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Sustainability in Web Dev
How to Optimize React Apps?
Jussi Pohjolainen
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Key Points
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Key Points
• Bundle Size and Network Usage
• Green Cloud Services
• Frontend Optimization
• Backend Optimization
• Monitoring and Analytics
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Bundle Size and Network Usage
• Smaller Bundles: Smaller files require less energy for transmission
and storage. This reduces the load on data centers, networks, and
user devices.
• Efficient Network Usage: Optimizing data transfer reduces the
energy demand of the network infrastructure, including routers,
switches, and undersea cables.
• Use lazy loading and tree-shaking.
• Optimize images and serve modern formats (e.g., WebP/AVIF).
• Implement code splitting.
• Use Content Delivery Networks (CDNs) for localized distribution.
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Green Cloud Services
• Green Cloud Providers
• Use cloud providers committed to renewable energy and carbon neutrality
(e.g., AWS with its sustainability initiatives, Google Cloud, or Azure with green
energy offsets).
• Energy Efficiency
• Choose data centers near your user base to reduce latency and energy used
for data transmission.
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Frontend Optimization
• Design lightweight websites that load quickly and reduce CPU usage
on user devices, especially mobile.
• Static Sites: Consider static site generation (e.g., Next.js, Hugo) to
reduce server load.
• Caching: Leverage browser caching and service workers for offline
support.
• Dark Mode: Reduce energy use, particularly on OLED screens.
• Minimalist Design: Simplify interfaces to use fewer assets.
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Backend Optimization
• Server Efficiency: Optimize API calls and avoid redundant requests.
• Database Queries: Minimize heavy queries and cache results where
appropriate.
• Serverless Architectures: Dynamically allocate resources to avoid
always-on server energy consumption.
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Monitoring and Analytics
• Use tools like Website Carbon Calculator to measure and track your
site's carbon footprint.
• Chrome Dev Tools
• Next.js analyzer
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In Overall
• Energy Efficiency: Optimize code, server infrastructure, and asset
delivery to minimize energy consumption and carbon emissions.
• Performance Optimization: Build fast-loading, resource-efficient
websites using techniques like caching, compression, and lazy
loading.
• Sustainable Hosting: Choose hosting providers powered by
renewable energy or with carbon offset programs.
• https:!//www.thegreenwebfoundation.org/tools/directory/
• Inclusive and Minimal Design: Create accessible, lightweight
websites optimized for all devices and internet speeds.
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React App Optimization
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Key Areas for Optimization
1. Efficient data fetching (SSR and CSR)
2. Reducing bundle size
3. Reducing number of renders
4. Monitoring performance using tools
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CSR
Step Interaction Description
1. Browser -> Server Request HTML + JS
The browser sends a request to the
server for the initial page load.
2. Server -> Browser Sends HTML Skeleton + JS
The server responds with a minimal
HTML file and associated JavaScript files.
3. Browser Loads and Executes JavaScript
The browser parses the HTML,
downloads, and executes JavaScript
files.
4. Browser -> API Fetches Data
The JavaScript running in the browser
makes API calls to fetch dynamic data.
5. API/Backend -> Browser Sends Data Response
The backend API responds with the
requested data in JSON or other formats.
6. Browser Renders Content
The JavaScript dynamically updates the
DOM using fetched data.
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SSR
Step Interaction Description
1. Browser -> Server Request HTML
The browser sends a request to the server
for the page.
2. Server -> Browser
Sends Fully Rendered
HTML
The server responds with the HTML
content already rendered.
3. Browser
Parses and Displays
HTML
The browser parses the HTML and displays
the fully rendered page.
4. Browser -> Server
Downloads JS (for
hydration)
The browser requests JavaScript files for
hydration (making the app interactive).
5. Browser
Executes JS (Hydrates
App)
The browser executes JavaScript to bind
event listeners and make the app
interactive.
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Client Side Rendering (CSR)
• Increases time to interactive (TTI) since data is fetched after the
initial page load.
• Poor SEO (Search Engine Optimization) because search engines
may not execute JavaScript to fetch the data.
• Higher client-side resource usage, affecting performance on
slower devices.
• Longer data fetching times on the client can lead to increased
energy usage on end-user devices
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Basic Client Side Rendering
import { useEffect, useState } from "react";
export default function Home() {
const [data, setData] = useState([]);
useEffect(() !=> {
fetch('https:!//example.com/data.json')
.then((res) !=> res.json())
.then((json) !=> setData(json));
}, []);
return (
);
}
Data List!
-
{data.map((item, index) !=> (
- {item.name}! ))} !
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Server Side Rendering (SSR)
• Improves time to interactive (TTI) since the data is already rendered when
page loads
• Better SEO as search engines, caa crawl pre-rendered content
• Reduces the load on the client-side, improving performance on low-end
devices
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Tooling: Vite (React)
• Primary Use Case: Fast, modern development experience for single-page
applications (SPAs).
• Focus: Vite is a build tool designed to replace Webpack for front-end
projects. It provides a highly optimized development environment with
lightning-fast hot module replacement (HMR).
• Use Case: Ideal for small to medium SPAs or projects where you want full
control over the project structure and configuration.
• Key Feature: Extremely fast due to its use of native ES modules in development
and pre-bundling using esbuild.
