Dynamic routes with parameters in Astro - Step-by-Step Execution

Concept Flow - Dynamic routes with parameters

User visits URL

↓

Match route pattern

↓

Extract parameter from URL

↓

Pass parameter to page/component

↓

Render page with parameter data

↓

Show dynamic content based on parameter

When a user visits a URL, Astro matches the dynamic route pattern, extracts parameters, and renders the page using those parameters.

Execution Sample

Astro

src/pages/blog/[slug].astro
---
const { slug } = Astro.params;
---
<h1>Post: {slug}</h1>

This Astro page uses a dynamic route to capture the 'slug' parameter from the URL and display it in the page.

Execution Table

Step	URL Visited	Route Pattern Matched	Parameter Extracted	Rendered Output
1	/blog/hello-world	/blog/[slug]	slug = 'hello-world'	<h1>Post: hello-world</h1>
2	/blog/astro-tutorial	/blog/[slug]	slug = 'astro-tutorial'	<h1>Post: astro-tutorial</h1>
3	/blog/	No match	No parameter	404 or fallback page

💡 Execution stops when no matching dynamic route pattern is found or page is rendered.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
slug	undefined	'hello-world'	'astro-tutorial'	undefined

Key Moments - 2 Insights

Why does the page show 404 when visiting /blog/ without a slug?

Because the dynamic route pattern /blog/[slug] expects a parameter after /blog/. Without it, no route matches, as shown in execution_table step 3.

How does Astro know what part of the URL is the parameter?

Astro uses the [slug] in the filename to capture that part of the URL as a parameter, as seen in execution_table steps 1 and 2 where slug is extracted.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'slug' at step 2?

A 'astro-tutorial'

B 'hello-world'

C undefined

D No parameter

💡 Hint

Check the 'Parameter Extracted' column at step 2 in the execution_table.

At which step does the route pattern fail to match?

A Step 2

B Step 3

C Step 1

D None, all match

💡 Hint

Look at the 'Route Pattern Matched' column for each step in the execution_table.

If the URL was /blog/my-first-post, what would the rendered output be?

A 404 page

B <h1>Post: blog</h1>

C <h1>Post: my-first-post</h1>

D <h1>Post: slug</h1>

💡 Hint

Refer to how the slug parameter is used in the rendered output in the execution_table.

Concept Snapshot

Dynamic routes in Astro use filenames with [param] to capture URL parts.
Astro extracts the parameter from the URL and passes it to the page.
Use Astro.params to access the parameter inside the page.
If URL doesn't match pattern, Astro shows 404 or fallback.
This enables pages to show dynamic content based on URL parts.

Full Transcript

Dynamic routes with parameters in Astro let you create pages that change based on the URL. When a user visits a URL like /blog/hello-world, Astro looks for a matching route pattern like /blog/[slug]. It extracts the slug parameter from the URL and passes it to the page. The page can then use this slug to show content specific to that URL. If the URL does not include the parameter part, like just /blog/, Astro does not find a match and shows a 404 page. This way, you can build flexible pages that respond to different URLs easily.

Practice

1. What does a filename like [id].astro represent in Astro?

easy

A. A dynamic route that captures the id parameter from the URL

B. A static page named literally [id]

C. A component that cannot receive parameters

D. A special Astro configuration file

Solution

1. Step 1: Understand dynamic route syntax

   In Astro, square brackets in filenames like [id].astro define a dynamic route parameter named id.

2. Step 2: Recognize the purpose of the parameter

   This parameter captures part of the URL, allowing the page to change based on the URL segment.

3. Final Answer:

   A dynamic route that captures the id parameter from the URL -> Option A

4. Quick Check:

   Dynamic route = filename with [param] [OK]

Hint: Look for square brackets in filename for dynamic parameters [OK]

Common Mistakes:

• Thinking [id].astro is a static file
• Confusing parameter with component name
• Assuming parameters are optional by default

2. Which is the correct way to access a dynamic route parameter named slug inside an Astro component?

easy

A. Astro.query.slug

B. Astro.route.slug

C. Astro.params.slug

D. Astro.props.slug

Solution

1. Step 1: Recall Astro parameter access

   Astro provides dynamic route parameters inside the component via Astro.params.

