Optimizing the output bundle size in Javascript

Anton Ioffe - September 10th 2023 - 14 minutes read

In today's fast-paced digital world, optimizing web applications for speed and efficiency is no longer an option but a necessity. This crucial need for performance optimization leads us to the main focus of this article: reducing and optimizing the JavaScript bundle size in modern web development. With the increasing demand for improved user experiences and speedy load times, overlooking the importance of maintaining an optimized JavaScript bundle size could be perilous.

Throughout this comprehensive guide, various advanced strategies and invaluable tools for bundle size optimization will be discussed extensively. Dig into the specifics of prominent concepts such as code splitting, tree shaking, lazy loading, and learn how dynamic imports play a major role in bundle size optimization. We will explore the significant role Webpack plays and how it contributes to this process, alongside a guide to essential tools like webpack-bundle-analyzer for inspecting and analyzing bundle sizes.

Moreover, framework-specific strategies for Angular, Vue.js, and React will also be delved into, offering insights on how to drastically reduce bundle size while improving load time in the context of these popular frameworks. This article promises to enrich your understanding and provide practical techniques that you can bring into your web development projects. Let's begin this journey towards optimal bundle size in Javascript.

Grasping the Importance of Bundle Size in Javascript

In modern web development, one key factor that often gets overlooked amidst various performance-enhancing techniques is the bundle size of the Javascript files your server delivers to end-users. Bundle size is performance critical since large bundles result in longer load times, deteriorating user experience. It's essential to keep an eagle eye on your bundle sizes throughout your JavaScript application's lifecycle.

Bundle Size Benchmarks

While it's challenging to prescribe 'one size fits all' optimal bundle size, industry benchmarks can serve as a useful guide. Ideally, a bundle size after undergoing minification (removing unnecessary or redundant data from the code) and gzip compression (reducing file size for faster network transfers) should not exceed 200KB. Anything beyond 500KB merits an investigation into the cause of such bulk, and bundles larger than 1MB largely necessitate a corrective action to reduce their size. Remember, these benchmarks are relevant for high-speed internet. Users with slower connections would have significantly lower limits.

Bundle Size Example

Consider the following JavaScript code commonly seen in projects where the utility library Lodash is used:

import _ from 'lodash';

function myComponent() {
    let myElement = document.createElement('div');

    // Lodash, a known contributor to large bundle sizes
    myElement.innerHTML = _.join(['Hello', 'webpack'], ' ');
    return myElement;


In this example, we've included the entire Lodash library for use in our file, even though we only needed one method. This is a common mistake that can lead to undesirably large bundle sizes.

Instead, we can just import the join method from Lodash, as shown below:

import { join } from 'lodash';

function myComponent() {
    let myElement = document.createElement('div');

    myElement.innerHTML = join(['Hello', 'webpack'], ' ');
    return myElement;


By only importing the join function from Lodash, the bundle size can be significantly reduced.

Reducing bundle size is more than just paring down your code. It's about creating a codebase that efficiently delivers the functionality your users need, without unnecessary elements. Keep in mind that large bundle sizes can negatively affect an application's performance and load times, so awareness of bundle size from the beginning of a project can save time and resources down the line. Have you ever experienced extended load times due to large bundle sizes? How did you tackle this in your JavaScript applications?

Techniques for Bundle Size Optimization

Code Splitting

Code splitting is a strategy that can dramatically reduce the size of a JavaScript output bundle. The idea is to only load the portions of code that are needed at any given moment, which decreases the initial load time.

For instance, if you have a large application with multiple pages, you could split the code for each page into its own bundle. The code for each page would then only be loaded when the user visits that page.

To achieve this, modern frameworks like React make use of dynamic imports. Instead of importing all modules at the beginning, you can use the import() function to dynamically import a module when it is needed.

Here's an example using React and React Router:

import React, { lazy, Suspense } from 'react';
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

const HomePage = lazy(() => import('./HomePage'));
const AboutPage = lazy(() => import('./AboutPage'));

function App() {
  return (
      <Suspense fallback={<div>Loading...</div>}>
           <Route exact path="/" component={HomePage}/>
           <Route path="/about" component={AboutPage}/>

Notice the lazy() function. This enables the dynamic imports. The Suspense component is used to display some fallback content to the user while the requested component is being loaded.

