bun-src/docs/runtime/index.mdx

---
title: Bun Runtime
description: Execute JavaScript/TypeScript files, package.json scripts, and executable packages with Bun's fast runtime.
---

import Run from "/snippets/cli/run.mdx";

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

Bun uses the [JavaScriptCore engine](https://developer.apple.com/documentation/javascriptcore), 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](https://twitter.com/jarredsumner/status/1499225725492076544) 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.

```bash terminal icon="terminal"
bun run index.js
```

Bun supports TypeScript and JSX with no configuration. Bun transpiles every file on the fly with its native [transpiler](/runtime/transpiler) before running it.

```bash terminal icon="terminal"
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.

```bash terminal icon="terminal"
bun index.tsx
bun index.js
```

### `--watch`

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

```bash terminal icon="terminal"
bun --watch run index.tsx
```

<Note>
When using `bun run`, put Bun flags like `--watch` immediately after `bun`.

```bash
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.

</Note>

## Run a `package.json` script

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

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

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

```json package.json icon="file-json"
{
  // ... other fields
  "scripts": {
    "clean": "rm -rf dist && echo 'Done.'",
    "dev": "bun server.ts"
  }
}
```

Use `bun run <script>` to execute these scripts.

```bash terminal icon="terminal"
bun run clean
rm -rf dist && echo 'Done.'
```

```txt
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](/runtime/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>

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.

```bash terminal icon="terminal"
bun run dev
```

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

```bash terminal icon="terminal"
bun run
```

```txt
...
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](<https://en.wikipedia.org/wiki/Shebang_(Unix)>) to indicate that they should be executed with `node`.

```js cli.js icon="/icons/javascript.svg"
#!/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.

```bash terminal icon="terminal"
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

```bash terminal icon="terminal"
bun run --filter 'ba*' <script>
```

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

See [`--filter`](/pm/filter#running-scripts-with-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.

```bash terminal icon="terminal"
echo "console.log('Hello')" | bun run -
```

```txt
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:

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

```txt
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.

```bash terminal icon="terminal"
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.

```ts console.ts icon="/icons/typescript.svg"
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.

```bash terminal icon="terminal"
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:

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

---

<Run />