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Rollup Feature Request: No Entrypoint JS Copy To Output

Alex Johnson
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Rollup Feature Request: No Entrypoint JS Copy To Output

Currently, when using Rollup with certain plugins to bundle JavaScript projects, an entry point JavaScript file is often copied to the output directory. However, in some scenarios, this behavior is undesirable. This article explores a feature request to provide an option to prevent Rollup from copying the entry point JS file to the output directory, offering more flexibility in build configurations.

The Problem: Unnecessary Entry Point Copying

When building web applications with Rollup, developers often use an HTML template as the main entry point. This template includes inline JavaScript that initializes the application. However, Rollup typically requires a separate JavaScript entry point file, even if its content is meant to be inlined into the HTML. This leads to an unnecessary copy of the throw-away entry point script in the output directory, which can clutter the project structure and potentially cause confusion. Let's explore how this issue arises and why it's a pain point for developers.

Defining the HTML Template

Many modern web projects use an HTML template as the primary entry point. This template often includes a <script> tag with the type="module" attribute, which imports the main application logic. For example, consider the following HTML template:

<html>
    <body>
        <script type="module">
            import CMS, { init } from '@sveltia/cms';
            init();
        </script>
    </body>
</html>

In this case, the HTML file serves as the entry point, and the JavaScript code within the <script> tag initializes the application using modules like @sveltia/cms. The goal is to produce a single HTML file with all dependencies inlined, making deployment straightforward.

The Need for a JavaScript Entry Point

Rollup, by default, requires a JavaScript entry point. To work around this, developers often create a separate JS file that contains the same code as the inline script in the HTML template. This file is then specified as the entry point in the Rollup configuration. For example, the main.js file might look like this:

import CMS, { init } from '@sveltia/cms';
init();

Configuration Challenges

Achieving the desired output—a single HTML file with inlined scripts—requires a custom Rollup configuration. Developers often need to define a rollup.config.js file that includes specific plugins and settings to handle the HTML template and inline the script. This configuration can become quite complex, involving custom logic to read the HTML template, replace placeholders with the bundled JavaScript, and output the final HTML file.

The Unwanted Copy

Despite these efforts, Rollup still copies the throw-away entry point script (main.js in this example) to the output directory. This is because Rollup is configured to process and output this file, even though it's only used as a means to an end. The presence of this extra file can be undesirable for several reasons:

  • Clutter: It adds an unnecessary file to the output directory, making it less clean and organized.
  • Confusion: It might confuse developers or other tools that expect the output directory to contain only the essential files.
  • Maintenance: It requires extra steps to clean up the output directory and remove the throw-away script.

Why This Feature Request Matters

Providing an option to prevent Rollup from copying the entry point JS file to the output directory would address these issues and offer developers more control over their build process. It would simplify the configuration, reduce clutter, and streamline the deployment process. This feature would be particularly useful for projects that rely on HTML templates as the primary entry point and aim to produce a single, self-contained HTML file.

Existing Solutions and Their Limitations

Before diving into the proposed solution, it's worth examining existing approaches and their limitations. The original poster (OP) mentioned trying two different methods to achieve the desired outcome, each with its own set of challenges. Let's break down these approaches.

Custom Rollup Configuration

The first approach involved defining a tricky custom rollup.config.js. This configuration would:

  • Define the HTML template.
  • Allow inlining the script.
  • Specify a JS entry point.

While this method can achieve the desired result of inlining the script into the HTML, it comes with several drawbacks:

  • Complexity: The configuration can become quite complex, requiring a deep understanding of Rollup's internals and plugin ecosystem.
  • Maintainability: Custom configurations are often harder to maintain and update, especially when Rollup or its plugins are upgraded.
  • Duplication: It requires defining the same script logic in both the HTML template and the JS entry point, leading to duplication and potential inconsistencies.

