Vercel Adds Bun Support: What Changes For Next.js Developers

Hello HaWkers, exciting news for those working with Next.js and seeking maximum performance: Vercel now supports the Bun runtime in beta. This means your Next.js applications and serverless functions can run on Bun's ultra-fast engine, built in Zig.

For those unfamiliar, Bun is a JavaScript runtime that promises to be significantly faster than Node.js in various operations. Let's understand what this integration means in practice and how you can take advantage of it.

What Is Bun

Bun is an all-in-one JavaScript runtime that was built from scratch with a focus on speed and developer experience.

Main features:

Runtime: Drop-in replacement for Node.js
Bundler: Alternative to webpack, esbuild, rollup
Package Manager: Replacement for npm, yarn, pnpm
Test Runner: Integrated testing framework
Transpiler: Native support for TypeScript and JSX

The big difference with Bun is that it was written in Zig (a low-level language focused on performance) and uses JavaScriptCore (Safari's engine) instead of V8.

Performance Numbers

Bun's benchmarks are impressive when compared to Node.js.

Performance comparison:

Operation	Node.js	Bun	Improvement
Cold Start	300ms	50ms	6x faster
npm install	25s	5s	5x faster
File I/O	1x	3x	3x faster
HTTP Server	1x	2.5x	2.5x faster
SQLite	1x	4x	4x faster

For serverless functions on Vercel, the cold start reduction is especially significant, as it directly impacts the end user experience.

Why Bun Is Faster

Technical reasons:

Zig: Low-level language without garbage collector
JavaScriptCore: Engine optimized for different workloads
Native APIs: APIs implemented directly in native code
Modern architecture: Designed for current hardware and usage patterns

Bun on Vercel: How It Works

Bun integration on Vercel is available in beta, allowing you to run your Next.js functions on this new runtime.

Basic Configuration

To enable Bun in your Vercel project, add the configuration to vercel.json:

{
  "functions": {
    "app/**/*.ts": {
      "runtime": "bun@1.0"
    },
    "api/**/*.ts": {
      "runtime": "bun@1.0"
    }
  }
}

You can also configure via environment variable:

# In Vercel dashboard or .env
VERCEL_FUNCTION_RUNTIME=bun@1.0

For Next.js Projects

In Next.js projects, configuration can be done in next.config.js:

/** @type {import('next').NextConfig} */
const nextConfig = {
  experimental: {
    // Enable Bun runtime for API routes
    serverComponentsExternalPackages: [],
  },
};

module.exports = nextConfig;

And in vercel.json:

{
  "framework": "nextjs",
  "functions": {
    "app/api/**/*.ts": {
      "runtime": "bun@1.0",
      "memory": 1024,
      "maxDuration": 10
    }
  }
}

Ideal Use Cases

Not all applications benefit equally from Bun. Here are the scenarios where gains are most expressive.

1. API Routes with Heavy Processing

Routes that do significant data processing benefit from Bun's raw speed:

// app/api/process-data/route.ts
import { NextRequest, NextResponse } from 'next/server';

export const runtime = 'edge'; // or 'nodejs' with Bun

export async function POST(request: NextRequest) {
  const data = await request.json();

  // Intensive processing
  const processed = data.items.map((item: any) => {
    // Complex transformations
    return {
      ...item,
      computed: expensiveComputation(item.value),
      hash: generateHash(item.id),
    };
  });

  // Bun executes this significantly faster
  const aggregated = processed.reduce((acc: any, item: any) => {
    acc[item.category] = (acc[item.category] || 0) + item.computed;
    return acc;
  }, {});

  return NextResponse.json({ result: aggregated });
}

function expensiveComputation(value: number): number {
  // Complex calculation simulation
  let result = value;
  for (let i = 0; i < 1000; i++) {
    result = Math.sqrt(result * result + i);
  }
  return result;
}

function generateHash(id: string): string {
  // Bun has faster native APIs for crypto
  return Bun.hash(id).toString(16);
}

