Execute JavaScript/TypeScript files, package.json scripts, and executable packages with Bun's fast runtime.

The Bun Runtime is designed to start fast and run fast.

Bun uses the JavaScriptCore engine, developed by Apple for Safari. JavaScriptCore usually starts and runs faster than V8, the engine used by Node.js and Chromium-based browsers. Bun's transpiler and runtime are written in Rust. On Linux, Bun starts 4x faster than Node.js.

Command	Time
`bun hello.js`	`5.2ms`
`node hello.js`	`25.1ms`

The benchmark runs a Hello World script on Linux.

Run a file

Use bun run to execute a source file.

bun run index.js

Bun supports TypeScript and JSX with no configuration. Bun transpiles every file on the fly with its native transpiler before running it.

bun run index.js
bun run index.jsx
bun run index.ts
bun run index.tsx

Alternatively, you can omit the run keyword and use the "naked" command; it behaves identically.

bun index.tsx
bun index.js

`--watch`

To run a file in watch mode, use the --watch flag.

bun --watch run index.tsx

When using bun run, put Bun flags like --watch immediately after bun.

bun --watch run dev # ✔️ do this
bun run dev --watch # ❌ don't do this

bun ignores flags at the end of the command and passes them through to the "dev" script itself.

`--check`

Bun runs TypeScript files without checking their types. To type check a file and everything it imports before Bun runs it, use the --check flag.

bun --check index.tsx

If there is a type error, Bun prints it, exits with code 1, and does not run the file. See bun check.

Run a `package.json` script

Compare to npm run <script> or yarn <script>

bun [bun flags] run <script> [script flags]

Your package.json can define named "scripts" that correspond to shell commands.

package.json

{
  // ... other fields
  "scripts": {
    "clean": "rm -rf dist && echo 'Done.'",
    "dev": "bun server.ts"
  }
}

Use bun run <script> to execute these scripts.

bun run clean
rm -rf dist && echo 'Done.'

Done.

Bun executes the script command in a subshell. On Linux & macOS, it checks for the following shells in order, using the first one it finds: bash, sh, zsh. On Windows, it uses the Bun Shell to support bash-like syntax and many common commands.

⚡️ The startup time for npm run on Linux is roughly 170ms; with Bun it is 6ms.

You can also run scripts with the shorter command bun <script>. If a built-in bun command has the same name, the built-in command takes precedence; use the explicit bun run <script> to run your package script instead.

bun run dev

To see a list of available scripts, run bun run without any arguments.

bun run

...
package.json scripts (2 found):
  $ bun run clean
    rm -rf dist && echo 'Done.'

  $ bun run dev
    bun server.ts

Bun respects lifecycle hooks. For instance, bun run clean runs preclean and postclean, if defined. If the pre<script> fails, Bun does not run the script itself.

`--bun`

It's common for package.json scripts to reference locally-installed CLIs like vite or next. These CLIs are often JavaScript files marked with a shebang to indicate that they should be executed with node.

cli.js

#!/usr/bin/env node

// do stuff

By default, Bun respects this shebang and executes the script with node. The --bun flag overrides it: the CLI runs with Bun instead of Node.js.

bun run --bun vite

Filtering

In a monorepo, the --filter argument runs a script in many packages at once.

bun run --filter <pattern> <script> executes <script> in every package selected by <pattern>. The pattern can be a package name glob, a ./path, a {dir} directory or a dependency relation like foo.... For example, if you have subdirectories containing packages named foo, bar and baz, running

bun run --filter 'ba*' <script>

executes <script> in both bar and baz, but not in foo.

See --filter.

`bun run -` to pipe code from stdin

bun run - reads JavaScript, TypeScript, TSX, or JSX from stdin and executes it without writing to a temporary file first.

echo "console.log('Hello')" | bun run -

Hello

You can also use bun run - to redirect files into Bun. For example, to run a .js file as if it were a .ts file:

echo "console.log!('This is TypeScript!' as any)" > secretly-typescript.js
bun run - < secretly-typescript.js

This is TypeScript!

bun run - treats all input as TypeScript with JSX support.

