bun.sh/docs/runtime/index.md

Bun is a new JavaScript runtime designed to be a faster, leaner, more modern replacement for Node.js.

## Speed

Bun is designed to start fast and run fast. It's transpiler and runtime are written in Zig, a modern, high-performance language. On Linux, this translates into startup times [4x faster](https://twitter.com/jarredsumner/status/1499225725492076544) than Node.js. Performance sensitive APIs like `Buffer`, `fetch`, and `Response` are heavily profiled and optimized. Under the hood Bun uses the [JavaScriptCore engine](https://developer.apple.com/documentation/javascriptcore), which is developed by Apple for Safari. It starts and runs faster than V8, the engine used by Node.js and Chromium-based browsers.

## File types

Bun natively supports TypeScript and JSX out of the box.

```bash
$ bun server.tsx
```

<!-- Before execution, Bun internally transforms all source files to vanilla JavaScript using its fast native transpiler. The transpiler looks at the files extension to determine how to handle it. -->

<!--

every file before execution. It's transpiler  can directly run TypeScript and JSX `{.js|.jsx|.ts|.tsx}` files directly. During execution, Bun internally transpiles all files (including `.js` files) to vanilla JavaScript with it's fast native transpiler. -->

<!-- A loader determines how to map imports &amp; file extensions to transforms and output. -->

<!-- Currently, Bun implements the following loaders: -->

<!-- {% table %}

- Extension
- Transforms
- Output (internal)

---

- `.js`
- JSX + JavaScript
- `.js`

---

- `.jsx`
- JSX + JavaScript
- `.js`

---

- `.ts`
- TypeScript + JavaScript
- `.js`

---

- `.tsx`
- TypeScript + JSX + JavaScript
- `.js`

---

- `.mjs`
- JavaScript
- `.js`

---

- `.cjs`
- JavaScript
- `.js`

---

- `.mts`
- TypeScript
- `.js`

---

- `.cts`
- TypeScript
- `.js`


{% /table %} -->

Source files can import a `*.json` or `*.toml` file to load its contents as a plain old JavaScript object.

```ts
import pkg from "./package.json";
import bunfig from "./bunfig.toml";
```

As of v0.5.2, experimental support has been for the [WebAssembly System Interface](https://github.com/WebAssembly/WASI) (WASI), you can run `.wasm` binaries.

```bash
$ bun ./my-wasm-app.wasm
# if the filename doesn't end with ".wasm"
$ bun run ./my-wasm-app.whatever
```

{% callout %}
**Note** — WASI support is based on [wasi-js](https://github.com/sagemathinc/cowasm/tree/main/packages/wasi-js). Currently, it only supports WASI binaries that use the `wasi_snapshot_preview1` or `wasi_unstable` APIs. Bun's implementation is not optimized for performance, but if this feature gets popular, we'll definitely invest time in making it faster.
{% /callout %}

Support for additional file types can be implemented with [Plugins](/docs/runtime/plugins).

## Node.js compatibility

Long-term, Bun aims for complete Node.js compatibility. Most Node.js packages already work with Bun out of the box, but certain low-level APIs like `dgram` are still unimplemented. Track the current compatibility status at [Ecosystem > Node.js](/docs/ecosystem/nodejs).

Bun implements the Node.js module resolution algorithm, so dependencies can still be managed with `package.json`, `node_modules`, and CommonJS-style imports.

{% callout %}
**Note** — We recommend using Bun's [built-in package manager](/docs/cli/install) for a performance boost over other npm clients.
{% /callout %}

## Web-standard

<!-- When prudent, Bun attempts to implement Web-standard APIs instead of introducing new APIs. Refer to [Runtime > Web APIs](/docs/web-apis) for a list of Web APIs that are available in Bun. -->

Some Web APIs aren't relevant in the context of a server-first runtime like Bun, such as the [DOM API](https://developer.mozilla.org/en-US/docs/Web/API/HTML_DOM_API#html_dom_api_interfaces) or [History API](https://developer.mozilla.org/en-US/docs/Web/API/History_API). Many others, though, are broadly useful outside of the browser context; when possible, Bun implements these Web-standard APIs instead of introducing new APIs.

The following Web APIs are partially or completely supported.

{% table %}

---

- HTTP
- [`fetch`](https://developer.mozilla.org/en-US/docs/Web/API/fetch) [`Response`](https://developer.mozilla.org/en-US/docs/Web/API/Response) [`Request`](https://developer.mozilla.org/en-US/docs/Web/API/Request) [`Headers`](https://developer.mozilla.org/en-US/docs/Web/API/Headers) [`AbortController`](https://developer.mozilla.org/en-US/docs/Web/API/AbortController) [`AbortSignal`](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal)

---

- URLs
- [`URL`](https://developer.mozilla.org/en-US/docs/Web/API/URL) [`URLSearchParams`](https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams)

