Implementing lazy loading using Rollup.js for better user experience

18 Sep 2023

In today’s fast-paced digital world, users expect websites and applications to load quickly and perform efficiently. One effective way to improve the user experience is by implementing lazy loading, which loads only the necessary resources when they are needed. In this blog post, we will explore how to implement lazy loading with Rollup.js, a popular JavaScript module bundler.

What is Lazy Loading?

Lazy loading is a technique used to defer the loading of non-critical resources, such as images, scripts, or stylesheets, until they are needed. By loading these resources on-demand, we can significantly reduce initial page load times and improve overall performance.

Why Use Rollup.js?

Rollup.js is a powerful JavaScript module bundler that offers several benefits for lazy loading implementation:

Efficient Bundle Optimization: Rollup.js optimizes and packs JavaScript modules efficiently, reducing bundle sizes and ensuring faster load times.
Tree-Shaking: Rollup.js can eliminate unused code (dead code) during the bundling process, further reducing bundle sizes, and improving performance.
Code Splitting: Rollup.js supports code splitting, allowing us to split our application into smaller chunks that can be loaded on-demand, as required.

Now let’s dive into implementing lazy loading using Rollup.js.

Step 1: Install Rollup.js

To get started, make sure you have Node.js installed on your system. Then, in your project directory, run the following command to install Rollup.js:

npm install rollup --save-dev

Step 2: Configure Rollup.js

Next, we need to create a Rollup config file (e.g., rollup.config.js) in the project root directory and define our build configuration. Here’s an example configuration for lazy loading:

// rollup.config.js
export default {
  input: 'src/index.js',
  output: {
    dir: 'dist',
    format: 'es',
  },
};

In this configuration, we specify the entry point of our application (src/index.js) and the output directory for our bundled code (dist). We set the format to es (ES Module) to support modern browsers.

Step 3: Implement Lazy Loading

To implement lazy loading, we need to split our code into smaller chunks that can be loaded on-demand. Rollup.js provides multiple ways to achieve this, such as dynamic import, code splitting plugins, or using the import() function.

Here’s an example using dynamic import syntax:

// src/index.js
document.addEventListener('DOMContentLoaded', () => {
  const button = document.getElementById('lazyLoadButton');
  button.addEventListener('click', async () => {
    const module = await import('./lazyModule.js');
    module.default();
  });
});

In this example, we import a separate module (lazyModule.js) only when the user clicks a specific button with the id lazyLoadButton. This way, the lazyModule.js code is loaded and executed only when needed.

Step 4: Build and Bundle

Once everything is set up, we can build our project using Rollup.js. Open the terminal and run the following command:

npx rollup --config rollup.config.js

Rollup.js will read the configuration file, bundle our code, and generate the output in the specified directory (dist).

Conclusion

Implementing lazy loading with Rollup.js can significantly enhance the user experience by reducing initial page load times. By deferring the loading of non-critical resources until they are needed, we ensure a faster and more optimized website or application.

Remember to continuously monitor and optimize your lazy loading strategy to achieve the best results. Experiment with different code splitting techniques and analyze the impact on performance using tools like Lighthouse or WebPageTest.

#webdevelopment #lazyloading