Why Dynamic routes with [param] in NextJS? - Purpose & Use Cases

The Big Idea

Discover how one simple file can handle hundreds of unique URLs effortlessly!

The Scenario

Imagine building a website where you have hundreds of user profiles, each with a unique URL like /user/john or /user/mary. You try to create a separate page file for each user manually.

The Problem

Manually creating a file for every user is impossible to maintain. Every time a new user joins, you must add a new file. It's slow, error-prone, and your project quickly becomes cluttered and hard to update.

The Solution

Dynamic routes with [param] let you create one flexible page that automatically handles all user URLs. You write code once, and Next.js fills in the details based on the URL, making your app scalable and easy to manage.

Before vs After

✗ Before

pages/user/john.js
pages/user/mary.js

✓ After

pages/user/[username].js

What It Enables

This lets you build scalable websites that respond to many different URLs with just one dynamic page, saving time and reducing errors.

Real Life Example

A blog site where each post has its own URL like /post/how-to-cook or /post/learn-nextjs, all handled by a single dynamic route page.

Key Takeaways

Manual page creation for each URL is not practical.

Dynamic routes use [param] to handle many URLs with one page.

This makes your app scalable, cleaner, and easier to maintain.

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