• Setup: Minimal and unopinionated. It focuses on speed and leaves the choice of
additional tools and libraries up to the developer.
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Tooling: Next.js
• Primary Use Case: Production-ready framework for server-rendered and
static websites.
• Focus: Next.js is a React framework offering opinionated conventions and
built-in features like server-side rendering (SSR), static site generation (SSG),
and API routes.
• Use Case: Best for large-scale projects, content-heavy websites, or
applications requiring advanced routing, SSR, SSG, or backend functionality.
• Key Feature: Comes with a built-in development server that supports SSR, SSG,
and client-side rendering (CSR).
• Setup: Opinionated and comprehensive, providing everything out of the box for
a complete full-stack solution.
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Comparison
Feature Next.js Vite
Ease of Use Very easy; minimal setup needed. Requires manual setup for SSR.
Routing Built-in file-based routing. Requires custom routing setup.
Production Readiness
Built-in optimizations and
deployments. Requires manual optimization.
Development Speed Fast but slightly slower than Vite. Extremely fast dev server.
Customizability Limited to framework conventions. Highly customizable.
Community Support Large, active community. Smaller but growing rapidly.
Streaming SSR Fully supported out-of-the-box.
Supported but needs
configuration.
Use Case Best for medium to large-scale apps.
Best for lightweight/customized
apps.
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App Router
Enables React 18 server components, new routing
mechanism, new pattern for fetch() and replaces the "older"
methods like getServerSideProps
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Next.js: SSR
export async function getServerSideProps() {
const res = await fetch('https:!//example.com/data.json');
const content = await res.json();
const props = { props: { data: content } }
return props;
}
export default function Home({ data }) {
return (
);
}
getServerSideProps i
s a special Next.js
function used to fetch
data on the server
side for each request to
the page. App Router is
disabled
It runs on the server at
request time and
passes the fetched data
as props to the React
component.
this is passed as a
props to the
component
Data List!
-
{data.map((item, index) !=> (
- {item.name}! ))} !
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React 18 (and Next.js): SSR
export default async function Home() {
const res = await fetch('https:!//example.com/data.json');
const data = await res.json();
return (
);
}
When component is async
and it becomes ssr
component
Does not use browser-
specific logic like useState,
useE=ect
Data List!
-
{data.map((item, index) !=> (
- {item.name}! ))} !
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Key features of SSR
• Data Fetching at Request Time
• Every time a user requests the page, getServerSideProps runs on the
server.
• This ensures the page always displays the latest data.
• SEO Benefits
• The page is pre-rendered with the fetched data on the server.
• Search engines can index the fully rendered HTML, improving SEO.
• Secure Fetching
• API calls or sensitive logic (like using API keys) are handled securely on
the server.
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Developer Control over SSR Optimization
• SSR gives developers greater control over the application's
infrastructure compared to CSR, which offloads rendering entirely
to client devices.
• CSR: The carbon footprint depends mainly entirely on the user's
browser and device.
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Real-World Considerations
• Global Traffic: For international users, SSR can increase the carbon
footprint if the server is centralized and far from the user. Using edge
servers mitigates this.
• High-Load Applications: SSR's carbon footprint increases with
traffic, making caching and scaling critical for reducing environmental
impact.
• Device Impact in CSR: CSR shifts the rendering workload to client
devices, which may be inefficient for lower-end or battery-powered
devices, leading to higher energy consumption per user.
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SSR vs CSR
• SSR is generally better for reducing the carbon footprint in scenarios
where:
• The application serves static or semi-dynamic content.
• Users rely on low-powered devices or poor internet connections.
• Fast, efficient data delivery is critical.
• CSR may be more efficient in scenarios where:
• The application involves complex interactivity or frequent data updates.
• The user base primarily uses high-performance devices.
• Client-side caching can significantly reduce repeated server requests.
• For sustainability, a hybrid approach (e.g., Static Site Generation or Server-
Side Rendering with Hydration) often provides the best balance, leveraging
the strengths of both SSR and CSR.
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Assignment 01
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Reducing Bundle Size
Code Splitting, Image Optimization, Tree Shaking, Reduce Dependencies
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Key Areas for Optimization
1. Efficient data fetching (SSR and CSR)
2. Reducing bundle size
• Code Splitting
• Image Optimization
• Tree Shaking
• Reduce Dependencies
3. Reducing number of renders
4. Monitoring performance using tools
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Why Bundle Size Matters for Carbon Footprint
• Data Transfer Energy Usage: Data transfer requires energy, and smaller
bundles mean less data being transferred. This reduces energy usage in:
• User Devices: Less processing power needed to parse and render the content.
• Data Centers: Reduced bandwidth and cooling energy requirements.
• Improved Device Longevity:
• Efficient websites cause less strain on end-user devices, extending their lifespan by
reducing unnecessary processing and heat generation.
• Better User Experience:
• Faster load times reduce bounce rates, encouraging sustainable practices like delivering
only the necessary resources.
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Code Splitting
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Reducing Bundle Size in React
• Code Splitting
• Image Optimization
• Tree Shaking
• Reduce Dependencies
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Default Behaviour
• In a typical React application, all components are bundled
together into one or more JavaScript files (called bundles) during
the build process.
• When a user accesses the application, the browser downloads
the entire bundle upfront, including components that might not be
immediately needed.
