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How to reduce unused JavaScript in Next.js

Web performance is a crucial factor for the success of any online business or service. A fast and responsive website can improve user satisfaction, engagement, and conversion rates. However, a slow and sluggish website can frustrate users, increase bounce rates, and damage your reputation.

A considerable number of developers opt for Next.js, a widely used framework for creating swift and scalable web applications with React. Next.js provides several features and optimizations to improve your web page’s performance and user experience, such as automatic code splitting, prefetching, dynamic imports, and more.

However, even with Next.js, one common issue can still affect your web performance: unused JavaScript.  This blog post will explain unused JavaScript, how it affects Next.js applications, and how to reduce unused javascript using simple techniques. By the end of this post, you will learn how to optimize your Next.js applications and make them faster and more efficient.

What is unused JavaScript?

Unused JavaScript is code sections within a website or web application downloaded by the browser but never executed.

Next.js reduces unused Javascript

Unused JavaScript can arise from various factors, such as changes in project requirements, obsolete functionalities, or the integration of external libraries. Let’s explore unused JavaScript and its implications for web performance.

To grasp the concept of unused JavaScript, imagine a web application as a collection of code files, functions, and libraries. When a user accesses a webpage, the browser retrieves and runs the required JavaScript code to deliver the intended functionalities. However, not all the downloaded JavaScript code may be relevant or needed for that specific page or user interaction.

According to Chrome Developers, unused JavaScript can slow down your page load speed in two ways:

  • Render-blocking JavaScript necessitates full download, parsing, compilation, and evaluation by the browser before proceeding with the remaining tasks for page rendering.
  • Despite being non-blocking, asynchronous JavaScript still vies for bandwidth during download, leading to notable performance impacts.

Transmitting superfluous code over the network unnecessarily consumes data, especially for mobile users with capped data plans.

Unused JavaScript can occur in different forms

  • Entire code blocks: Entire codes might be written for specific scenarios or no longer used features. These blocks could include event listeners, functions, or conditional logic.
  • Unused functions or variables: Certain functions or variables may have been created within a codebase but are no longer called or referenced. These unused entities contribute to the accumulation of unused JavaScript.
  • Redundant libraries: In some cases, web developers include third-party libraries or scripts that offer a wide range of functionalities. However, not all these functionalities may be utilized, resulting in unused JavaScript code.
  • Deprecated features: Web technologies and standards evolve, and certain features or APIs depreciate. If the codebase includes JavaScript code that relies on deprecated elements, it can be considered unused as it no longer aligns with current best practices.
  • Conditional features: Web applications often incorporate conditionally displayed elements based on specific criteria, such as user roles or device capabilities. The associated JavaScript code becomes unused if the conditions for displaying a particular feature are not met.

Common sources of unused JavaScript

Unused JavaScript can originate from various sources within a web application:

  • Third-Party libraries: Integrating third-party libraries can provide additional functionality and save development time. However, if the entire library or only specific features are not utilized, it leads to unused JavaScript.
  • Legacy code: Over time, codebases may evolve, and certain features may become obsolete or redundant. However, the corresponding JavaScript code might still be present, contributing to unused JavaScript.
  • Conditional features: Websites often include conditionally displayed features based on specific criteria. The associated JavaScript code becomes unused if the conditions for displaying a particular segment are unmet.
  • Abandoned experiments: Developers may implement experimental features or alternative approaches to solve specific problems during the development process. The associated JavaScript code becomes unused if these experiments are unsuccessful or later abandoned.
  • External scripts and embeds: Websites often integrate external scripts, such as analytics tracking codes, social media widgets, or advertising scripts. However, if these scripts are no longer required or their functionalities are no longer utilized, they contribute to unused JavaScript.
  • Plugin or module dependencies: Web applications rely on plugins or modules to extend functionality. If a plugin or module is removed or disabled, but its associated JavaScript code remains, it becomes unused JavaScript.
  • Code modifications and refactoring: During the development lifecycle, code modifications, refactoring, and feature changes may result in unused JavaScript. When code is altered or removed, remnants of the previous implementation may remain, leading to new code.

By identifying and addressing these familiar sources of unused JavaScript, developers can improve the performance and efficiency of their web applications. In the following sections, we will explore practical strategies to measure, remove, and optimize unused JavaScript in Next.js applications, enhancing web performance.

Effects of unused JavaScript in Next.js

Unused JavaScript can significantly affect Next.js applications, impacting their performance and user experience. In this section, we will explore how unused JavaScript specifically affects Next.js applications and discuss the features and optimizations provided by Next.js to minimize its impact.

1. Increased loading time

Unused JavaScript adds unnecessary overhead to the initial page load, increasing loading times for Next.js applications. When the browser downloads and parses unused JavaScript code, it takes up valuable network bandwidth and processing power, delaying the rendering and display of the page content. This can cause user frustration and result in a suboptimal user experience.

Unused JavaScript in Next.js increases load times

Next.js Mitigation: Next.js offers automatic code splitting, a feature that splits the JavaScript code into smaller, more manageable chunks. By only loading the necessary code for a specific page or route, Next.js reduces the amount of unused JavaScript that needs to be downloaded, resulting in faster loading times and improved performance.

