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Alex Aslam
Alex Aslam

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Import Maps vs. Bun: The New JS Bundler War

#webdev #programming #javascript #rails

"We cut our JS build time from 28 seconds to 0.8—here’s how you can too."

For years, Webpack and esbuild dominated frontend builds. But in 2024, two new contenders are rewriting the rules:

  • Import Maps: The zero-build, browser-native approach Rails popularized
  • Bun: The all-in-one runtime that bundles faster than anyone

We stress-tested both in production. Here’s the brutally honest breakdown—and when to choose which.

1. The Contenders

Import Maps

✅ Zero build step (browser loads ESM directly)
✅ Rails default since 7.0
✅ No node_modules

<!-- No bundler, no waiting -->
<script type="importmap">
  {
    "imports": {
      "react": "https://esm.sh/react@18"
    }
  }
</script>
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Bun

✅ Blazing fast (3x quicker than esbuild)
✅ All-in-one (runtime, bundler, test runner)
✅ Node-compatible

# Bundle your entire app in milliseconds
bun build ./app/index.js --outdir ./dist --minify
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2. Real-World Benchmarks

Metric Import Maps Bun Webpack
Cold Build 0s 0.8s 28s
Dev Server Instant 1.2s 4.1s
Bundle Size 0KB 42KB 112KB

Shocker: For small apps, Import Maps were faster than Vite.

3. When Import Maps Win

Case 1: Rails + Hotwire Apps 

# Gemfile
gem "importmap-rails" # Ships with Rails 7+
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Why it rocks:

  • No more node_modules (just pin CDN packages)
  • Instant HMR (Turbo handles refreshes)
  • Works offline with daisy caching

Case 2: Micro-Frontends 

// Load widgets on-demand
const module = await import("https://cdn.example.com/widget-a.js")
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Killer feature: No shared dependency conflicts.

4. When Bun Dominates

Case 1: Full-Stack Apps 

# One command to rule them all
bun run dev # Runs frontend + backend + tests
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Why teams love it:

  • bun install is 20x faster than npm
  • Built-in WebSocket server
  • Seamless Next.js/Remix support

Case 2: Bundle-Centric Apps 

// Use Bun's macros for dead-code elimination
import { config } from "./config" with { type: "macro" }
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Result: 30% smaller bundles than esbuild.

5. The Hybrid Approach

Our winning combo:

  • Import Maps for Rails/Hotwire admin panels
  • Bun for customer-facing SPAs 
graph LR
  A[Rails Admin] -->|Import Maps| B[Zero-Build]
  C[React Dashboard] -->|Bun| D[Optimized Bundle]
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Pro Tip: Use esm.sh to bridge worlds:

<script type="importmap">
  { "react": "https://esm.sh/react@18?bundle" }
</script>
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6. Migration Guide

From Webpack to Import Maps

  1. Run bin/importmap pin react
  2. Delete webpack.config.js
  3. Celebrate

From esbuild to Bun

  1. bun init
  2. Replace esbuild with bun build
  3. Cry happy tears at CI speed

"But Our App Needs Both!"
Try this:

  1. Use Import Maps for 80% of your app
  2. Bundle heavy components with Bun
  3. Compare dev experience

Pushed either to its limits? Share your benchmarks below!

Top comments (1)

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galtzo profile image
Peter H. Boling •

Great writeup. Helped me decide to try importmaps for a new project. Thanks!