A common mistake when implementing code splitting is not considering fallback scenarios properly. Ensure that a suitable fallback component is provided to handle the scenario when the desired code chunk is being fetched.

Tree Shaking

Tree shaking is a technique that involves eliminating unused or dead code from the final bundle. The concept of "shaking" the dependency tree comes from considering the codebase like a tree, and "shaking" off the dead leaves (unused code).

To enable tree shaking, it's important to structure your codebase with modular units. If you write a module that exports ten functions, but only two are ever imported elsewhere in your code, a good bundler equipped with tree shaking capabilities can exclude the eight unused functions from the final bundle.

Here is an example showcasing a common mistake:

// utils.js
export function usedFunction() {
   // This one is used.

export function unusedFunction() {
   // This one is not.

If unusedFunction is never imported, it would still end up in the final bundle if not for tree shaking capabilities. With tree shaking, only usedFunction would be part of the final bundle.

Lazy Loading

Lazy loading is another technique used to optimize the bundle size. Instead of loading all the components or modules at once, we load them as they are needed. Lazy loading can dramatically increase performance and reduce initial loading times, especially in large applications.

Similar to code splitting, lazy loading also leverages dynamic imports. Here's a simple example of a component being lazy-loaded:

import React, { lazy, Suspense } from 'react';

const LazyLoadedComponent = lazy(() => import('./LazyLoadedComponent'));

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <LazyLoadedComponent />

One common pitfall when using lazy loading is failing to handle loading state properly. If the component takes some time to load and there's no fallback, the user might end up seeing a blank or broken page for a brief period.

Ponder Over

  • Could you identify portions of your codebase that can be split into different chunks and loaded lazily?
  • Are there parts of your codebase that aren't being used and can be shaken off from the final bundle?
  • How do you handle fallback scenarios when implementing dynamic imports and lazy loading?
  • How does the nature of your application or component structure influence your decisions about where and how to apply these techniques?
  • How do you ensure that the benefits of these optimizations aren't negated by other bottlenecks in your application?

Webpack's Role in Bundle Size Optimization

Webpack, a highly configurable bundler, is key in optimizing the output bundle. As developers, efficiently configuring Webpack aids bundle size optimization.

Improving Webpack's Performance

Optimizing Webpack boosts build time and decreases output bundle size. The two key techniques that aid performance improvement include lazy loading and code splitting.

Lazy Loading

Lazy loading defers the initialization of objects until they are needed. This process reduces the startup cost and boosts your app's performance.

Correct Usage of Dynamic Imports for Lazy Loading
// Using dynamic imports to achieve lazy loading
const loadComponent = (componentName) => import(`./components/${componentName}`);

Key benefits of this function are reduced memory consumption and swifter load times. However, misapplication can lead to complex code structures or create performance issues.

Common Mistake: Using Static Imports Instead of Dynamic Ones

A frequent mishap includes using static over dynamic imports, as illustrated below:

// Common mistake: using static imports instead of dynamic ones
import Component from './components/Component';

const loadComponent = (componentName) => Component;

In this incorrect code example, Component is imported and returned regardless of the input parameter. This practice defeats the benefits of dynamic loading and incorrectly inflates the bundle size.

The corrected code using dynamic imports would look like this:

// Correct way: using dynamic imports 
const loadComponent = (componentName) => import(`./components/${componentName}`);

Here, the import statement is functionally used to load a module only when the function is invoked.

Code Splitting

Webpack provides code splitting as a tool to increase your app's performance. Code splitting partitions your code into multiple bundles that can be loaded in segments or concurrently.

Decomposing the Entire Application into Separate Features
// Using require.ensure to decouple features

require.ensure([], function() {
  // Modules required only when this function is called
  const utils = require('./utils');
Breaking Libraries Apart from the Main Application
// Code splitting to separate vendors from main application, considering all modules inside the node_modules folder as vendors

const config = {
  optimization: {
    splitChunks: {
      cacheGroups: {
        vendor: {
          test: /[\\/]node_modules[\\/]/,
          name: 'vendors',
          chunks: 'all',
Dividing Components into Reusable Chunks
// Dividing components into reusable chunks using import()

const componentChunk = (componentName) => import(`./components/${componentName}`);

Common Mistake: Not Implementing Code Splitting

// Entire application as a single bundle 
const appCode = require('./app');

Lacking code splitting, modules bulk together in a single bundle. The consequent large bundle size is demanding to load efficiently.

