Build output analysis in NextJS - Mini Project: Build & Apply

Build Output Analysis in Next.js

📖 Scenario: You are working on a Next.js project. You want to understand how the build output looks and how to analyze it to improve your app's performance.

🎯 Goal: Learn how to generate and analyze the build output of a Next.js app using the next build command and inspect the generated files.

📋 What You'll Learn

Create a simple Next.js app with a basic page

Add a configuration variable to enable build output tracing

Run the build command and capture the output files

Analyze the build output files and add a summary comment

💡 Why This Matters

🌍 Real World

Understanding build output helps developers optimize Next.js apps for faster loading and smaller bundles.

💼 Career

Knowing how to analyze and configure build output is important for frontend engineers working with Next.js in production environments.

1

Create a basic Next.js page

Create a file called pages/index.js with a React functional component named HomePage that returns an <h1> element with the text "Welcome to Next.js Build Analysis".

NextJS

// Create the HomePage component in pages/index.js
// Your code here

Hint

This is a simple React functional component that returns a heading.

2

Add build output tracing configuration

In the root of your project, create or update next.config.js to export an object with experimental: { outputFileTracing: true } to enable build output tracing.

NextJS

export default function HomePage() {
  return Welcome to Next.js Build Analysis;
}

// Create next.config.js with outputFileTracing enabled
// Your code here

Hint

This setting helps Next.js track which files are used in the build output.

3

Run the build and inspect output files

Run next build in your terminal to generate the production build. Then, in your project folder, locate the .next/server/pages/index.js file and note that it contains the server-side code for your page.

NextJS

export default function HomePage() {
  return Welcome to Next.js Build Analysis;
}

// next.config.js
module.exports = {
  experimental: {
    outputFileTracing: true
  }
};

// Run `next build` in terminal and check .next/server/pages/index.js
// Your code here

Hint

Use your terminal to run next build and then explore the .next/server/pages folder.

4

Add a summary comment about build output

Add a comment at the end of pages/index.js explaining that the build output files include server and client bundles, and that output tracing helps reduce bundle size by including only used files.

NextJS

export default function HomePage() {
  return Welcome to Next.js Build Analysis;
}

// Add a summary comment about build output here
// Your code here

Hint

Write a clear comment summarizing what you learned about the build output.

Practice

1. What is the main purpose of running npx next build in a Next.js project?

easy

A. To install all project dependencies

B. To start the development server with hot reloading

C. To generate a new Next.js project

D. To create an optimized production build of the app

Solution

1. Step 1: Understand the command purpose

   npx next build compiles the app for production, optimizing files.

2. Step 2: Compare with other commands

   next dev starts development server, npm install installs packages, npx create-next-app creates a new project.

3. Final Answer:

   To create an optimized production build of the app -> Option D

4. Quick Check:

   Build command = production build [OK]

Hint: Build command creates production files, dev command runs server [OK]

Common Mistakes:

• Confusing build with development server start
• Thinking build installs dependencies
• Mixing build with project creation

2. Which of the following is the correct command to generate a detailed build size report in Next.js?

easy

A. npx next build --profile

B. npx next build

C. npx next build --stats

D. npx next start --report

Solution

1. Step 1: Identify the command for build analysis

   Next.js npx next build generates a bundle size report.

2. Step 2: Check other options

   --profile and --stats are not valid Next.js build flags; next start runs the server, not build.

3. Final Answer:

   npx next build -> Option B

4. Quick Check:

   Build command = detailed build report [OK]

Hint: next build generates bundle sizes [OK]

Common Mistakes:

• Using start command instead of build
• Confusing profile or stats flags
• Using invalid flags like --analyze

3. Given the following Next.js build output snippet:

Page                                                           Size     First Load JS
┌ ○ /                                                          1.2 kB         45.3 kB
├   /_app                                                      0 B           44.1 kB
└ ○ /about                                                     2.3 kB         46.4 kB

What does the 'First Load JS' column represent?

medium

A. The size of CSS files loaded for that page

B. The size of the page's HTML file

C. The total JavaScript size loaded on the first visit to that page

D. The size of images loaded on that page

Solution

1. Step 1: Understand the build output columns

   The 'First Load JS' shows the JavaScript bytes downloaded when the page loads initially.

2. Step 2: Differentiate from other assets

   HTML size is separate, CSS and images are not counted in this JS size column.

3. Final Answer:

   The total JavaScript size loaded on the first visit to that page -> Option C

4. Quick Check:

   First Load JS = initial JS bytes [OK]

Hint: First Load JS means initial JavaScript bytes loaded [OK]

Common Mistakes:

• Thinking it shows HTML or CSS size
• Confusing with image sizes
• Ignoring shared JS loaded by _app

4. You run npx next build but get an error: Module not found: Can't resolve 'xyz'. What is the best way to fix this?

medium

A. Install the missing package 'xyz' using npm or yarn

B. Delete the .next folder and rebuild

C. Change the import statement to a different module

D. Ignore the error and run next start

Solution

1. Step 1: Understand the error cause

   The error means the build cannot find the module 'xyz' because it is not installed or missing.

2. Step 2: Fix by installing the missing package

   Run npm install xyz or yarn add xyz to add the dependency.

3. Final Answer:

   Install the missing package 'xyz' using npm or yarn -> Option A

4. Quick Check:

   Module not found = install missing package [OK]

Hint: Missing module error means install that package [OK]

Common Mistakes:

• Deleting build folder without fixing dependencies
• Ignoring error and starting server anyway
• Changing imports without knowing correct module

5. You notice your Next.js build output shows a very large First Load JS size for the homepage. Which approach will best reduce this size?

hard

A. Use dynamic imports to load heavy components only when needed

B. Remove the _app.js file entirely

C. Increase the server RAM to speed up build

D. Disable JavaScript in the browser

Solution

1. Step 1: Identify cause of large JS size

   Large first load JS means too much code is bundled upfront, slowing page load.

2. Step 2: Apply code splitting with dynamic imports

   Dynamic imports let Next.js load heavy components only when needed, reducing initial JS size.

3. Step 3: Evaluate other options

   Removing _app.js breaks app structure, increasing RAM doesn't reduce bundle size, disabling JS breaks app.

4. Final Answer:

   Use dynamic imports to load heavy components only when needed -> Option A

5. Quick Check:

   Dynamic imports = smaller initial JS [OK]

Hint: Split code with dynamic imports to shrink initial JS [OK]

Common Mistakes:

• Deleting _app.js which is essential
• Thinking server RAM affects bundle size
• Disabling JavaScript breaks app functionality