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Behaviour with Lazy Loading
• When you use React.lazy to lazy-load a component, its code
is excluded from the initial bundle.Instead, the browser only
fetches the code for the lazy-loaded component when the
component is about to be rendered in the app.
• This reduces the initial load time of the application because only
the essential components are loaded upfront.
• Imagine you have a page with two sections: Home and Settings.
• The Home section is displayed by default when the app loads.
• The Settings section can be accessed by clicking a button.
• With lazy loading, settings is not downloaded by default
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Benefits of Lazy Loading
• Performance:
• Reduces the initial download size of the application.
• Improves the app's load time, especially for large applications.
• Efficiency:
• Saves network bandwidth by not loading unused components or pages.
=> Carbon Footprint
• Ideal for applications with rarely-used features or components.
• User Experience:
• Users interact with the app sooner, as the essential features load quickly.
• The "loading..." fallback provides a smooth transition for the lazy-loaded
components.
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!// Home.js
export default function Home() {
return
Welcome to the Home Page!!
; } !// Settings.js export default function Settings() { returnSettings Page!
; } !// App.js import React, { useState } from "react"; import Home from "./Home"; import Settings from "./Settings"; function App() { const [showSettings, setShowSettings] = useState(false); return (
setShowSettings(!showSettings)}>
Toggle Settings
!
{showSettings ? : }
!
);
}
export default App;
Both Home.js and Settings.js
are imported statically at the
top of App.js.
During the build process, the
code for both components is
bundled into the initial
JavaScript file.
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No content
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!// Home.js
export default function Home() {
return
Welcome to the Home Page!!
; } !// Settings.js export default function Settings() { returnSettings Page!
; } !// App.js import React, { useState, Suspense } from "react"; !// Lazy load the Settings component const Settings = React.lazy(() !=> import("./Settings")); function App() { const [showSettings, setShowSettings] = useState(false); return (
setShowSettings(!showSettings)}>
Toggle Settings
!
Loading!!...!
}>
{showSettings ? : }
!
!
);
}
export default App;
The Home component is
statically imported, so it is
included in the initial bundle
and available immediately.
The Settings component is not
included in the initial bundle
because it is lazily loaded
using React.lazy.
While the browser fetches the
Settings component, the
Loading...
specified
in the Suspense component is
displayed.
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No content
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No content
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Lazy Loading in Next.js
• React.lazy is designed for client-side rendering, it does not SSR
• Dynamic import for components support CSR and SSR
• Automatic route-based code splitting
• Lazy loading of images using ![]()
component
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Next.js: Dynamic import for components
• React.lazy
• Designed for client-side rendering only. It does not support server-side
rendering (SSR).
• If you use React.lazy in a Next.js app that uses SSR, the server will not pre-
render the lazy-loaded component, potentially breaking SSR or returning an
empty page.
• Next.js dynamic
• Supports both SSR and client-side rendering (CSR).
• You can configure it to enable or disable SSR for a specific component using
the ssr option.
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Next.js dynamic vs React.lazy
Feature React.lazy Next.js dynamic
Where it can be
used Always outside the component.
Can be used both inside
and outside the
component.
Supports SSR No, client-side only. Yes, if ssr: true is specified.
Conditional
Imports Not possible.
Possible (e.g., inside a
button click handler).
Fallback Rendering Requires Suspense for fallback.
Customizable fallback via
the loading option.
Flexibility Limited to static, client-side usage.
Highly flexible: supports
SSR, CSR, and runtime
decisions.
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import dynamic from "next/dynamic";
import { useState } from "react";
export async function getServerSideProps() {
const response = await fetch("https:!//jsonplaceholder.typicode.com/users");
const data = await response.json();
return {
props: {
customers: data,
},
};
}
console.log("app loaded");
export default function App({ customers }) {
const [showTable, setShowTable] = useState(false);
const CustomerTable = dynamic(() !=> import("!../components/CustomerTable"), {
ssr: false, !// Disable SSR
loading: () !=>
);
}
Loading customer data!!...!
, }); return (Customer Information!
setShowTable((prev) !=> !prev)}> {showTable ? "Hide Customers" : "Show Customers"} ! {showTable !&& } !Slide 45
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console.log("Table loaded");
function CustomerTable({ data }) {
if (!Array.isArray(data) !|| data.length !!=== 0) {
return
No customer data available.!
; } return ( ! ! {data.map((customer) !=> ( ! ))} ! ! ID! Name! Email! Phone! {customer.id}! {customer.name}! {customer.email}! {customer.phone}! ); } export default CustomerTable;Slide 46
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!// App.js
import React, { useState } from "react";
import dynamic from "next/dynamic";
import Main from "!../components/Main";
export async function getServerSideProps(context) {
console.log("SSR");
const showTable = context.query.showtable !!=== "true";
let customers = [];
if (showTable) {
console.log("Fetch api");
const response = await fetch("https:!//jsonplaceholder.typicode.com/users");
customers = await response.json();
}
return {
props: {
customers,
initialShowTable: showTable,
},
};
}
console.log("Home loaded");
function Home({ customers, initialShowTable }) {
const [showTable, setShowTable] = useState(initialShowTable);
const CustomerTable = dynamic(() !=> import("!../components/CustomerTable"), {
ssr: true,
loading: () !=>
);
}
export default Home;
ssr is true
conditional
fetching
Loading customer data!!...!
