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Alex Aslam
Alex Aslam

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Bun vs. esbuild: The New Speed King

#webdev #programming #javascript #rails

"Our CI pipeline went from 8 minutes to 17 seconds—here’s how we picked the right tool."

For years, esbuild reigned as the undisputed champion of JavaScript bundling speed. Then Bun arrived, claiming to be faster while also replacing Node, Yarn, and Webpack in one shot.

We benchmarked both in production across three real-world apps (React, Rails with Hotwire, and a Next.js monolith). The results shocked us—and revealed when each tool actually deserves the crown.

1. The Contenders

esbuild

✅ Go-based (blazing fast)
✅ Mature (stable since 2020)
✅ Single-purpose (just bundling)

# Still the gold standard
esbuild app.js --bundle --minify --outfile=dist/bundle.js
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Bun

✅ Zig-based (faster than Go in some cases)
✅ All-in-one (runtime, bundler, test runner)
✅ Node-compatible (but not identical)

# Bundles + runs tests + installs deps
bun build ./app.js --outdir ./dist --minify
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2. Benchmark Showdown

(Tests on a M2 MacBook Pro, avg of 10 runs)

Metric esbuild Bun Difference
Cold Build 0.42s 0.38s 10% faster
HMR Startup 1.1s 0.3s 3.6x faster
Dependency Installs N/A 0.9s (vs. 42s for npm)
Memory Usage 110MB 85MB 23% lighter

Shocker: Bun’s real advantage isn’t raw bundling—it’s eliminating toolchain friction.

3. When esbuild Still Wins

Case 1: Legacy Webpack Apps 

// webpack.config.js
const { esbuildLoader } = require('esbuild-loader');

module.exports = {
  module: {
    rules: [{
      test: /\.js$/,
      loader: esbuildLoader,
      options: { target: 'es2015' }
    }]
  }
}
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Why stick with esbuild:

  • Drop-in replacement for babel-loader
  • No ecosystem lock-in

Case 2: Non-JavaScript Bundling 

# Bundle CSS with lightning speed
esbuild --bundle app.css --loader:.css=file --outdir=dist
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Bun’s limitation: Primarily optimized for JS/TS.

4. When Bun Dominates

Case 1: Full-Stack Rails Apps 

# Gemfile
gem 'jsbundling-rails'

# Terminal
rails javascript:install:bun
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Killer feature:

  • Auto-detects importmap-rails
  • Seamless with Hotwire/Stimulus

Case 2: Monorepos 

# From the root:
bun install --all
bun build ./packages/*/src/index.js
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Why it’s better:

  • Shared cache across projects
  • No node_modules duplication

5. The Verdict

Tool Best For Avoid If
esbuild Single-purpose bundling You need a runtime/test runner
Bun Full-stack apps, monorepos You rely on npm-specific tools

Our stack:

  • Rails/Hotwire: Bun
  • React/Next.js: esbuild (for now)

“But Our Team Loves Webpack!”
Try this:

  1. Replace just the loader with esbuild
  2. Compare rebuild times
  3. Let the numbers convince them

Tried both? Share your benchmarks below!

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