2. File Operations

Reading and writing files are significantly faster in Bun:

// app/api/files/route.ts
export async function GET() {
  // Bun.file is up to 3x faster than Node's fs
  const file = Bun.file('./data/large-dataset.json');
  const content = await file.json();

  // Processing
  const filtered = content.filter((item: any) => item.active);

  // Writing is also optimized
  await Bun.write('./data/filtered.json', JSON.stringify(filtered));

  return Response.json({ count: filtered.length });
}

3. Embedded SQLite

Bun has native SQLite support that is 4x faster:

// app/api/db/route.ts
import { Database } from 'bun:sqlite';

const db = new Database('./local.db');

export async function GET(request: Request) {
  const url = new URL(request.url);
  const search = url.searchParams.get('q') || '';

  // SQLite queries are extremely fast in Bun
  const results = db
    .query('SELECT * FROM products WHERE name LIKE ?')
    .all(`%${search}%`);

  return Response.json(results);
}

export async function POST(request: Request) {
  const { name, price } = await request.json();

  const result = db
    .query('INSERT INTO products (name, price) VALUES (?, ?)')
    .run(name, price);

  return Response.json({ id: result.lastInsertRowid });
}

Limitations and Considerations

Despite the benefits, there are important limitations to consider.

Compatibility

Packages that may have issues:

Native modules compiled for Node.js
Packages that depend on Node-specific APIs
Some cryptography libraries
ORMs with native bindings (some versions)

Checking Compatibility

Before migrating, test your application locally:

# Install Bun
curl -fsSL https://bun.sh/install | bash

# Test your project
cd your-project
bun install
bun run dev

# Check compatibility
bun pm untrusted

Gradual Migration Strategy

I recommend an incremental approach:

{
  "functions": {
    "api/fast-endpoints/**/*.ts": {
      "runtime": "bun@1.0"
    },
    "api/legacy/**/*.ts": {
      "runtime": "nodejs20.x"
    }
  }
}

This way you can migrate route by route, testing compatibility.

Bun vs Node.js vs Deno

For context, let's compare the three main JavaScript runtimes.

Feature Comparison

Feature	Node.js	Bun	Deno
Package Manager	npm/yarn/pnpm	Integrated	npm compat
TypeScript	Via transpiler	Native	Native
Bundler	External	Integrated	External
Test Runner	External	Integrated	Integrated
Performance	Baseline	Faster	Similar to Node
npm Compatibility	Full	Almost full	Partial
Maturity	Very high	Medium	High
Vercel Support	Yes	Beta	Deno Deploy

When to Use Each

Node.js: Projects requiring maximum compatibility and mature ecosystem.

Bun: New projects focused on performance and modern DX.

Deno: Projects prioritizing security and simplicity.

The Future of JavaScript Runtime

Competition between runtimes is accelerating innovation.

Observed Trends

1. Feature Convergence:

Node.js is incorporating Bun APIs (for example, native fetch)
Bun is improving Node compatibility
All are adopting native TypeScript

2. Edge Computing Focus:

Smaller and faster runtimes
Optimized cold starts
Global distribution

3. Developer Experience:

All-in-one tools
Zero configuration
More intuitive commands

What to Expect for 2026

Area	Expectation
Bun	Version 2.0 with stability
Node.js	More APIs inspired by Bun
Vercel	Bun support in production
Performance	Cold starts < 20ms

Conclusion

Adding Bun support by Vercel represents an important milestone for the JavaScript ecosystem. Next.js developers now have another powerful option to optimize their applications.

Key points:

Bun offers significant performance gains
Integration with Vercel is in functional beta
Migration should be done gradually
Not all packages are compatible yet
Ideal for new features and APIs

For existing projects, I recommend starting by enabling Bun on new or low-risk routes. For new projects, consider Bun as the first option if performance is a priority.

If you want to dive deeper into performance for web applications, I recommend checking out another article: ECMAScript 2025: New JavaScript Features where you will discover the optimizations that arrived natively in the language.