, }); return (Customer Information!
{showTable ? : } !Slide 47
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Next.js Routes
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Next.js: Lazy Loading Routes
• Next.js automatically performs route-based code splitting,
meaning each page is bundled separately.
• When a user navigates to a new route, the corresponding JavaScript
for that page is fetched dynamically.
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Next.js: Routing
• Each file in the pages/ directory corresponds to a route.
• For example:
• pages/index.js → /
• pages/about.js → /about
• pages/contact.js → /contact
• You don’t need a separate router configuration like in React apps
with react-router-dom.
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Example: pages/page1.js
export default function Page1() {
return (
);
}
Welcome to Page 1!
This is the content of Page 1.!
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Best practice: Separate components
• Reusability: Components can be reused across multiple pages or
other components.
• Readability: Smaller components make your code easier to read,
maintain, and debug.
• Separation of Concerns: Keeps your pages focused on routing
and layout while moving reusable logic and presentation to
components.
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pages/page1.js
import Message from '!../components/Message';
export default function Page1() {
return (
);
}
Welcome to Page 1!
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components/Message.js
export default function Message({ text }) {
return
{text}!
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App Router
SSR and App Router
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App Router
• The App Router in Next.js (introduced in version 13) is a new
paradigm for structuring applications using React Server Components
(RSC)
• It is an alternative to the traditional Pages Router, enabling a more
flexible and modular approach to building applications.
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Key Features of the App Router
• React Server Components (RSC)
• Allows components to run on the server by default, reducing client-side
JavaScript and improving performance.
• Components in the App Router are server-rendered unless explicitly marked
as client components.
• File-Based Routing with Nested Layouts
• Introduces nested folder-based routing with support for colocating layouts,
pages, and components.
• url1/page.js, url2/page.js
• Data Fetching at the Component Level
• Provides built-in data fetching methods (e.g., fetch) directly in React Server
Components.
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app/posts/page.js
import { Suspense } from 'react';
import PostList from './PostList';
export default function PostsPage() {
return (
);
}
Posts!
Loading posts!!...!}> ! !Slide 58
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app/posts/PostList.js
async function fetchPosts() {
const response = await fetch('https:!//jsonplaceholder.typicode.com/posts');
if (!response.ok) {
throw new Error('Failed to fetch posts');
}
return response.json();
}
export default async function PostList() {
const posts = await fetchPosts();
return (
-
{posts.slice(0, 5).map(post !=> (
-
{post.title}!
{post.body}!
!
))}
!
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SSR on by default
• In the Next.js App Router, components are server components
by default
• sent as HTML to the client
• To explicitly mark a component as a client component, you need to
add the special directive "use client" at the top of the file.
• Client components can use React hooks (e.g., useState, useEffect) and
interact with browser APIs (like localStorageor window).
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Image Optimization
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Reducing Bundle Size in React
• Code Splitting
• Image Optimization
• Tree Shaking
• Reduce Dependencies
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Image Optimization in React
• Next-gen Formats: Use modern formats like WebP, which provide
better compression and quality compared to traditional formats like
JPEG or PNG.
• SVG for Vector Graphics: Use SVG for icons and simple graphics, as
they scale well and have smaller file sizes.
• Use the loading="lazy" attribute in ![]()
tags.
• Libraries like React Lazy Load or built-in features in frameworks like
Next.js (next/image) can simplify this process.
• Serve appropriately sized images based on the user's device and
screen resolution.
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Native Lazy Loading
• Example
• 
• Libraries like React Lazy Load or built-in features in frameworks like
Next.js (next/image) can simplify this process.
• Libraries like React Lazy Load or built-in features in frameworks like
Next.js (next/image) can simplify this process.
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Use Responsive Images
• Serve appropriately sized images based on the user's device and
screen resolution.
• Use the srcset attribute for ![]()
tags to define multiple image
sizes.
• Tools like sharp (Node.js library) can generate different image
sizes during the build process.
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Common Responsive Design Breakpoints
• Extra small devices: max-width: 575px (phones).
• Small devices: 576px to 767px (small tablets).
• Medium devices: 768px to 991px (larger tablets).
• Large devices: 992px to 1199px (laptops).
• Extra-large devices: 1200px+ (desktops).
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HTML5.1: Example
![Responsive Example]()
Define the width of
the image in pixels,
so 575w => 575px
if viewport width is less than
575px then..
.. the browser assumes that
image will take up to 575 CSS
pixels of space
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Scenario 1: Viewport Width = 500px, DPR = 1
• max-width: 575px applies because 500px ≤ 575px.
• The browser assumes the image will occupy 575 CSS pixels.
• Required resolution = 575px × 1 = 575px.
• The browser fetches the 575w image from the srcset.
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Scenario 1: Viewport Width = 500px, DPR = 2
• max-width: 575px applies because 500px ≤ 575px.
• The browser assumes the image will occupy 575 CSS pixels.
• Required resolution = 575px × 2 = 1150px.
• The browser fetches the 1200w image from the srcset, 992w is too
small
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HTML5.1: Example
For users who do not scroll to
the bottom of the page,
images that remain off-screen
are never loaded.
