jarle/Bun Bun Runtime

Bun Runtime

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.

Under the hood, Bun uses the JavaScriptCore engine, which is developed by Apple for Safari. In most cases, the startup and running performance is faster than V8, the engine used by Node.js and Chromium-based browsers. Its transpiler and runtime are written in Zig, a modern, high-performance language. On Linux, this translates into startup times 4x faster than Node.js.

| Command | Time | | --------------- | -------- | | bun hello.js | 5.2ms | | node hello.js | 25.1ms |

This benchmark is based on running a simple 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 out of the box. Every file is transpiled on the fly by Bun's fast native transpiler before being executed.

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

<Note> 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

Flags that occur at the end of the command will be ignored and passed through to the "dev" script itself. </Note>

Run a package.json script

<Note> Compare to `npm run <script>` or `yarn <script>` </Note>

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

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

{
  // ... 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.'

Cleaning...
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 bun shell to support bash-like syntax and many common commands.

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

Scripts can also be run with the shorter command bun <script>, however if there is a built-in bun command with the same name, the built-in command takes precedence. In this case, use the more explicit bun run <script> command to execute your package script.

bun run dev

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

bun run

quickstart scripts:

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

 bun run dev
   bun server.ts

2 scripts

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

#!/usr/bin/env node

// do stuff

By default, Bun respects this shebang and executes the script with node. However, you can override this behavior with the --bun flag. For Node.js-based CLIs, this will run the CLI with Bun instead of Node.js.

bun run --bun vite

Filtering

In monorepos containing multiple packages, you can use the --filter argument to execute scripts in many packages at once.

Use bun run --filter <name_pattern> <script> to execute <script> in all packages whose name matches <name_pattern>. For example, if you have subdirectories containing packages named foo, bar and baz, running

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

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

Find more details in the docs page for filter.

bun run - to pipe code from stdin

bun run - lets you read JavaScript, TypeScript, TSX, or JSX from stdin and execute 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!

For convenience, all code is treated as TypeScript with JSX support when using bun run -.

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

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

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

This causes the garbage collector to run more frequently, which can slow down execution. However, it can be useful in environments with limited memory. Bun automatically 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, so this is mostly useful for cases where you want to make the heap size grow more slowly.

Resolution order

Absolute paths and paths starting with ./ or .\\ are always executed as source files. Unless using bun run, running a file with an allowed extension will prefer the file over a package.json script.

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

1. package.json scripts, eg bun run build
2. Source files, eg bun run src/main.js
3. Binaries from project packages, eg bun add eslint && bun run eslint
4. (bun run only) System commands, eg bun run ls

---

CLI Usage

bun run <file or script>

General Execution Options

<ParamField path="--silent" type="boolean"> Don't print the script command </ParamField> <ParamField path="--if-present" type="boolean"> Exit without an error if the entrypoint does not exist </ParamField> <ParamField path="--eval" type="string"> Evaluate argument as a script. Alias: <code>-e</code> </ParamField> <ParamField path="--print" type="string"> Evaluate argument as a script and print the result. Alias: <code>-p</code> </ParamField> <ParamField path="--help" type="boolean"> Display this menu and exit. Alias: <code>-h</code> </ParamField>

Workspace Management

<ParamField path="--elide-lines" type="number" default="10"> Number of lines of script output shown when using --filter (default: 10). Set to 0 to show all lines </ParamField> <ParamField path="--filter" type="string"> Run a script in all workspace packages matching the pattern. Alias: <code>-F</code> </ParamField> <ParamField path="--workspaces" type="boolean"> Run a script in all workspace packages (from the <code>workspaces</code> field in <code>package.json</code>) </ParamField> <ParamField path="--parallel" type="boolean"> Run multiple scripts or workspace scripts concurrently with prefixed output </ParamField> <ParamField path="--sequential" type="boolean"> Run multiple scripts or workspace scripts one after another with prefixed output </ParamField> <ParamField path="--no-exit-on-error" type="boolean"> When using <code>--parallel</code> or <code>--sequential</code>, continue running other scripts when one fails </ParamField>

