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✨ .astro Syntax

Astro comes with its own server-side, component-based templating language. Think of it as HTML enhanced with the full power of JavaScript.

Learning a new syntax can be intimidating, but the .astro format has been carefully designed with familiarity in mind. It borrows heavily from patterns you likely already know—components, Frontmatter, and JSX-like expressions. We're confident that this guide will help you feel comfortable writing .astro files in no time.

The .astro format

If you're already familiar with HTML or JavaScript, you'll likely feel comfortable with .astro files right away.

Think of .astro as component-oriented HTML. Components are reusable, self-contained blocks of HTML and CSS that belong together.

<!-- This is a valid Astro component -->
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <main>
      <h1>Hello world!</h1>
    </main>
  </body>
</html>
<!-- This is also a valid Astro component! -->
<main>
  <h1>Hello world!</h1>
</main>

Developers have come up with a myriad of different techniques for composing blocks of HTML over the years, but far and away the most successful has been JSX.

We love JSX! In fact, .astro files borrow the highly-expressive expression syntax directly from JSX.

<!-- This is an Astro component with expressions! -->
<main>
  <h1>Hello {name}!</h1>
  <ul>
    {items.map((item) => (
      <li>{item}</li>
    ))}
  </ul>
  <h2 data-hint={`Use JS template strings when you need to mix-in ${"variables"}.`}>So good!</h2>
</main>

.astro files also borrow the concept of Frontmatter from Markdown. Instead of introducing a new HTML-oriented import and export syntax, .astro just uses JavaScript.

---
// This area is TypeScript (and therefore JavaScript)!
import MyComponent from './MyComponent.astro'
---

<html lang="en">
  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
  </head>
  <body>
    <MyComponent></MyComponent>
  </body>
</html>

Data and Props

.astro components can define local variables inside of the Frontmatter script. These are automatically exposed to the content below.

---
let name = 'world';
---

<main>
    <h1>Hello {name}!</h1>
</main>

.astro components can also accept props when they are rendered. Public props can be marked using the export keyword.

Local values are overwritten when props are passed, otherwise they are considered the default value.

---
export let greeting = 'Hello';
export let name;
---

<main>
    <h1>{greeting} {name}!</h1>
</main>

Fragments

At the top-level of an .astro file, you may render any number of elements.

<!-- Look, no Fragment! -->
<div id="a" />
<div id="b" />
<div id="c" />

Inside of an expression, you must wrap multiple elements in a Fragment. Fragments must open with <> and close with </>.

<div>
  {[0, 1, 2].map((id) => (
    <>
      <div id={`a-${id}`} />
      <div id={`b-${id}`} />
      <div id={`c-${id}`} />
    </>
  ))}
</div>

.astro versus .jsx

.astro files can end up looking very similar to .jsx files, but there are a few key differences. Here's a comparison between the two formats.

Feature Astro JSX
File extension .astro .jsx or .tsx
User-Defined Components <Capitalized> <Capitalized>
Expression Syntax {} {}
Spread Attributes {...props} {...props}
Boolean Attributes autocomplete === autocomplete={true} autocomplete === autocomplete={true}
Inline Functions {items.map(item => <li>{item}</li>)} {items.map(item => <li>{item}</li>)}
IDE Support WIP - VS Code Phenomenal
Requires JS import No Yes, jsxPragma (React or h) must be in scope
Fragments Automatic top-level, <> inside functions Wrap with <Fragment> or <>
Multiple frameworks per-file Yes No
Modifying <head> Just use <head> Per-framework (<Head>, <svelte:head>, etc)
Comment Style <!-- HTML --> {/* JavaScript */}
Special Characters &nbsp; {'\xa0'} or {String.fromCharCode(160)}
Attributes dash-case camelCase

URL resolution

It’s important to note that Astro won’t transform HTML references for you. For example, consider an <img> tag with a relative src attribute inside src/pages/about.astro:

<!-- ❌ Incorrect: will try and load `/about/thumbnail.png` -->
<img src="./thumbnail.png" />

Since src/pages/about.astro will build to /about/index.html, you may not have expected that image to live at /about/thumbnail.png. So to fix this, choose either of two options:

Option 1: Absolute URLs

<!-- ✅ Correct: references public/thumbnail.png -->
<img src="/thumbnail.png" />

The recommended approach is to place files within public/*. This references a file it public/thumbnail.png, which will resolve to /thumbnail.png at the final build (since public/ ends up at /).

Option 2: Asset import references

---
//  ✅ Correct: references src/thumbnail.png
import thumbnailSrc from './thumbnail.png';
---

<img src={thumbnailSrc} />

If you’d prefer to organize assets alongside Astro components, you may import the file in JavaScript inside the component script. This works as intended but this makes thumbnail.png harder to reference in other parts of your app, as its final URL isn’t easily-predictable (unlike assets in public/*, where the final URL is guaranteed to never change).

TODO: Composition (Slots)