10 RollupJS Best Practices

RollupJS is a popular JavaScript module bundler that helps developers create optimized bundles of code. It’s a great tool for creating smaller, more efficient JavaScript bundles, but it can be tricky to use. To make the most of RollupJS, it’s important to follow best practices for setting up and configuring the bundler.

In this article, we’ll discuss 10 RollupJS best practices that will help you get the most out of the bundler. We’ll cover topics like setting up the configuration file, using plugins, and optimizing your bundles. By following these best practices, you can ensure that your bundles are as efficient and optimized as possible.

1. Use the rollup-plugin-peer-deps-external plugin to ensure any peer dependencies are bundled together

Peer dependencies are packages that a project depends on, but which are expected to be provided by the consumer of the package. This means that peer dependencies should not be bundled with the main package, as they will already exist in the consuming application. However, if these peer dependencies are not included in the bundle, then the code may fail at runtime due to missing modules. The rollup-plugin-peer-deps-external plugin solves this problem by automatically detecting and including any peer dependencies in the bundle.

To use the plugin, it must first be installed via npm or yarn. Then, it can be added to the Rollup configuration file as a plugin. Once configured, the plugin will detect any peer dependencies declared in the package.json file and include them in the bundle. This ensures that all necessary modules are present when the code is executed, avoiding potential runtime errors.

2. Use the rollup-plugin-terser plugin for minifying your code

The rollup-plugin-terser plugin is a great choice for minifying your code because it uses the Terser library, which is an advanced and highly optimized JavaScript minifier. It can reduce the size of your code by up to 70%, making it much more efficient and faster to load. Additionally, it supports ES6+ syntax, so you don’t have to worry about compatibility issues.

Using the rollup-plugin-terser plugin with RollupJS is easy. All you need to do is install the plugin via npm or yarn, then add it to your Rollup configuration file. Once that’s done, you can specify the options you want to use when minifying your code, such as whether to compress or mangle variables, and how aggressive you want the minification process to be. Finally, just run Rollup and the plugin will take care of the rest.

3. Utilize tree shaking and dynamic imports to reduce bundle size

Tree shaking is a process of eliminating unused code from the bundle. It works by analyzing the static structure of ES2015 modules and determining which pieces are used and which can be removed. This helps to reduce the size of the final bundle, as only the necessary code is included.

Dynamic imports allow for splitting up bundles into smaller chunks that can be loaded on demand. This means that instead of loading all the code upfront, only the code needed for the current page or feature will be loaded. This reduces the initial load time and overall bundle size.

To use tree shaking and dynamic imports with RollupJS, you need to configure your project’s rollup.config.js file. You’ll need to add plugins such as rollup-plugin-terser and @rollup/plugin-commonjs to enable tree shaking and dynamic imports respectively. Additionally, you should set the format option in the output section to “es” so that the generated code is compatible with tree shaking.

4. Prefer es modules over commonjs when possible

Es modules are the official standard for JavaScript modules, and they offer a number of advantages over commonjs. Es modules are statically analyzable, meaning that RollupJS can analyze them at build time to determine which parts of your code are used and which aren’t. This allows RollupJS to create smaller bundles by only including the necessary code. Additionally, es modules support tree-shaking, which is the process of removing unused code from the bundle. Commonjs does not support tree-shaking, so it’s important to use es modules when possible.

Using es modules with RollupJS is easy. All you need to do is make sure your files have the .mjs extension instead of the .js extension. Then, you can import and export modules as usual. For example, if you wanted to import a module called myModule.mjs, you would write:

import myModule from ‘./myModule.mjs’;

5. Leverage Rollup’s watch mode to speed up development

When using RollupJS, watch mode allows the bundler to detect changes in source files and automatically re-bundle them. This means that developers no longer have to manually run the build command every time they make a change; instead, Rollup will do it for them. This saves time and makes development more efficient.

To enable watch mode, all you need to do is add the –watch flag when running the rollup command. This will start watching your source files for any changes and automatically rebuild whenever something changes. Additionally, you can also specify which files should be watched by adding the -w option followed by a comma-separated list of file paths.

6. Set a sourcemap type in your configuration file

Sourcemaps are a way to map the code in your bundled file back to its original source. This is especially useful for debugging, as it allows you to trace errors and warnings back to their origin. Setting a sourcemap type in your configuration file ensures that RollupJS will generate sourcemaps when bundling your code.

The two main types of sourcemaps supported by RollupJS are inline and separate. Inline sourcemaps are embedded directly into the bundle, while separate sourcemaps are stored in an external file. The choice between these two depends on the size of your bundle and how much control you need over the sourcemap. If your bundle is small or if you want more control over the sourcemap, then using a separate sourcemap is recommended. On the other hand, if your bundle is large, then using an inline sourcemap may be preferable.

To set a sourcemap type in your configuration file, you can use the “sourcemap” option. For example, to generate an inline sourcemap, you would add the following line to your configuration file:

“sourcemap”: “inline”

7. Specify an output format such as umd or iife

UMD (Universal Module Definition) is a format that allows code to be used in multiple module systems, such as CommonJS and AMD. This makes it easier for developers to use the same code across different environments without having to rewrite or refactor their code. On the other hand, IIFE (Immediately Invoked Function Expression) is a format that wraps your code into an anonymous function expression which is immediately invoked when loaded. This helps protect your code from being accessed by external scripts and also provides better performance due to its self-executing nature.

To specify an output format with RollupJS, you can use the “output” option in the configuration file. You can set the “format” property of this option to either umd or iife depending on your needs. Additionally, you can also set the “name” property if you want to give your bundle a specific name. Once you have specified the output format, you can then run the rollup command to generate the bundled code.

8. Take advantage of Rollup’s external option to exclude certain packages from the bundle

The external option allows you to specify which packages should not be included in the bundle, and instead be loaded from a CDN or other external source. This is beneficial because it reduces the size of your bundle by only including the code that you actually need. It also helps with caching since the external libraries are cached separately from your own code.

To use this feature, simply add an array of package names to the external property in your Rollup configuration file. For example, if you wanted to exclude React from your bundle, you would add “react” to the external array. You can then include the library via a script tag in your HTML page, or import it directly into your JavaScript files.

9. Make use of named exports to prevent conflicts with global variables

Named exports allow you to explicitly name the variables that are being exported from a module, which helps prevent conflicts with global variables. This is because when using named exports, each variable has its own unique identifier and can be imported into other modules without any risk of overriding existing global variables.

To use named exports in RollupJS, you need to make sure your code is written in ES6 syntax. Then, you can use the export keyword followed by the variable name to specify what should be exported from the module. For example:

export const myVariable = ‘value’;

Once this is done, you can then import the variable into another module using the same syntax. For example:

import {myVariable} from ‘./moduleName’;

10. Utilize the rollup-plugin-babel plugin to compile down ES2015+ syntax

The rollup-plugin-babel plugin allows developers to write modern JavaScript code and transpile it down into a version of JavaScript that is supported by all browsers. This ensures that the code will be compatible with older browsers, while still allowing developers to take advantage of the latest features available in ES2015+ syntax.

Using this plugin is simple; just install it via npm or yarn, then add it as a plugin to your RollupJS configuration file. Once added, you can specify which versions of JavaScript you want to target, and the plugin will automatically transpile your code accordingly. Additionally, you can also configure Babel’s options such as presets and plugins for more advanced use cases.