Rollup.js is a popular JavaScript module bundler that allows you to efficiently bundle your code for the web. While Rollup provides excellent support for standard setups, there may be instances where you need to implement custom input/output handling for unconventional setups. In this blog post, we will explore how to accomplish this in Rollup.js.

Understanding the Rollup Input/Output System

Before diving into custom handling, let’s first understand how Rollup’s input/output system works. In Rollup, the input is the entry point of your application. Rollup analyzes the imports and exports in your code and builds a dependency graph. It then determines the output files based on the configuration provided.

By default, Rollup handles common file types like JavaScript (.js) and TypeScript (.ts) out of the box. However, for unconventional setups that involve non-standard file types or custom processing logic, we need to extend Rollup’s functionality.

Customizing Input Handling

To customize input handling in Rollup.js, we need to create a custom plugin. A plugin is a function that receives an input file and returns an object representing the transformed output file. Here’s an example of a custom input handling plugin for Rollup:

import { createFilter } from 'rollup-pluginutils';

export default function customInput({ include, exclude } = {}) {
  const filter = createFilter(include, exclude);

  return {
    name: 'custom-input',
    resolveId(id) {
      // Custom logic to resolve input file
      if (filter(id)) {
        return id;
      }
    },
    load(id) {
      // Custom logic to load input content
      if (filter(id)) {
        // Read and return the content of the input file
        return readFileContents(id);
      }
    }
  };
}

In this example, the customInput plugin checks if an input file matches the specified include and exclude patterns. If it does, it resolves the input file path using custom logic in the resolveId method and loads the content of the file using the load method.

Customizing Output Handling

Similarly, we can customize the output handling in Rollup.js using a custom plugin. This allows us to define how the bundled output will be written to the file system or any other destination. Here’s an example of a custom output handling plugin for Rollup:

export default function customOutput({ dest } = {}) {
  return {
    name: 'custom-output',
    writeBundle(outputOptions, bundle) {
      // Custom logic to handle output
      // Write the bundled content to the specified destination
      writeFileContents(dest, bundle[inputFileName].source.code);
    }
  };
}

In this example, the customOutput plugin takes a dest parameter specifying the destination file path. The writeBundle method is called after Rollup has finished generating the bundle. Inside this method, you can define custom logic to handle the output. You can write the bundled content to the specified destination using the writeFileContents function.

Configuring Rollup with Custom Input/Output Handling

To configure Rollup with custom input/output handling, you need to include your plugins in the Rollup configuration file. Here’s an example of how you can update your Rollup configuration:

import customInput from './custom-input-plugin';
import customOutput from './custom-output-plugin';

export default {
  input: 'src/main.js',
  output: {
    file: 'dist/bundle.js',
    format: 'umd'
  },
  plugins: [
    customInput(), // Custom input handling plugin
    customOutput({ dest: 'dist/bundle.custom' }) // Custom output handling plugin
  ]
};

In this configuration, we import and include the custom input handling plugin created earlier using the customInput() function. We also import and include the custom output handling plugin using the customOutput() function. The dest option specifies the destination file path for the output.

Conclusion

Rollup.js provides a flexible and extensible architecture that allows you to implement custom input/output handling for unconventional setups. By creating custom plugins, you can easily extend Rollup’s functionality and seamlessly integrate non-standard file types or custom processing logic into your build process. With the knowledge gained from this blog post, you can now adapt Rollup.js to better suit your unique requirements. Happy bundling!

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