2. Step 2: Use the parameter name as key

   To get the value of slug, use Astro.params.slug.

3. Final Answer:

   Astro.params.slug -> Option C

4. Quick Check:

   Access params with Astro.params.[name] [OK]

Hint: Use Astro.params to get dynamic route values [OK]

Common Mistakes:

• Using Astro.route or Astro.query instead of Astro.params
• Trying to access parameters via props
• Forgetting to use the exact parameter name

3. Given a file named src/pages/blog/[post].astro with this code snippet:

---
const postId = Astro.params.post;
---
Post ID: {postId}

What will be rendered when visiting /blog/42?

medium

A. Post ID: 42

B. Post ID: post

C. Post ID:

D. 404 Not Found

Solution

1. Step 1: Identify dynamic parameter value

   The URL /blog/42 matches the dynamic route [post].astro, so Astro.params.post is '42'.

2. Step 2: Render the parameter in the template

   The component outputs <h1>Post ID: {postId}</h1>, so it shows Post ID: 42.

3. Final Answer:

   <h1>Post ID: 42</h1> -> Option A

4. Quick Check:

   URL segment fills param = rendered value [OK]

Hint: URL segment replaces [param] and shows in Astro.params [OK]

Common Mistakes:

• Expecting literal 'post' instead of parameter value
• Assuming empty or missing parameter
• Confusing 404 with dynamic route handling

4. You have a dynamic route file named [user].astro but when visiting /john you get a runtime error: Cannot read property 'user' of undefined. What is the likely cause?

medium

A. Astro.params is only available in server components

B. You forgot to use Astro.params.user and accessed Astro.params directly

C. Dynamic routes cannot have parameters named 'user'

D. The filename should be {user}.astro instead of [user].astro

Solution

1. Step 1: Understand the error message

   The error says Cannot read property 'user' of undefined, meaning Astro.params is undefined or not accessed properly.

2. Step 2: Check parameter access syntax

   Accessing Astro.params.user is correct. If code tries to access Astro.params.user but Astro.params is undefined, it means Astro.params was not used or imported correctly.

3. Final Answer:

   You forgot to use Astro.params.user and accessed Astro.params directly -> Option B

4. Quick Check:

   Always use Astro.params.[param] to avoid undefined errors [OK]

Hint: Always access parameters via Astro.params.[name] [OK]

Common Mistakes:

• Using curly braces instead of square brackets in filename
• Assuming Astro.params is always defined globally
• Misnaming the parameter in code

5. You want to create a nested dynamic route in Astro to handle URLs like /products/electronics/123. Which file structure and parameter access is correct?

hard

A. Create src/pages/products/[category]/id.astro and access with Astro.params.id

B. Create src/pages/products/[category-id].astro and access with Astro.params['category-id']

C. Create src/pages/products/category/id.astro and access with Astro.params.category and Astro.params.id

D. Create src/pages/products/[category]/[id].astro and access with Astro.params.category and Astro.params.id

Solution

1. Step 1: Understand nested dynamic routes

   To match URLs like /products/electronics/123, you need nested folders with dynamic parameters: [category] and [id].

2. Step 2: Correct file structure and parameter access

   The correct structure is src/pages/products/[category]/[id].astro. Inside, access parameters as Astro.params.category and Astro.params.id.

3. Final Answer:

   Create src/pages/products/[category]/[id].astro and access with Astro.params.category and Astro.params.id -> Option D

4. Quick Check:

   Nested folders with [param] = nested params [OK]

Hint: Use nested folders with [param] for multi-level dynamic routes [OK]

Common Mistakes:

• Combining multiple params in one filename incorrectly
• Using static folder names instead of dynamic
• Accessing parameters that don't exist in the route