Runtime & Process Control

<ParamField path="--bun" type="boolean"> Force a script or package to use Bun's runtime instead of Node.js (via symlinking node). Alias: <code>-b</code> </ParamField> <ParamField path="--shell" type="string"> Control the shell used for <code>package.json</code> scripts. Supports either <code>bun</code> or <code>system</code> </ParamField> <ParamField path="--smol" type="boolean"> Use less memory, but run garbage collection more often </ParamField> <ParamField path="--expose-gc" type="boolean"> Expose <code>gc()</code> on the global object. Has no effect on <code>Bun.gc()</code> </ParamField> <ParamField path="--no-deprecation" type="boolean"> Suppress all reporting of the custom deprecation </ParamField> <ParamField path="--throw-deprecation" type="boolean"> Determine whether or not deprecation warnings result in errors </ParamField> <ParamField path="--title" type="string"> Set the process title </ParamField> <ParamField path="--zero-fill-buffers" type="boolean"> Boolean to force <code>Buffer.allocUnsafe(size)</code> to be zero-filled </ParamField> <ParamField path="--no-addons" type="boolean"> Throw an error if <code>process.dlopen</code> is called, and disable export condition <code>node-addons</code> </ParamField> <ParamField path="--unhandled-rejections" type="string"> One of <code>strict</code>, <code>throw</code>, <code>warn</code>, <code>none</code>, or{" "} <code>warn-with-error-code</code> </ParamField> <ParamField path="--console-depth" type="number" default="2"> Set the default depth for <code>console.log</code> object inspection (default: 2) </ParamField>

Development Workflow

<ParamField path="--watch" type="boolean"> Automatically restart the process on file change </ParamField> <ParamField path="--hot" type="boolean"> Enable auto reload in the Bun runtime, test runner, or bundler </ParamField> <ParamField path="--no-clear-screen" type="boolean"> Disable clearing the terminal screen on reload when --hot or --watch is enabled </ParamField>

Debugging

<ParamField path="--inspect" type="string"> Activate Bun's debugger </ParamField> <ParamField path="--inspect-wait" type="string"> Activate Bun's debugger, wait for a connection before executing </ParamField> <ParamField path="--inspect-brk" type="string"> Activate Bun's debugger, set breakpoint on first line of code and wait </ParamField>

Dependency & Module Resolution

<ParamField path="--preload" type="string"> Import a module before other modules are loaded. Alias: <code>-r</code> </ParamField> <ParamField path="--require" type="string"> Alias of --preload, for Node.js compatibility </ParamField> <ParamField path="--import" type="string"> Alias of --preload, for Node.js compatibility </ParamField> <ParamField path="--no-install" type="boolean"> Disable auto install in the Bun runtime </ParamField> <ParamField path="--install" type="string" default="auto"> Configure auto-install behavior. One of <code>auto</code> (default, auto-installs when no node\_modules),{" "} <code>fallback</code> (missing packages only), <code>force</code> (always) </ParamField> <ParamField path="-i" type="boolean"> Auto-install dependencies during execution. Equivalent to --install=fallback </ParamField> <ParamField path="--prefer-offline" type="boolean"> Skip staleness checks for packages in the Bun runtime and resolve from disk </ParamField> <ParamField path="--prefer-latest" type="boolean"> Use the latest matching versions of packages in the Bun runtime, always checking npm </ParamField> <ParamField path="--conditions" type="string"> Pass custom conditions to resolve </ParamField> <ParamField path="--main-fields" type="string"> Main fields to lookup in <code>package.json</code>. Defaults to --target dependent </ParamField> <ParamField path="--preserve-symlinks" type="boolean"> Preserve symlinks when resolving files </ParamField> <ParamField path="--preserve-symlinks-main" type="boolean"> Preserve symlinks when resolving the main entry point </ParamField> <ParamField path="--extension-order" type="string" default=".tsx,.ts,.jsx,.js,.json"> Defaults to: <code>.tsx,.ts,.jsx,.js,.json</code> </ParamField>