2. Reduced interactivity and responsiveness

Unused JavaScript can negatively impact the interactivity and responsiveness of Next.js applications. When the browser executes unused JavaScript code, it takes away processing resources that could be utilized for other critical tasks, such as handling user interactions or updating the user interface. This can lead to sluggishness, unresponsive interactions, and a suboptimal user experience.

Next.js Mitigation: Next.js uses prefetching and dynamic imports to optimize JavaScript execution. Prefetching predicts and fetches upcoming route’s JavaScript code in the background, ensuring availability when the user navigates. Dynamic imports load JavaScript code on-demand, reducing initial load time and improving interactivity by fetching code when needed.

3. SEO implications

Unused JavaScript can also significantly impact the search engine optimization (SEO) of Next.js applications. Search engines consider page load times, overall performance, and user experience ranking factors. Slow-loading pages due to excessive unused JavaScript can lead to lower search engine rankings, reduced organic visibility, and a decreased flow of organic traffic to the website.

Unused JavaScript lengthens load times, harming SEO

Next.js Mitigation: Next.js provides performance optimizations such as automatic code splitting, which helps reduce the size of the JavaScript bundles and improve page load times. By minimizing unused JavaScript, Next.js enables better performance, improving SEO rankings and visibility in search engine results.

4. Increased resource consumption

Unused JavaScript consumes unnecessary system resources, such as memory and processing power. This can have a significant impact on the scalability and efficiency of Next.js applications, especially in high-traffic scenarios. The presence of unused JavaScript increases the overall footprint of the application, requiring more server resources to handle user requests and potentially leading to higher hosting costs.

Next.js Mitigation: By removing unused JavaScript, Next.js applications can optimize resource utilization and improve scalability. This ensures system resources are utilized efficiently, leading to better performance and cost-effective application hosting.

5. Code Complexity and Maintenance Challenges

Unused JavaScript adds unnecessary complexity to the codebase, making it more challenging to maintain, debug, and extend. It can obscure the actual functionality of the application, making it harder for developers to understand and modify the code. With time, this can lead to increased technical debt, reduced development velocity, and difficulties onboarding new team members.

Next.js Mitigation: Developers can keep the codebase clean and maintainable by regularly reviewing and removing unused JavaScript. This simplifies future development efforts, improves collaboration among team members, and reduces the risk of introducing bugs or regressions during code modifications.

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How to reduce unused JavaScript in Next.js

how to reduce unused javascript

Reducing unused JavaScript in Next.js applications is essential for optimizing performance and improving user experience. In this section, we will explore practical tips and best practices to minimize the presence of unused JavaScript.

By following these recommendations, you can streamline your codebase and ensure that only necessary JavaScript is included in your Next.js application.

  • Utilize next-unused: One effective tool for identifying and removing unused files in your Next.js project is next-unused. This tool analyzes your project’s codebase and detects not imported or referenced files, helping you identify unused JavaScript. By running next-unused regularly, you can keep your codebase clean and minimize unnecessary files.
  • Leverage dynamic imports: Next.js offers dynamic imports, a powerful feature allowing you to load components or modules only when needed. By dynamically importing JavaScript code, you can reduce the initial load time of your application, as these modules are fetched and executed on-demand. Identify areas where components or modules are not required immediately and utilize dynamic imports to improve performance.
  • Disable prefetching for unvisited links: Next.js provides prefetching capabilities, which fetches JavaScript code for links in the background to improve performance during navigation. However, not all links are equally likely to be visited by users. By selectively turning off prefetching using the prefetch=false attribute for links that are unlikely to be seen, you can prevent unnecessary fetching of JavaScript code and optimize your application’s performance.
  • Employ tree shaking: Leverage tree shaking, a technique supported by tools like Webpack, to eliminate unused exports from your JavaScript bundles. Tree shaking analyzes your code and removes any dead or new code paths during the build process, resulting in smaller, more optimized bundles. Configure your Next.js project to utilize Tree shaking and enjoy the benefits of reduced bundle sizes and improved performance.
  • Implement code splitting and lazy loading: Next.js offers built-in support for code splitting, allowing you to split your code into smaller chunks loaded on demand. By breaking down your JavaScript code into smaller, more manageable pieces, you can prioritize loading essential code first and defer loading non-essential code until needed. This reduces the initial load time and improves the overall performance of your Next.js application.
  • Analyze and optimize bundles: Consider utilizing tools like webpack-bundle-analyzer to analyze and optimize your JavaScript bundles. These tools visually represent your bundle’s contents, allowing you to identify larger modules or chunks that can be further optimized or split into smaller pieces. By optimizing your fortunes, you can reduce the overall size of your JavaScript code and enhance performance.


In conclusion, reducing unused JavaScript in Next.js applications is crucial to optimizing performance and delivering exceptional web experiences. Unused JavaScript can significantly impact page load times, user interactivity, and search engine rankings, ultimately affecting the overall success of your Next.js projects. 

By implementing strategies such as dynamic imports, code splitting, and Tree shaking, you can eliminate unnecessary code, improve loading speed, and enhance the responsiveness of your applications. Regular code maintenance and optimization should be integral to your development process to ensure that your Next.js projects remain efficient and user-friendly.

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Marketing Manager of Mageplaza. Summer is attracted by new things. She loves writing, travelling and photography. Perceives herself as a part-time gymmer and a full-time dream chaser.
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