Preventing Duplication: Webpack's Deduplication

Preventing duplication supports efficiency by avoiding output bundle redundancy. Webpack's SplitChunksPlugin segments chunks into reusable modules to avoid duplication.

// Here, the SplitChunksPlugin is utilized to prevent code duplication
const config = {
  optimization: {
    splitChunks: {
      chunks: 'all',

Another common mistake is mistakenly loading a module multiple times:

// Incorrect code: module loaded multiple times
import { something } from 'module';
import { somethingElse } from 'module';

The corrected version, in which the modules are correctly consolidated in one import statement, is shown below:

// Correctly consolidating different modules within one import statement
import { something, somethingElse } from 'module';

Webpack’s Role with Offending Libraries

Webpack can identify modules or libraries occupying excessive space to assist bundle size reduction. You can then refine, defer, or split these modules.

// Webpack can assist in scrutinizing the output bundle size

  performance: {
    hints: 'error', // Enum
    maxAssetSize: 200000, // 200kb
    maxEntrypointSize: 400000, // 400kb

With this configuration, an alert triggers when the asset size or entry point size crosses the set maximum limit. This feature helps review the output bundle for informed decisions.

Following Webpack’s optimization methods of lazy loading and code splitting, you can minimize your JavaScript application's bundle size and achieve faster load times. Deduplication helps prevent redundancy, reducing bundle size, and streamlining load times. Lastly, leveraging Webpack to manage offending libraries lets you identify bulky modules that may need refining. Using these techniques impacts your projects positively by potentially shrink the bundle size and ultimately delivering a better user experience.

Comprehensive Guide to Tools for Bundle Analysis and Optimization

One of the most efficient ways to improve your Javascript's performance in a modern web development environment is by scrutinizing and optimizing the size of your output bundle. Several tools exist to make this task a manageable one, including the impressive webpack-bundle-analyzer.

A Deep Dive Into webpack-bundle-analyzer

This tool gives you a visual and interactive overview of your bundle content by showing all the packed modules in a size-oriented treemap. With webpack-bundle-analyzer, you can immediately visualize and examine which parts of your code are taking up the most space.

Let's look at an example of how to run webpack-bundle-analyzer with your project.

// Inside your webpack.config.js file
const BundleAnalyzerPlugin = require('webpack-bundle-analyzer').BundleAnalyzerPlugin;

module.exports = {
    // Your webpack configs here...
    plugins: [
        new BundleAnalyzerPlugin()

Executing webpack as you typically would right after this configuration will automatically run webpack-bundle-analyzer on your project accretion. By default, it opens an interactive zoomable treemap packed with all the output bundles sorted by size.

Common Coding Mistakes

Including Unnecessary Packages

One common mistake developers make is including unnecessary packages in their bundle. For instance, including the entire lodash library when you only need a specific function. The correction would be to import only the functions you need, like so:

// Incorrect import, includes entire lodash library
import _ from 'lodash';

// Correct import, includes only the needed function
import isEqual from 'lodash/isEqual';

Ignoring Code Splitting

Ignorance of code splitting is another common oversight. When the entire app's code gets packed into a single bundle, it slows down the initial page load time. By splitting your code into separate smaller chunks that load asynchronously, you improve load time dramatically. It also means users only download the portions of code that they need.

// Incorrect chunking
import { Chart } from 'massive-chart-library';

// Correct code splitting
import(/* webpackChunkName: "chart" */ 'massive-chart-library').then(({ Chart }) => {
    // Use `Chart` here

When analyzing and optimizing your Javascript's output bundle size, asking the right questions can take you a long way. Does your bundle include redundant libraries? Could your large dependencies be swapped out for lighter alternatives? Could the use of code splitting provide a better user experience?

Mastering and utilizing an effective tool like webpack-bundle-analyzer to fully analyze your project’s bundle size, will vastly improve your web page's performance. In this resource-rich era of Javascript development, conquering your bundle size shouldn't be an insurmountable hurdle, but rather an opportunity for fine-tuning.