Pages with many images (e.g.,
galleries, news articles, or e-
commerce listings) benefit the
most from lazy loading.
Users can start interacting
with the page content (text,
buttons, links) sooner, as the
browser prioritizes visible
elements over off-screen
images.
For users who do not scroll to
the bottom of the page,
images that remain off-screen
are never loaded.
Pages with many images (e.g.,
galleries, news articles, or e-
commerce listings) benefit the
most from lazy loading.
Users can start interacting
with the page content (text,
buttons, links) sooner, as the
browser prioritizes visible
elements over off-screen
images.
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Optimize with Framework Features
• If you're using a React framework like Next.js, take advantage of
its built-in ![]()
component:
• Automatically optimizes images.
• Generates responsive sizes.
• Supports lazy loading and WebP.
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Next.js: Lazy Loading with ![]()
• Lazy loading images in Next.js can be efficiently achieved using
the ![]()
component, which is part of Next.js's built-in image
optimization features.
• The ![]()
component automatically supports lazy loading by
default, ensuring that images are only loaded when they enter the
viewport.
• This improves page load performance, reduces bandwidth usage, and
enhances user experience.
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WebP
• WebP is a modern image format developed by Google that provides
superior lossless and lossy compression for images on the web
• Designed to reduce image file sizes while maintaining high visual
quality, thereby improving website performance by reducing page
load times and bandwidth usage
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WebP: Key Features
• WebP images are typically 25–34% smaller than JPEG images and up
to 26% smaller than PNG images of comparable quality.
• WebP supports transparency (alpha channel) in both lossless and
lossy modes, unlike JPEG.
• WebP supports animated images, providing a more efficient
alternative to formats like GIF.
• Most modern browsers (Chrome, Firefox, Edge, and Safari 14+)
support WebP. However, fallback mechanisms may be needed for
older browsers.
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Tools
• Command-Line Tools: Use ImageMagick or cwebp for powerful,
scriptable workflows.
• Automated Workflows: Use sharp (Node.js) or Pillow (Python) for
integration into development pipelines.
• One-Off Conversions: Use GUI tools like XnConvert or online
tools like Squoosh for occasional use.
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Next.js ![]()
Example
import Image from 'next/image';
export default function Example() {
return (
!
);
}
Lazy Loading Example!
!Slide 78
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Next.js ![]()
Placeholder
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Benefits of Using ![]()
for Lazy Loading
• Improved Performance: Unnecessary images are not loaded
upfront, reducing initial page load time.
• SEO Friendly: The alt attribute ensures accessibility and improves
SEO.
• Responsive Design: The ![]()
component works seamlessly
with responsive layouts, adapting to different screen sizes.
• Out-of-the-box Support: Next.js handles all the complex parts,
such as creating optimized image formats, implementing lazy
loading, and adding responsive behavior.
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srcSet with ![]()
import Image from 'next/image';
export default function ResponsiveImage() {
return (
);
}
Automatically generate
multiple versions of
example-image.jpg at
various resolutions (e.g.,
480w, 1024w, etc.).
);
}
Automatically generate
multiple versions of
example-image.jpg at
various resolutions (e.g.,
480w, 1024w, etc.).
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Next.js generates..
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Tree Shaking
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Reducing Bundle Size in React
• Code Splitting
• Image Optimization
• Tree Shaking
• Reduce Dependencies
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Tree Shaking
• Tree shaking is a technique used to eliminate dead code—unused
parts of the codebase—from the final bundle during the build
process.
• This optimization reduces the size of the JavaScript bundle that is
delivered to the user, which can improve performance by speeding up
loading times and reducing resource usage.
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How Tree Shaking Works
• Modern JavaScript tools, like Webpack, Rollup, and Parcel, use static
analysis to inspect the dependency graph of the project.
• They analyze the import and export statements in ES6 modules to
determine which parts of the code are actually used.
• Code that is not referenced in the application (i.e., not "reachable"
during execution) is considered "dead code" and is removed from the
final bundle.
• After tree shaking, the resulting code is further minified by tools like
Terser or UglifyJS to reduce its size.
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In React Context
• React Components: Tree shaking can help exclude unused React
components if they are modularized properly (i.e., exported and
imported correctly).
• For example, if your project only uses Button from a library but not Card,
tree shaking can exclude Card if the library supports tree-shakeable exports.
• Utility Functions: Similarly, utility functions or hooks that are
imported but not used in the project will be removed during tree
shaking.
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Prerequisites for Tree Shaking
• ES Modules (ESM)
• Properly Configured Build Tools:
• Tools like Webpack or Rollup need to be setup up. Usually on by default.
• Library Support
• The libraries and dependencies you use must be designed to support tree
shaking, which means they need to export individual modules instead of
bundling everything into a single export.
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Example in React
• Tree-shakeable import
• import debounce from 'lodash/debounce';
• Not tree-shakeable
• import _ from 'lodash';
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Tree shaking in Next.js
• Tight Integration with Webpack or Turbopack
• Webpack
• Mature, well-supported, feature-rich, stable, reliable
• Slow build times, complex configuration
• Turbopack (The Future of Next.js)
• Claims to be 700x faster then Webpack during development for large apps
• Tree shaking and other optimizations are faster
• Developed by Vercel team who has done Next.js
• Simpler Configuration
• Future-proof
• Early stage!
