bun-runtime

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Translated

Bun as runtime, package manager, bundler, and test runner. When to choose Bun vs Node, migration notes, and Vercel support.

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bun-runtimenode-migrationvercel-deploymentjavascript-toolchainpackage-manager

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Bun Runtime

Bun is a fast all-in-one JavaScript runtime and toolkit: runtime, package manager, bundler, and test runner.

When to Use

  • Prefer Bun for: new JS/TS projects, scripts where install/run speed matters, Vercel deployments with Bun runtime, and when you want a single toolchain (run + install + test + build).
  • Prefer Node for: maximum ecosystem compatibility, legacy tooling that assumes Node, or when a dependency has known Bun issues.
Use when: adopting Bun, migrating from Node, writing or debugging Bun scripts/tests, or configuring Bun on Vercel or other platforms.

How It Works

  • Runtime: Drop-in Node-compatible runtime (built on JavaScriptCore, implemented in Zig).
  • Package manager: 
    bun install
    is significantly faster than npm/yarn. Lockfile is 
    bun.lock
    (text) by default in current Bun; older versions used 
    bun.lockb
    (binary).
  • Bundler: Built-in bundler and transpiler for apps and libraries.
  • Test runner: Built-in 
    bun test
    with Jest-like API.
Migration from Node: Replace 
node script.js
with 
bun run script.js
or 
bun script.js
. Run 
bun install
in place of 
npm install
; most packages work. Use 
bun run
for npm scripts; 
bun x
for npx-style one-off runs. Node built-ins are supported; prefer Bun APIs where they exist for better performance.
Vercel: Set runtime to Bun in project settings. Build: 
bun run build
or 
bun build ./src/index.ts --outdir=dist
. Install: 
bun install --frozen-lockfile
for reproducible deploys.

Examples

Run and install

bash
# Install dependencies (creates/updates bun.lock or bun.lockb)
bun install

# Run a script or file
bun run dev
bun run src/index.ts
bun src/index.ts

Scripts and env

bash
bun run --env-file=.env dev
FOO=bar bun run script.ts

Testing

bash
bun test
bun test --watch
typescript
// test/example.test.ts
import { expect, test } from "bun:test";

test("add", () => {
  expect(1 + 2).toBe(3);
});

Runtime API

typescript
const file = Bun.file("package.json");
const json = await file.json();

Bun.serve({
  port: 3000,
  fetch(req) {
    return new Response("Hello");
  },
});

Best Practices

  • Commit the lockfile (
    bun.lock
    or 
    bun.lockb
    ) for reproducible installs.
  • Prefer 
    bun run
    for scripts. For TypeScript, Bun runs 
    .ts
    natively.
  • Keep dependencies up to date; Bun and the ecosystem evolve quickly.