Build output analysis in NextJS

Introduction

Build output analysis helps you understand what files and code your Next.js app creates when you build it. This shows how your app is organized and how big each part is.

You want to see which parts of your app take the most space after building.

You need to find unused code or big files slowing down your app.

You want to optimize loading speed by checking what loads on the client.

You want to understand how Next.js splits your code into chunks.

You want to check if your images and assets are included correctly.

Syntax

NextJS

npx next build
npx next build --profile
npx next build && npx source-map-explorer '.next/static/chunks/*.js'

npx next build creates the production build of your app.

Using --profile or tools like source-map-explorer helps see detailed info about the build output.

Examples

This runs the build process and shows basic output in the terminal.

NextJS

npx next build

This runs the build and shows detailed timing and size info for each part.

NextJS

npx next build --profile

This builds the app and then opens a visual report to explore bundle sizes interactively.

NextJS

npx next build && npx source-map-explorer '.next/static/chunks/*.js'

Sample Program

This is what you see after running npx next build. It lists pages, their sizes, and shared JavaScript chunks. This helps you know which pages are bigger and what code is shared.

NextJS

/* Run in terminal */
npx next build

/* Sample output snippet */
info  - Creating an optimized production build...
info  - Compiled successfully
info  - Collecting page data...
Page                                                           Size     First Load JS
┌ ○ /                                                          1.2 kB         75.3 kB
├   /_app                                                      0 B            73.9 kB
├ ○ /about                                                     2.3 kB         75.4 kB
└ ● /index                                                     3.1 kB         75.3 kB
+ First Load JS shared by all                                  73.9 kB
  ├ chunks/framework.123abc.js                                 45.6 kB
  ├ chunks/main.456def.js                                      20.1 kB
  └ chunks/pages/index.789ghi.js                               8.2 kB

Page                                                           Size     First Load JS
┌ ○ /contact                                                   2.0 kB         75.3 kB

Done in 15.3s.

Important Notes

The build output shows sizes in kilobytes (kB) to help you spot large files.

Pages marked with ○ are static, ● means server-side rendered.

Use tools like source-map-explorer for a visual breakdown of your bundles.

Summary

Build output analysis helps you see what your Next.js app creates when building.

You can find big files and optimize your app's loading speed.

Use commands like npx next build and tools for detailed reports.

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