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Next.js: Turbopack
• In Turbopack, tree-shaking is designed to work automatically with minimal input from the
developer
• Turbopack optimizes the bundling process by making tree-shaking an integral part of its
architecture.
• Turbopack performs static analysis on your codebase, focusing on ESModules (ESM).
• It identifies and removes code that is not imported or referenced in the dependency graph.
• It uses the same principles as Webpack but implements them much faster and more efficiently, thanks
to its Rust-based architecture.
• Turbopack automatically detects unused code (dead code) and eliminates it from the final bundle.
• Turbopack deeply analyzes imports from third-party libraries, ensuring that only the necessary
modules are included.
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What developer needs to know?
• Use ESM syntax correctly – do not use CommonJS
• Choose Tree-Shakeable libraries
• They use ESM and offer module exports
• No dynamic imports
• if(condition) import ...
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Dependencies
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Reducing Bundle Size in React
• Code Splitting
• Image Optimization
• Tree Shaking
• Reduce Dependencies
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Reducing Dependencies
• Reducing dependencies refers to minimizing the number and size
of external libraries or packages included in your project
• Dependencies are third-party modules or libraries installed via npm
(or similar package managers)
• Each dependency contributes to the final size of your application
bundle
• A larger bundle can slow down page load times, especially on slower
networks or devices.
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Benefits
• Smaller bundle size: Every dependency added to your project
increases the JavaScript code that browsers need to download
and execute, potentially leading to slower performance.
• Reduced attack surface: Fewer dependencies mean fewer
opportunities for security vulnerabilities.
• Better maintainability: With fewer libraries, there’s less risk of
version conflicts and outdated or unsupported code.
• Carbon Footprint
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Strategies
• Evaluate whether you really need a specific library. Often, functionality can be
implemented natively in JavaScript or with a small utility function.
• Example: Instead of adding a library like lodash for a single utility function, use native
JavaScript methods like Array.map, Object.assign, etc.
• Ensure the libraries you include support tree-shaking, which removes unused
code during the build process.
• Use smaller, modular libraries
• date-fns over moment.js?
• Use tools like Webpack Bundle Analyzer or Vite's visualizer plugin to identify large
dependencies in your project
• Use tools like depcheck or manually check package.json for unused or outdated
dependencies and remove them.
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Next.js and Bundle size
Feature Description
Automatic Tree-Shaking Removes unused JavaScript from libraries.
Code Splitting Splits code by route, reducing initial load.
Dynamic Imports Load components or libraries on demand.
Optimized Images Reduces image sizes with modern formats.
Server-Side Rendering Reduces JavaScript required on the client.
Static Site Generation Minimizes client-side computation.
Bundle Analyzer Identifies large dependencies.
External Dependencies Loads heavy libraries via CDN.
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Reduce number of renders
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Key Areas for Optimization
1. Efficient data fetching (SSR and CSR)
2. Reducing bundle size
3. Reduce number of renders
- useMemo, useCallback, React.Memo
4. Monitoring performance using tools
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useMemo and useCallback hooks
• In React, useMemo and useCallback are hooks that help optimize
performance by preventing unnecessary computations and re-renders
• By reducing these inefficiencies, you indirectly lower the carbon
footprint of your application by using fewer computational resources
(CPU, memory, etc.), which in turn reduces energy consumption.
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useMemo
• useMemo is used to memoize the result of an expensive
computation so that it is only recomputed when its dependencies
change.
• const memoizedValue = useMemo(() !=> computeExpensiveValue(a, b), [a, b]);
• For expensive computations like sorting, filtering, or large array
transformations.
• When a calculated value is repeatedly used in your component
but doesn’t change frequently.
• Compute and memoize a derived value. It returns a value that is
cached until its dependencies change.
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"use client";
import React, { useState } from "react";
export default function FilteredListWithoutMemo({ items }) {
const [search, setSearch] = useState("");
const [random, setRandom] = useState(0);
const filteredItems = items.filter((item) !=> {
console.log("no useMemo", "filtering!!...");
return item.includes(search);
});
console.log("no useMemo", "render");
return (
setRandom(Math.random())}>{random}!
setSearch(e.target.value)} placeholder="Search items" !/>
);
}
This is run even if
button is clicked
setSearch(e.target.value)} placeholder="Search items" !/>
-
{filteredItems.map((item, index) !=> (
- {item}! ))} !
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"use client";
import React, { useState, useMemo } from "react";
export default function FilteredList({ items }) {
const [search, setSearch] = useState("");
const [random, setRandom] = useState(0); !// Start with a stable value
const filteredItems = useMemo(() !=> {
console.log("useMemo", "filtering");
return items.filter((item) !=> item.includes(search));
}, [items, search]);
console.log("useMemo", "render");
return (
setRandom(Math.random())}>{random}!
setSearch(e.target.value)} placeholder="Search items" !/>
);
}
Only run if items are
search is changed. If
button is pressed it
will us cached version
of filteredItems
setSearch(e.target.value)} placeholder="Search items" !/>
-
{filteredItems.map((item, index) !=> (
- {item}! ))} !
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useCallback
• useCallback is a React Hook that memoizes a function so that it
does not get re-created on every render unless its dependencies
change.