Using Open-WC Rollup Plugin

The OP also explored using the @open-wc/rollup-plugin. This plugin allows defining an HTML entry point directly, which aligns with the desired workflow. However, the OP encountered a limitation:

  • Inability to Inline Scripts: The plugin didn't provide a straightforward way to inline the script defined in the HTML entry point. This meant that the script would still be output as a separate file, defeating the purpose of having a single, self-contained HTML file.

Best of Both Worlds

The OP's request is essentially a plea for the best of both worlds: the ability to define an HTML entry point (like with @open-wc/rollup-plugin) combined with the ability to inline scripts (achievable with a custom Rollup configuration). This would provide a more streamlined and intuitive way to build web applications with Rollup, especially those that rely on HTML templates as the primary entry point.

Proposed Solution: A Rollup Plugin Enhancement

To address this issue, a new feature or enhancement to an existing Rollup plugin is needed. This enhancement would provide an option to prevent copying the entry point JS file to the output directory when using an HTML template as the main entry point. Here’s a breakdown of how this could be implemented:

Plugin Option

A new option, such as skipEntryPointCopy, could be added to the plugin's configuration. When set to true, this option would instruct the plugin not to copy the entry point JS file to the output directory.

// rollup.config.js
import htmlTemplate from 'rollup-plugin-html-template';

export default {
  input: 'src/index.js',
  output: {
    dir: 'dist',
    format: 'es',
  },
  plugins: [
    htmlTemplate({
      template: 'src/index.html',
      skipEntryPointCopy: true, // Add this option
    }),
  ],
};

Conditional Copying Logic

Inside the plugin, the logic for copying the entry point JS file would be made conditional based on the skipEntryPointCopy option. If the option is set to true, the plugin would skip the copying step. This could be implemented using a simple if statement in the plugin's code.

HTML Template Integration

The plugin would also need to handle the inlining of the script defined in the HTML template. This could be achieved by:

  • Reading the HTML template.
  • Extracting the <script> tag with type="module".
  • Bundling the JavaScript code within the <script> tag using Rollup.
  • Replacing the <script> tag in the HTML template with the bundled JavaScript.
  • Outputting the modified HTML template to the output directory.

Benefits of This Approach

This approach offers several benefits:

  • Simplicity: It provides a simple and intuitive way to control whether the entry point JS file is copied to the output directory.
  • Flexibility: It allows developers to choose the behavior that best suits their needs.
  • Clean Output: It ensures that the output directory contains only the essential files, reducing clutter and confusion.
  • Integration: It integrates seamlessly with existing Rollup workflows and plugins.

Real-World Use Cases

To illustrate the value of this feature request, let's consider a few real-world use cases where it would be particularly beneficial.

Single-Page Applications (SPAs)

In many SPAs, the entire application is loaded and initialized from a single HTML file. The JavaScript code is often inlined into the HTML or loaded as a separate module. In such cases, the entry point JS file is only used to kickstart the application and is not needed in the output directory.

Web Components

Web components are reusable UI elements that can be used in any web application. They are often distributed as a single JavaScript file that defines the component's behavior. When building web components with Rollup, developers might want to create a simple HTML file that demonstrates the component. In this scenario, the entry point JS file is only used to load the component in the HTML file and is not needed in the output directory.

Static Site Generators (SSGs)

SSGs generate static HTML files from templates and data. They often use JavaScript to add interactivity and dynamic content to the generated pages. When using Rollup to bundle the JavaScript code for an SSG, developers might want to inline the code into the HTML templates. In this case, the entry point JS file is only used to bundle the code and is not needed in the output directory.

Conclusion

The feature request to provide an option to prevent Rollup from copying the entry point JS file to the output directory is a valuable enhancement that would offer developers more control and flexibility in their build process. It would simplify the configuration, reduce clutter, and streamline the deployment process, especially for projects that rely on HTML templates as the primary entry point. By implementing a plugin option or conditional copying logic, Rollup can become even more versatile and user-friendly for a wide range of web development scenarios.

For more information on Rollup and its capabilities, visit the official Rollup documentation. This resource provides comprehensive details on how to configure and use Rollup for various projects.

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