element = 'div'
const Component = MyComponent;
---
<Element>Hello!</Element> <!-- renders as <div>Hello!</div> -->
<Component /> <!-- renders as <MyComponent /> -->

When using dynamic tags:

  • Variable names must be capitalized. For example, use Element, not element. Otherwise, Astro will try to render your variable name as a literal HTML tag.

  • Hydration directives are not supported. When using client:* hydration directives, Astro needs to know which components to bundle for production, and the dynamic tag pattern prevents this from working.

Fragments

Astro supports using either <Fragment> </Fragment> or the shorthand <> </>.

Fragments can be useful to avoid wrapper elements when adding set:* directives, as in the following example:

src/components/SetHtml.astro
---
const htmlString = '<p>Raw HTML content</p>';
---
<Fragment set:html={htmlString} />

Differences between Astro and JSX

Astro component syntax is a superset of HTML. It was designed to feel familiar to anyone with HTML or JSX experience, but there are a couple of key differences between .astro files and JSX.

Attributes

In Astro, you use the standard kebab-case format for all HTML attributes instead of the camelCase used in JSX. This even works for class, which is not supported by React.

example.astro
<div className="box" dataValue="3" />
<div class="box" data-value="3" />

Multiple Elements

An Astro component template can render multiple elements with no need to wrap everything in a single <div> or <>, unlike JavaScript or JSX.

src/components/RootElements.astro
---
// Template with multiple elements
---
<p>No need to wrap elements in a single containing element.</p>
<p>Astro supports multiple root elements in a template.</p>

Comments

In Astro, you can use standard HTML comments or JavaScript-style comments.

example.astro
---
---
<!-- HTML comment syntax is valid in .astro files -->
{/* JS comment syntax is also valid */}