`bun run --console-depth`

Control the depth of object inspection in console output with the --console-depth flag.

bun --console-depth 5 run index.tsx

--console-depth sets how deeply Bun displays nested objects in console.log() output. The default depth is 2. Higher values show more nested properties but may produce verbose output for complex objects.

console.ts

const nested = { a: { b: { c: { d: "deep" } } } };
console.log(nested);
// With --console-depth 2 (default): { a: { b: { c: [Object] } } }
// With --console-depth 4: { a: { b: { c: { d: 'deep' } } } }

`bun run --smol`

In memory-constrained environments, use the --smol flag to reduce memory usage at a cost to performance.

bun --smol run index.tsx

--smol makes the garbage collector run more frequently, which can slow down execution. Bun adjusts the garbage collector's heap size based on the available memory (accounting for cgroups and other memory limits) with and without the --smol flag. The flag is therefore mostly useful when you want the heap to grow more slowly.

Resolution order

Bun always executes absolute paths and paths starting with ./ or .\\ as source files. Unless you use bun run, a name with an allowed extension resolves to the file rather than a package.json script.

When a package.json script and a file have the same name, bun run prefers the script. The full resolution order is:

package.json scripts: bun run build
Source files: bun run src/main.js
Binaries from project packages: bun add eslint && bun run eslint
(bun run only) System commands: bun run ls

CLI Usage

bun run <file or script>

General Execution Options

--silent boolean

Don't print the script command

--if-present boolean

Exit without an error if the entrypoint does not exist

--eval string

Evaluate argument as a script. Alias: -e

--print string

Evaluate argument as a script and print the result. Alias: -p

--help boolean

Display this menu and exit. Alias: -h

Workspace Management

--elide-lines number default: 0

Truncate each script's output to its last N lines when using --filter. Default 0 shows all lines

--filter string

Run a script in all workspace packages matching the pattern. Alias: -F

--workspaces boolean

Run a script in all workspace packages (from the workspaces field in package.json)

--parallel boolean

Run multiple scripts or workspace scripts concurrently with prefixed output

--sequential boolean

Run multiple scripts or workspace scripts one after another with prefixed output

--no-exit-on-error boolean

When using --parallel or --sequential, continue running other scripts when one fails

Runtime & Process Control

--bun boolean

Force a script or package to use Bun's runtime instead of Node.js (via symlinking node). Alias: -b

--shell string

Control the shell used for package.json scripts. Supports either bun or system

--interactive boolean

Open the Node.js-compatible REPL (node:repl). When combined with -e, starts the REPL and then evaluates the script. Under --interactive, -e is raw JavaScript (matching{" "} node -i -e). Use bun repl for TypeScript. Distinct from bun repl, which is Bun's native REPL.

--smol boolean

Use less memory, but run garbage collection more often

--expose-gc boolean

Expose gc() on the global object. Has no effect on Bun.gc()

--no-deprecation boolean

Silence all deprecation warnings

--throw-deprecation boolean

Throw deprecation warnings as errors

--title string

Set the process title

--zero-fill-buffers boolean

Force Buffer.allocUnsafe(size) to be zero-filled

--no-addons boolean

Throw an error if process.dlopen or bun:ffi cc() is called, and disable export condition node-addons

--no-ffi-cc boolean

Throw an error if bun:ffi cc() is called (disables the C compiler)

--unhandled-rejections string

One of strict, throw, warn, none, or{" "} warn-with-error-code

--console-depth number default: 2

Set the default depth for console.log object inspection (default: 2)

Development Workflow

--check boolean

Type check before running. Nothing runs if there are type errors. See bun check

--watch boolean

Automatically restart the process on file change

--watch-kill-signal string default: SIGTERM

Signal whose handlers run when --watch restarts the process

--hot boolean

Enable auto reload in the Bun runtime, test runner, or bundler

--no-clear-screen boolean

Disable clearing the terminal screen on reload when --hot or --watch is enabled

Debugging

--inspect string

Activate Bun's debugger

--inspect-wait string

Activate Bun's debugger, wait for a connection before executing

--inspect-brk string

Activate Bun's debugger, set breakpoint on first line of code and wait

Dependency & Module Resolution

--preload string

Import a module before Bun loads other modules. Alias: -r

--require string

Alias of --preload, for Node.js compatibility

--import string

Alias of --preload, for Node.js compatibility

--no-install boolean

Disable auto install in the Bun runtime

--install string default: auto

Configure auto-install behavior. One of auto (default, auto-installs when no node_modules),{" "} fallback (missing packages only), force (always)