Transpilation & Language Features

<ParamField path="--tsconfig-override" type="string"> Specify custom <code>tsconfig.json</code>. Default <code>\$cwd/tsconfig.json</code> </ParamField> <ParamField path="--define" type="string"> Substitute K:V while parsing, e.g. <code>--define process.env.NODE\_ENV:"development"</code>. Values are parsed as JSON. Alias: <code>-d</code> </ParamField> <ParamField path="--drop" type="string"> Remove function calls, e.g. <code>--drop=console</code> removes all <code>console.\*</code> calls </ParamField> <ParamField path="--loader" type="string"> Parse files with <code>.ext:loader</code>, e.g. <code>--loader .js:jsx</code>. Valid loaders: <code>js</code>,{" "} <code>jsx</code>, <code>ts</code>, <code>tsx</code>, <code>json</code>, <code>toml</code>, <code>text</code>,{" "} <code>file</code>, <code>wasm</code>, <code>napi</code>. Alias: <code>-l</code> </ParamField> <ParamField path="--no-macros" type="boolean"> Disable macros from being executed in the bundler, transpiler and runtime </ParamField> <ParamField path="--jsx-factory" type="string"> Changes the function called when compiling JSX elements using the classic JSX runtime </ParamField> <ParamField path="--jsx-fragment" type="string"> Changes the function called when compiling JSX fragments </ParamField> <ParamField path="--jsx-import-source" type="string" default="react"> Declares the module specifier to be used for importing the jsx and jsxs factory functions. Default: <code>react</code> </ParamField> <ParamField path="--jsx-runtime" type="string" default="automatic"> <code>automatic</code> (default) or <code>classic</code> </ParamField> <ParamField path="--jsx-side-effects" type="boolean"> Treat JSX elements as having side effects (disable pure annotations) </ParamField> <ParamField path="--ignore-dce-annotations" type="boolean"> Ignore tree-shaking annotations such as <code>@**PURE**</code> </ParamField>

Networking & Security

<ParamField path="--port" type="number"> Set the default port for <code>Bun.serve</code> </ParamField> <ParamField path="--fetch-preconnect" type="string"> Preconnect to a URL while code is loading </ParamField> <ParamField path="--max-http-header-size" type="number" default="16384"> Set the maximum size of HTTP headers in bytes. Default is 16KiB </ParamField> <ParamField path="--dns-result-order" type="string" default="verbatim"> Set the default order of DNS lookup results. Valid orders: <code>verbatim</code> (default), <code>ipv4first</code>,{" "} <code>ipv6first</code> </ParamField> <ParamField path="--use-system-ca" type="boolean"> Use the system's trusted certificate authorities </ParamField> <ParamField path="--use-openssl-ca" type="boolean"> Use OpenSSL's default CA store </ParamField> <ParamField path="--use-bundled-ca" type="boolean"> Use bundled CA store </ParamField> <ParamField path="--redis-preconnect" type="boolean"> Preconnect to <code>\$REDIS\_URL</code> at startup </ParamField> <ParamField path="--sql-preconnect" type="boolean"> Preconnect to PostgreSQL at startup </ParamField> <ParamField path="--user-agent" type="string"> Set the default User-Agent header for HTTP requests </ParamField>

Global Configuration & Context

<ParamField path="--env-file" type="string"> Load environment variables from the specified file(s) </ParamField> <ParamField path="--cwd" type="string"> Absolute path to resolve files & entry points from. This just changes the process' cwd </ParamField> <ParamField path="--config" type="string"> Specify path to Bun config file. Default <code>\$cwd/bunfig.toml</code>. Alias: <code>-c</code> </ParamField>

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