---

- Streams
- [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream) [`WritableStream`](https://developer.mozilla.org/en-US/docs/Web/API/WritableStream) [`TransformStream`](https://developer.mozilla.org/en-US/docs/Web/API/TransformStream) [`ByteLengthQueuingStrategy`](https://developer.mozilla.org/en-US/docs/Web/API/ByteLengthQueuingStrategy) [`CountQueuingStrategy`](https://developer.mozilla.org/en-US/docs/Web/API/CountQueuingStrategy) and associated classes

---

- Blob
- [`Blob`](https://developer.mozilla.org/en-US/docs/Web/API/Blob)

---

- WebSockets
- [`WebSocket`](https://developer.mozilla.org/en-US/docs/Web/API/WebSocket)

---

- Encoding and decoding
- [`atob`](https://developer.mozilla.org/en-US/docs/Web/API/atob) [`btoa`](https://developer.mozilla.org/en-US/docs/Web/API/btoa) [`TextEncoder`](https://developer.mozilla.org/en-US/docs/Web/API/TextEncoder) [`TextDecoder`](https://developer.mozilla.org/en-US/docs/Web/API/TextDecoder)

---

- Timeouts
- [`setTimeout`](https://developer.mozilla.org/en-US/docs/Web/API/setTimeout) [`clearTimeout`](https://developer.mozilla.org/en-US/docs/Web/API/clearTimeout)

---

- Intervals
- [`setInterval`](https://developer.mozilla.org/en-US/docs/Web/API/setInterval)[`clearInterval`](https://developer.mozilla.org/en-US/docs/Web/API/clearInterval)

---

- Crypto
- [`crypto`](https://developer.mozilla.org/en-US/docs/Web/API/Crypto) [`SubtleCrypto`](https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto)
  [`CryptoKey`](https://developer.mozilla.org/en-US/docs/Web/API/CryptoKey)

---

- Debugging

- [`console`](https://developer.mozilla.org/en-US/docs/Web/API/console) [`performance`](https://developer.mozilla.org/en-US/docs/Web/API/Performance)

---

- Microtasks
- [`queueMicrotask`](https://developer.mozilla.org/en-US/docs/Web/API/queueMicrotask)

---

- Errors
- [`reportError`](https://developer.mozilla.org/en-US/docs/Web/API/reportError) [`ResolveError`](https://developer.mozilla.org/en-US/docs/Web/API/ResolveError)
  [`BuildError`](https://developer.mozilla.org/en-US/docs/Web/API/BuildError)

---

- User interaction
- [`alert`](https://developer.mozilla.org/en-US/docs/Web/API/Window/alert) [`confirm`](https://developer.mozilla.org/en-US/docs/Web/API/Window/confirm) [`prompt`](https://developer.mozilla.org/en-US/docs/Web/API/Window/prompt) (intended for interactive CLIs)

<!-- - Blocking. Prints the alert message to terminal and awaits `[ENTER]` before proceeding. -->
<!-- - Blocking. Prints confirmation message and awaits `[y/N]` input from user. Returns `true` if user entered `y` or `Y`, `false` otherwise.
- Blocking. Prints prompt message and awaits user input. Returns the user input as a string. -->

---

- Realms
- [`ShadowRealm`](https://github.com/tc39/proposal-shadowrealm)

---

- Events
- [`EventTarget`](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget)
  [`Event`](https://developer.mozilla.org/en-US/docs/Web/API/Event) [`ErrorEvent`](https://developer.mozilla.org/en-US/docs/Web/API/ErrorEvent) [`CloseEvent`](https://developer.mozilla.org/en-US/docs/Web/API/CloseEvent) [`MessageEvent`](https://developer.mozilla.org/en-US/docs/Web/API/MessageEvent)

---

{% /table %}

## Bun-native APIs

Bun exposes a set of Bun-specific APIs on the `Bun` global object and through a number of built-in modules. These APIs represent the canonical "Bun-native" way to perform some common development tasks. They are all heavily optimized for performance. Click the link in the left column to view the associated documentation.

{% table %}

- Topic
- APIs

---

- [HTTP](/docs/api/http)
- `Bun.serve`

---

- [File I/O](/docs/api/file-io)
- `Bun.file` `Bun.write`

---

- [Processes](/docs/api/spawn)
- `Bun.spawn` `Bun.spawnSync`

---

- [TCP](/docs/api/tcp)
- `Bun.listen` `Bun.connect`

---

- [Transpiler](/docs/api/transpiler)
- `Bun.Transpiler`

---

- [Routing](/docs/api/file-system-router)
- `Bun.FileSystemRouter`

---

- [HTMLRewriter](/docs/api/html-rewriter)
- `HTMLRewriter`

---

- [Utils](/docs/api/utils)
- `Bun.peek` `Bun.which`

---

- [SQLite](/docs/api/sqlite)
- `bun:sqlite`

---

- [FFI](/docs/api/ffi)
- `bun:ffi`

---

- [DNS](/docs/api/dns)
- `bun:dns`

---

- [Testing](/docs/api/test)
- `bun:test`

---

- [Node-API](/docs/api/node-api)
- `Node-API`

---

{% /table %}