-i boolean

Auto-install dependencies during execution. Equivalent to --install=fallback

--prefer-offline boolean

Skip staleness checks for packages in the Bun runtime and resolve from disk

--prefer-latest boolean

Use the latest matching versions of packages in the Bun runtime, always checking npm

--conditions string

Pass custom conditions to resolve

--main-fields string

Main fields to lookup in package.json

--preserve-symlinks boolean

Preserve symlinks when resolving files

--preserve-symlinks-main boolean

Preserve symlinks when resolving the main entry point

--extension-order string default: .tsx,.ts,.jsx,.cts,.cjs,.js,.mjs,.mts,.json,.node

Defaults to: .tsx,.ts,.jsx,.cts,.cjs,.js,.mjs,.mts,.json,.node

Transpilation & Language Features

--tsconfig-override string

Specify custom tsconfig.json. Default $cwd/tsconfig.json

--define string

Substitute K:V while parsing, e.g. --define process.env.NODE_ENV:"development". Bun parses values as JSON. Alias: -d

--drop string

Remove function calls, e.g. --drop=console removes all console.* calls

--loader string

Parse files with .ext:loader, e.g. --loader .js:jsx. Valid loaders: js,{" "} jsx, ts, tsx, json, toml, text,{" "} file, wasm, napi. Alias: -l

--no-macros boolean

Disable macro execution in the bundler, transpiler and runtime

--jsx-factory string

Changes the function called when compiling JSX elements using the classic JSX runtime

--jsx-fragment string

Changes the function called when compiling JSX fragments

--jsx-import-source string default: react

Declares the module specifier used to import the jsx and jsxs factory functions. Default: react

--jsx-runtime string default: automatic

automatic (default) or classic

--jsx-side-effects boolean

Treat JSX elements as having side effects (disable pure annotations)

--ignore-dce-annotations boolean

Ignore tree-shaking annotations such as @PURE

Networking & Security

--port number

Set the default port for Bun.serve

--fetch-preconnect string

Preconnect to a URL while code is loading

--max-http-header-size number default: 16384

Set the maximum size of HTTP headers in bytes. Default is 16KiB

--dns-result-order string default: verbatim

Set the default order of DNS lookup results. Valid orders: verbatim (default), ipv4first,{" "} ipv6first

--use-system-ca boolean

Use the system's trusted certificate authorities

--use-openssl-ca boolean

Use OpenSSL's default CA store

--use-bundled-ca boolean

Use bundled CA store

--redis-preconnect boolean

Preconnect to $REDIS_URL at startup

--sql-preconnect boolean

Preconnect to PostgreSQL at startup

--user-agent string

Set the default User-Agent header for HTTP requests

Global Configuration & Context

--env-file string

Load environment variables from the specified file(s)

--cwd string

Absolute path to resolve files & entrypoints from. This only changes the process' cwd

--config string

Specify path to Bun config file. Default $cwd/bunfig.toml. Alias: -c

Examples

Run a JavaScript or TypeScript file:

bun run ./index.js
bun run ./index.tsx

Run a package.json script:

bun run dev
bun run lint

Bun Runtime

Run a file

--watch

--check

Run a package.json script

--bun

Filtering

bun run - to pipe code from stdin

bun run --console-depth

bun run --smol

Resolution order

CLI Usage

General Execution Options

Workspace Management

Runtime & Process Control

Development Workflow

Debugging

Dependency & Module Resolution

Transpilation & Language Features

Networking & Security

Global Configuration & Context

Examples

`--watch`

`--check`

Run a `package.json` script

`--bun`

`bun run -` to pipe code from stdin

`bun run --console-depth`

`bun run --smol`