• useMemo memoizes a result of computation
• This helps optimize performance, especially in scenarios where
functions are passed as props to child components or used
in useEffect.
• To prevent that a new function instance is created every time the
component re-renders.
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"use client";
import React, { useState, useCallback } from "react";
export default function useCallback1() {
const [count, setCount] = useState(0);
const increment1 = useCallback(() !=> {
setCount((prev) !=> prev + 1);
}, []);
const increment2 = () !=> {
setCount((prev) !=> prev + 1);
};
return (
Increment Count - useCallback!
Increment Count - NO useCallback!
);
}
increment1 – created
only once for r—
renders. increment2 -
created on every re-
render
Count: {count}!
!Slide 107
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"use client";
import React, { useState, useCallback, useRef } from "react";
export default function useCallback1() {
const [count, setCount] = useState(0);
const increment1 = useCallback(() !=> {
setCount((prev) !=> prev + 1);
}, []);
const increment2 = () !=> {
setCount((prev) !=> prev + 1);
};
!// Use refs to store the previous references
const previousIncrement1Ref = useRef(increment1);
const previousIncrement2Ref = useRef(increment2);
!// Compare the previous and current references
console.log(
"increment1:",
previousIncrement1Ref.current !!=== increment1
);
console.log(
"increment2:",
previousIncrement2Ref.current !!=== increment2
);
!// Update the refs for the next render
previousIncrement1Ref.current = increment1;
previousIncrement2Ref.current = increment2;
console.log("LargeComponent rendered");
return (
Increment Count - useCallback!
Increment Count - NO useCallback!
);
}
increment1 – created
only once for r—
renders. increment2 -
created on every re-
render
Count: {count}!
!Slide 108
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Higher-Order Component (HOC) in React
• A Higher-Order Component (HOC) is a function that takes a
component as an input and returns a new component. It is a
pattern used in React to reuse component logic.
• HOCs are not a feature of React itself but a design pattern that
emerges from the compositional nature of React components.
• const EnhancedComponent =
HigherOrderComponent(BaseComponent);
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import React from "react";
!// Higher-Order Component
function withLogging(WrappedComponent) {
return function EnhancedComponent(props) {
console.log("Props:", props);
return ;
};
}
!// Base Component
function MyComponent({ name }) {
return
Hello, {name}!!
; } !// Enhanced Component const MyComponentWithLogging = withLogging(MyComponent); export default function App() { return ; }Slide 110
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React.memo?
• React.memo is a higher-order component (HOC) in React
that memoizes a functional component.
• It prevents the component from re-rendering unnecessarily
by skipping renders when its props haven’t changed.
• By default, React re-renders child components whenever a parent re-
renders, even if the child's props are the same.
• When you wrap a functional component with React.memo,
React shallowly compares the component's new and previous
props.
• const MemoizedComponent = React.memo(MyComponent);
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"use client";
import React, { useState, memo } from "react";
function Child({ count }) {
console.log("Child component rendered");
return
Child Count: {count}!
; } const ChildComponentMemorizied = React.memo(Child); function Parent() { const [count, setCount] = useState(0); const [otherState, setOtherState] = useState(0); console.log("Parent component rendered"); return (
setCount((prev) !=> prev + 1)}>
Increment Count
!
setOtherState((prev) !=> prev + 1)}>
Update Other State
!
!
);
}
When other state
changes, child is not
re-rendered!
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React.memo and useCallback?
• useCallback comes into play when you pass functions as
props to a child component wrapped with React.memo
• Even if the logic of the function remains the same, React creates a
new function reference on every render
• This causes React.memo to treat the function prop as "changed,"
resulting in unnecessary re-renders of the child component.
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Child
function Child({ count, onClick }) {
console.log("Child component rendered");
return (
);
}
const ChildComponentMemorized = React.memo(Child);
Child Count: {count}!
Say Hello From Child! !Slide 114
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function Parent() {
const [count, setCount] = useState(0);
const [otherState, setOtherState] = useState(0);
!// Function defined inline (re-created on every render)
const handleClick = () !=> {
alert("Hello from child component");
};
console.log("Parent component rendered");
return (
setCount((prev) !=> prev + 1)}>
Increment Count
!
setOtherState((prev) !=> prev + 1)}>
Update Other State {otherState}
!
!
);
}
New function
instance in every
render
Passing new props
on every render,
child re-renders
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function Parent() {
const [count, setCount] = useState(0);
const [otherState, setOtherState] = useState(0);
!// Function defined inline (re-created on every render)
const handleClick = useCallback(() !=> {
alert("Hello from child component");
}, []);
console.log("Parent component rendered");
return (
setCount((prev) !=> prev + 1)}>
Increment Count
!
setOtherState((prev) !=> prev + 1)}>
Update Other State {otherState}
!
!
