Dynamic routes with [param] in NextJS

Introduction

Dynamic routes let your app show different pages based on the URL. This helps create pages for many items without making each page by hand.

You want to show a user profile page for each user using their ID in the URL.

You have a blog and want each post to have its own page based on the post's slug.

You need product pages that change depending on the product ID in the URL.

You want to build a details page for events where the event ID is part of the URL.

Syntax

NextJS

app/[param]/page.jsx

// or in pages directory
pages/[param].jsx

The folder or file name uses square brackets [param] to mark a dynamic part.

The param name is the variable you use to get the value from the URL.

Examples

This example shows a dynamic route where id is the URL part you want to read.

NextJS

app/[id]/page.jsx

export default function Page({ params }) {
  return <p>ID is {params.id}</p>;
}

Here, slug is the dynamic part of the URL for a blog post.

NextJS

pages/[slug].jsx

export default function Post({ params }) {
  return <h1>Post: {params.slug}</h1>;
}

Sample Program

This component shows a product page. The productId comes from the URL, so if you visit /product/123, it shows "Product ID: 123".

NextJS

app/product/[productId]/page.jsx

export default function ProductPage({ params }) {
  return (
    <main>
      <h1>Product Details</h1>
      <p>Product ID: {params.productId}</p>
    </main>
  );
}

Important Notes

Dynamic route parameters are always strings from the URL.

Use the params object in your component to access the dynamic parts.

Dynamic routes help keep your app organized and scalable.

Summary

Dynamic routes use square brackets [param] in file or folder names.

The params object gives you the dynamic value from the URL.

This lets you create many pages with one component by changing content based on the URL.

Practice

1. What does the file name [id].js inside the pages folder in Next.js represent?

easy

A. A static page named literally 'id'

B. A dynamic route where id is a variable part of the URL

C. A special API route for handling requests

D. A CSS module file for styling

Solution

1. Step 1: Understand Next.js dynamic route naming

   Files with names in square brackets like [id].js represent dynamic routes where the part inside brackets is a variable.

2. Step 2: Interpret the meaning of [id].js

   This means the URL can have any value in place of id, and the page will render accordingly.

3. Final Answer:

   A dynamic route where id is a variable part of the URL -> Option B

4. Quick Check:

   Dynamic route = [param] file name [OK]

Hint: Square brackets mean dynamic URL part [OK]

Common Mistakes:

• Thinking it's a static page named 'id'
• Confusing with API routes
• Assuming it's a styling file

2. Which of the following is the correct way to access the dynamic parameter slug in a Next.js page component inside pages/[slug].js?

easy

A. const { slug } = props.params;

B. const slug = params.slug;

C. const slug = useRouter().query.slug;

D. const slug = window.location.slug;

Solution

1. Step 1: Recall how to get dynamic params in Next.js client-side

   Next.js provides the useRouter hook to access route parameters via router.query.

2. Step 2: Identify correct syntax

   The correct way is const slug = useRouter().query.slug; to get the dynamic slug value.

3. Final Answer:

   const slug = useRouter().query.slug; -> Option C

4. Quick Check:

   useRouter().query.param gets dynamic route param [OK]

Hint: Use useRouter().query.param for dynamic params [OK]

Common Mistakes:

• Trying to destructure from props.params directly
• Using window.location which doesn't have slug
• Using undefined variable params

3. Given the file pages/blog/[postId].js with this code:

import { useRouter } from 'next/router';
export default function Post() {
  const router = useRouter();
  return <div>Post ID: {router.query.postId}</div>;
}

What will be rendered when the user visits /blog/42?

medium

A. Post ID: postId

B. Error: router is undefined

C. Post ID: undefined

D. Post ID: 42

Solution

1. Step 1: Understand dynamic route matching

   The file [postId].js matches URLs like /blog/42 and sets postId to '42'.

2. Step 2: Check how router.query.postId is used

   The component uses router.query.postId to display the dynamic value, so it will show '42'.

3. Final Answer:

   Post ID: 42 -> Option D

4. Quick Check:

   Dynamic param value shown = 42 [OK]

Hint: router.query.param shows URL dynamic value [OK]

Common Mistakes:

• Expecting literal 'postId' string output
• Assuming undefined because of initial render
• Thinking router is not defined

4. You have a dynamic route file pages/product/[productId].js. The component tries to access params.productId directly but gets an error. What is the likely cause?

medium

A. The component should use useRouter().query.productId instead of params.productId

B. The file name should be productId.js without brackets

C. Dynamic routes do not support parameters

D. You must import params from 'next/params'

Solution

1. Step 1: Identify how to access dynamic params in client components

   In Next.js, dynamic route params are accessed via useRouter().query in client components, not directly from params.

2. Step 2: Understand why params.productId causes error

   Using params.productId without proper context or props causes undefined errors.

3. Final Answer:

   The component should use useRouter().query.productId instead of params.productId -> Option A

4. Quick Check:

   Use useRouter().query for dynamic params [OK]

Hint: Use useRouter().query, not params directly [OK]

Common Mistakes:

• Trying to access params without props or hooks
• Renaming file incorrectly
• Expecting automatic import of params

5. You want to create a dynamic route in Next.js that matches URLs like /user/123/profile. Which folder and file structure inside app directory correctly implements this using dynamic segments?

hard

A. app/user/[id]/profile/page.js

B. app/user/profile/[id].js

C. app/user/[id]-profile/page.js

D. app/user/[id]/profile.js

Solution

1. Step 1: Understand Next.js app router folder structure

   Each folder represents a URL segment. Dynamic segments use square brackets like [id].

2. Step 2: Map URL /user/123/profile to folders

   The URL has three parts: 'user', dynamic '123', and 'profile'. So folders should be user/[id]/profile with a page.js inside.

3. Final Answer:

   app/user/[id]/profile/page.js -> Option A

4. Quick Check:

   Dynamic folder inside URL path = [param] folder [OK]

Hint: Dynamic segments are folders with [param] in app router [OK]

Common Mistakes:

• Placing dynamic param after static folder incorrectly
• Using dash instead of folder for dynamic segment
• Putting dynamic param in wrong folder level