esbuild and Rollup both bundle your JavaScript, but they approach it from fundamentally different philosophies, making one a better fit for certain library development workflows than the other.
Let’s see esbuild in action, building a simple library.
# Install esbuild
npm install --save-dev esbuild
# Create a simple library file
echo "export const greet = (name) => \`Hello, \${name}!\`;" > src/index.js
# Build the library with esbuild
npx esbuild src/index.js --bundle --outfile=dist/bundle.js --format=esm --platform=neutral --minify
This produces dist/bundle.js:
// dist/bundle.js
var greet=(e=>`Hello, ${e}!`);export{greet};
Now, let’s see Rollup do the same, but with a bit more configuration to emulate a typical library setup.
# Install Rollup and its Node.js resolution plugin
npm install --save-dev rollup @rollup/plugin-node-resolve
# Create a rollup.config.js file
cat <<EOF > rollup.config.js
import resolve from '@rollup/plugin-node-resolve';
export default {
input: 'src/index.js',
output: {
file: 'dist/bundle.js',
format: 'esm',
name: 'MyLibrary', // Name for UMD/IIFE formats
sourcemap: true
},
plugins: [
resolve() // Helps Rollup find modules in node_modules
]
};
EOF
# Build the library with Rollup
npx rollup -c
This produces dist/bundle.js and dist/bundle.js.map. The bundle.js might look something like this (depending on Rollup version and exact config):
// dist/bundle.js
var greet = (name) => `Hello, ${name}!`;
export { greet };
The core problem both solve is taking your source code, which might be spread across multiple files and use modern JavaScript features, and transforming it into a single (or few) files that browsers or Node.js can understand, often with optimizations. But how they get there is where they diverge.
esbuild is built in Go and prioritizes raw speed. It uses a more direct, less AST-manipulative approach to bundling. Think of it as a highly optimized compiler that quickly understands your code’s dependencies and outputs a bundled file. Its strength lies in its incredible build times, making it fantastic for rapid iteration during development or for projects where build speed is paramount. It’s less focused on complex code transformations or highly customized plugin ecosystems.
Rollup, on the other hand, is built in JavaScript and is designed with tree-shaking and modularity as first-class citizens. It works by building an Abstract Syntax Tree (AST) of your code, then performing various transformations and optimizations on that tree before generating output. This AST-centric approach makes Rollup incredibly powerful for code analysis, dead code elimination (tree-shaking), and supporting a rich plugin ecosystem. It’s often the preferred choice for libraries because it excels at producing clean, small bundles that only include the code consumers actually use.
For library development, Rollup’s strength in tree-shaking is often the deciding factor. When you publish a library, you want to ensure that your users only download the parts of your library they import. Rollup’s meticulous AST traversal allows it to accurately identify and remove unused code. esbuild, while fast, can sometimes be less aggressive with tree-shaking, potentially leading to larger bundles for consumers if not configured carefully.
Consider this: if you’re building a library that might have many optional features, Rollup’s ability to produce highly optimized, minimal bundles for each import is a significant advantage. A user importing only one small function from your library shouldn’t have to download your entire codebase.
Rollup’s plugin API is also a major draw for library authors. Want to integrate with TypeScript, PostCSS, or specific code transformations? Rollup has a mature ecosystem of plugins that make these integrations seamless. esbuild can do some of these things, but its plugin API is newer and less comprehensive, often requiring you to pre-transpile your code with another tool before feeding it to esbuild.
The real magic of Rollup for libraries comes from its output.manualChunks configuration. This allows you to explicitly define how your library’s output should be split into separate files, which can be incredibly useful for managing dependencies or providing different entry points. For example, you could split out a heavy charting component into its own chunk, only loaded by users who explicitly need it.
// rollup.config.js excerpt for manual chunks
output: {
// ... other output options
manualChunks: {
'heavy-chart': ['src/components/chart.js'],
'utils': ['src/utils/math.js', 'src/utils/string.js']
}
}
While esbuild is undeniably faster for many tasks, especially during local development or for application bundling, Rollup’s deliberate, AST-driven approach and its mature plugin ecosystem make it the more robust and often preferred choice for publishing libraries where bundle size and code efficiency for end-users are critical. The ability to precisely control output and ensure maximal tree-shaking is a powerful differentiator for library authors.
The next hurdle you’ll likely face is managing different output formats (CommonJS, ESM, UMD, IIFE) and ensuring compatibility across various JavaScript environments.