);
}
Same function
instance in every
render
Passing same props
on every render,
child does not re-
render
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Tools for Analysing
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Key Areas for Optimization
1. Efficient data fetching (SSR and CSR)
2. Reducing bundle size
3. Reduce number of renders
4. Monitoring performance using tools
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Tools for Analysis
• Chrome DevTools
• Lighthouse for performance metrics
• Network tab for analyzing requests
• React DevTools
• Identifying unnecessary re-renders
• Next.js Bundle Analyzer
• Monitoring and optimizing bundle size
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Chrome Devtools: Lighthouse
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Report: Performance metrics
• Speed Index - How quickly content is visually displayed during loading
• First Contentful Paint (FCP)
• Largest Contentful Paint (LCP)
• Time to Interactive (TTI)
• Cumulative Layout Shift (CLS)
• Total Blocking Time (TBT)
• Time to Interactive (TTI)
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Energy Impact
• Less Processing Power
• Faster-loading pages reduce CPU and GPU usage, minimizing energy
consumption on both the client device (user’s browser) and the server.
• Fewer Resources Downloaded
• Optimizing images, scripts, and other assets reduces bandwidth usage,
lowering energy demands in data centers and network infrastructure.
• Reduced Idle Time
• Efficient performance avoids unnecessary delays, allowing devices to return
to low-power or idle states sooner.
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Performance Metrics
Metric Weight
Largest Contentful Paint 25 %
Total Blocking Time 25 %
Cumulative Layout Shift 15 %
Speed Index 15 %
First Contentful Paint 10 %
Time to Interactive 10 %
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Network Tab
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Network Tab
• Filter requests
• Use filters like XHR (AJAX requests), JS, CSS, Img, etc., to narrow down
specific resources.
• Columns
• Name, status, type, size, time, waterfall
• Throttling
• Simulate slower connections (e.g., "Fast 3G" or "Offline") from the dropdown
menu to test performance under varying conditions.
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React Dev Tools
• React DevTools is a browser extension (available for Chrome and
Firefox) that allows you to inspect and debug the component tree of
a React application
• It provides insights into the structure of your app, state, props, and
component performance.
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Components tab
• Inspect Component Tree
• View Props and State
• Edit Props or State (For Debugging)
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Profiler tab
• Record Renders
• Click "Start profiling" to record interactions and renders
• It shows a timeline of rendering activity
• Analyze Performance
• Identify which components take the most time to render.
• Look for unnecessary re-renders (components rendering when they don’t
need to).
• Optimize Rendering
• Use the Profiler data to optimize expensive components
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Next.js Bundle Analyzer
• The Next.js Bundle Analyzer is a tool that helps you visualize the
size and structure of your Next.js application’s JavaScript bundles.
• Generates an interactive treemap of your bundles, allowing you to
identify large or unnecessary dependencies and optimize your app's
performance
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Why Use Next.js Bundle Analyzer?
• Identify Large Dependencies
• Find out which libraries or modules are contributing the most to your bundle
size.
• Optimize Performance:
• Reducing the bundle size improves loading times, especially for users on
slower networks or devices.
• Debug Code Splitting:
• Visualize how your code is split into chunks and ensure unnecessary code is
not loaded upfront.
• Detect Unused Code:
• Spot unused or redundant code that can be removed to reduce bundle size.
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Install: npm install @next/bundle-analyzer
• Configure next.config.mjs
import withBundleAnalyzer from "@next/bundle-analyzer";
const withAnalyzer = withBundleAnalyzer({
enabled: process.env.ANALYZE !!=== "true", !// Enable analyzer only when ANALYZE=true
openAnalyzer: true, !// Automatically open the report in the browser
});
const nextConfig = {
reactStrictMode: false, !// Other Next.js configurations
};
export default withAnalyzer(nextConfig);
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Add script and open reports
• "analyze": "ANALYZE=true npm run build"
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nodejs.html
• This report shows the JavaScript bundle used in the Node.js
runtime during server-side rendering (SSR).
• It includes code required to generate server-rendered pages or
handle API routes.
• If your app heavily relies on SSR or has API routes, this report helps
you optimize server-side code and dependencies.
• Useful for identifying large server-only libraries that could slow down
server-side rendering.
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Edge Computing
• Node.js server can be in Australia – when doing fetch api takes
time if doing it from Finland
• With edge computing you can spread small servers all around the
world so that you can access them faster
• Now if doing the connection, you connect to a small server in Finland
instead of Australia
• Edge is fast, but it has limitations
• they can’t handle big tasks like running a full database or doing heavy
calculations.
• You redirect users, authentication
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edge.html
• This report shows the JavaScript bundle used when deploying your
app to Edge environments, such as Vercel Edge Functions or other
Content Delivery Networks (CDNs).
• Edge functions are often used for running server-side logic closer to
the user, improving performance.
• If you’re deploying your app to an edge network (e.g., Vercel,
Cloudflare Workers), this report helps you identify and optimize the
code that runs in the Edge runtime.
• Ensure minimal and efficient dependencies to meet size and
execution time limits often imposed by edge environments.
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client.html
• This report shows the JavaScript bundle sent to the client-side
(browser).
• It includes all code required for hydration (converting server-
rendered HTML into a fully interactive React app) and any other
client-side logic.
• This is the most critical report for optimizing frontend
performance because it affects the loading speed and interactivity
of your app.
• Large bundles here can cause slow load times, especially on slower
networks.
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Conclusion
• nodejs.html: Represents the server-side code responsible for
generating pages or API responses on the Node.js backend.
• edge.html: Represents server-side code optimized for edge
networks, where server-side logic is executed closer to the end user.
• client.html: Represents client-side JavaScript that runs in the
browser, responsible for interactivity, hydration, and user experience.
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