Framework Specific Bundle Size Optimizations

Angular Specific Bundle Size Optimizations

Angular, created by Google, is a full-featured JavaScript framework favored for its robustness. However, its larger production bundle size compared to other frameworks often becomes a concern. Luckily, Angular developers are provided with various strategies and approaches to optimize the bundle size.

One key strategy is ahead-of-time (AOT) compilation, as opposed to just-in-time (JIT) compilation. AOT compilation, as the name suggests, occurs before the browser actually renders the application. It translates HTML and TypeScript code into efficient JavaScript code during the build phase itself, thus reducing the workload at runtime.

The AOT compiler is instrumental in eliminating dead code and unnecessary imports, resulting in a significant reduction in the bundle size. However, the AOT compilation process can result in longer build times.

Common mistake: Omitting the --prod flag when building for production. Correct approach: Always use the --prod flag when building for production which enables AOT compilation.

// Incorrect
ng build

// Correct
ng build --prod

In addition to a reduced bundle size, the Angular CLI also provides options like lazy loading and differential serving that further enhance application performance.

Vue.js Specific Bundle Size Optimizations

Vue.js, which is lightweight by default, provides developers with ways to further foster efficiency. The Vue CLI tool offers us various ways to analyze and integrate optimizations to the bundle size.

One approach to consider is dynamic imports or code splitting. Vue router allows the split of codebase into 'chunks' which only load when needed. This drastically improves the initial load time.

Common mistake: Importing the whole component without splitting. Correct approach: Using import() function for dynamic imports.

// Incorrect
import MyComponent from './MyComponent.vue';

// Correct
const MyComponent = () => import('./MyComponent.vue');

Vue also offers an array of third-party libraries that are lighter and more efficient than the standard versions, such as vue-the-mask instead of V-mask.

React Specific Bundle Size Optimizations

React, maintained by Facebook, is one of the most widely used JavaScript libraries for building UIs. Like others, React too provides several methods for optimizing the production bundle size.

One default strategy is to use production builds instead of development builds for deployment. This may seem obvious, but it's quite common to miss. Production builds include optimizations like minification which significantly reduce the bundle size.

Common mistake: Not using react's production build for deployment. Correct approach: Using npm run build to create an optimized build of your application.

# Incorrect
npm run start

# Correct
npm run build

An efficient approach is using React's built-in component lazy loading feature, 'React.lazy'. It’s a function that lets you dynamically import components, with retry logic and error handling, essentially returning a Promise.

Common mistake: Importing the whole react component without lazy loading. Correct approach: Using React.lazy() function for dynamic component imports.

// Incorrect
import MyComponent from './components/MyComponent';

// Correct
const MyComponent = React.lazy(() => import('./components/MyComponent'));

Keep in mind, to use 'React.lazy' with the components loaded, the Suspense component must be used within the component tree. It's a React component that renders fallback UI while waiting for the dynamically imported component to load.

import React, { Suspense } from 'react';
const MyComponent = React.lazy(() => import('./components/MyComponent'));

function MyLazyLoadedApp() {
  return (
      <Suspense fallback={<div>Loading...</div>}>
        <MyComponent />

By applying 'React.lazy', you can drastically reduce your initial bundle size by not including large components that aren't immediately needed.

While analyzing the bundle size, it's crucial to focus on techniques that are more likely to yield significant results. This typically includes practices such as eliminating dead code and unnecessary imports, code minification, and taking advantage of async loading.


The article focuses on the importance of optimizing JavaScript bundle size in modern web development. With the growing need for better user experience and quicker load times, proper bundle size can drastically enhance the performance of a web application. The article discusses various techniques for bundle size optimisation such as code splitting, tree shaking, lazy loading, and the use of dynamic imports. It also highlights the importance of a good bundler like Webpack which aids in this process.

Additionally, the article provides a comprehensive guide to useful tools like webpack-bundle-analyzer for scrutiny and analysis and strategies specific to JavaScript frameworks like Angular, Vue.js, and React. These approaches foster reduced bundle size, a must for every developer aiming for an optimized JavaScript application.

Now for the challenging task: Analyze a project of yours. Identify the parts that are causing bloat and apply the learned techniques to optimize the bundle size. Make use of webpack-bundle-analyzer to identify the bulky parts. Additionally, review your implementation of code splitting, tree shaking and lazy loading. Can you improve it anyway? Record the changes in load time and performance before and after the optimization and share your observations.

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