Dynamic routes with [param] in NextJS - Performance & Optimization

Performance: Dynamic routes with [param]

MEDIUM IMPACT

This affects page load speed and rendering performance by controlling how pages are generated and served based on URL parameters.

Serving pages based on URL parameters efficiently

NextJS

export async function getStaticPaths() {
  return { paths: [{ params: { param: 'a' } }, { params: { param: 'b' } }], fallback: false };
}

export async function getStaticProps({ params }) {
  const data = await fetchData(params.param);
  return { props: { data } };
}

export default function Page({ data }) {
  return <div>{data.content}</div>;
}

Pre-generating pages at build time avoids client fetch delays and speeds up initial paint.

📈 Performance GainImproves LCP by 300-500ms, reduces client CPU and network load

Serving pages based on URL parameters efficiently

NextJS

export default function Page({ params }) {
  const [data, setData] = React.useState(null);
  React.useEffect(() => {
    fetch(`/api/data?param=${params.param}`)
      .then(res => res.json())
      .then(data => setData(data));
  }, [params.param]);
  if (!data) return <div>Loading...</div>;
  return <div>{data.content}</div>;
}

// No getStaticPaths or getServerSideProps used

Fetching data on client side causes slower initial load and delays Largest Contentful Paint.

📉 Performance CostBlocks rendering until client fetch completes, increasing LCP by 300-500ms

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Client-side data fetch in dynamic route	Minimal DOM nodes	1 reflow after data loads	High paint cost due to delayed content	[X] Bad
Static generation with getStaticProps and getStaticPaths	Minimal DOM nodes	Single reflow on initial paint	Low paint cost, fast content display	[OK] Good

Rendering Pipeline

Dynamic routes with [param] influence how Next.js fetches and renders page content either at build time or on request, affecting the critical rendering path.

→Server-side Rendering

→Static Generation

→Network

→Paint

⚠️ BottleneckNetwork and Server-side Rendering delay if data fetching is done client-side

Core Web Vital Affected

LCP

This affects page load speed and rendering performance by controlling how pages are generated and served based on URL parameters.

Optimization Tips

1Pre-generate dynamic pages with getStaticProps and getStaticPaths when possible.

2Avoid client-side data fetching for initial page content to improve LCP.

3Use fallback options wisely to balance build time and user experience.

Performance Quiz - 3 Questions

What is the main performance benefit of using getStaticProps with dynamic routes in Next.js?

A Delays rendering until user interaction

B Fetches data on client side for faster interaction

C Pre-generates pages to reduce client load and improve LCP

D Increases bundle size for all pages

DevTools: Performance

How to check: Record page load in Performance tab, look for long network fetches and delayed Largest Contentful Paint event

What to look for: Long gaps before LCP event indicate client-side data fetching delays; faster LCP means better performance

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