bun-types

The Bun bundler implements a set of default loaders out of the box. As a rule of thumb, the bundler and the runtime both support the same set of file types out of the box. `.js` `.cjs` `.mjs` `.mts` `.cts` `.ts` `.tsx` `.jsx` `.toml` `.json` `.txt` `.wasm` `.node` `.html` Bun uses the file extension to determine which built-in _loader_ should be used to parse the file. Every loader has a name, such as `js`, `tsx`, or `json`. These names are used when building [plugins](https://bun.sh/docs/bundler/plugins) that extend Bun with custom loaders. You can explicitly specify which loader to use using the 'loader' import attribute. ```ts import my_toml from "./my_file" with { loader: "toml" }; ``` ## Built-in loaders ### `js` **JavaScript**. Default for `.cjs` and `.mjs`. Parses the code and applies a set of default transforms like dead-code elimination and tree shaking. Note that Bun does not attempt to down-convert syntax at the moment. ### `jsx` **JavaScript + JSX.**. Default for `.js` and `.jsx`. Same as the `js` loader, but JSX syntax is supported. By default, JSX is down-converted to plain JavaScript; the details of how this is done depends on the `jsx*` compiler options in your `tsconfig.json`. Refer to the TypeScript documentation [on JSX](https://www.typescriptlang.org/docs/handbook/jsx.html) for more information. ### `ts` **TypeScript loader**. Default for `.ts`, `.mts`, and `.cts`. Strips out all TypeScript syntax, then behaves identically to the `js` loader. Bun does not perform typechecking. ### `tsx` **TypeScript + JSX loader**. Default for `.tsx`. Transpiles both TypeScript and JSX to vanilla JavaScript. ### `json` **JSON loader**. Default for `.json`. JSON files can be directly imported. ```ts import pkg from "./package.json"; pkg.name; // => "my-package" ``` During bundling, the parsed JSON is inlined into the bundle as a JavaScript object. ```ts var pkg = { name: "my-package", // ... other fields }; pkg.name; ``` If a `.json` file is passed as an entrypoint to the bundler, it will be converted to a `.js` module that `export default`s the parsed object. {% codetabs %} ```json#Input { "name": "John Doe", "age": 35, "email": "johndoe@example.com" } ``` ```js#Output export default { name: "John Doe", age: 35, email: "johndoe@example.com" } ``` {% /codetabs %} ### `toml` **TOML loader**. Default for `.toml`. TOML files can be directly imported. Bun will parse them with its fast native TOML parser. ```ts import config from "./bunfig.toml"; config.logLevel; // => "debug" // via import attribute: // import myCustomTOML from './my.config' with {type: "toml"}; ``` During bundling, the parsed TOML is inlined into the bundle as a JavaScript object. ```ts var config = { logLevel: "debug", // ...other fields }; config.logLevel; ``` If a `.toml` file is passed as an entrypoint, it will be converted to a `.js` module that `export default`s the parsed object. {% codetabs %} ```toml#Input name = "John Doe" age = 35 email = "johndoe@example.com" ``` ```js#Output export default { name: "John Doe", age: 35, email: "johndoe@example.com" } ``` {% /codetabs %} ### `text` **Text loader**. Default for `.txt`. The contents of the text file are read and inlined into the bundle as a string. Text files can be directly imported. The file is read and returned as a string. ```ts import contents from "./file.txt"; console.log(contents); // => "Hello, world!" // To import an html file as text // The "type' attribute can be used to override the default loader. import html from "./index.html" with { type: "text" }; ``` When referenced during a build, the contents are inlined into the bundle as a string. ```ts var contents = `Hello, world!`; console.log(contents); ``` If a `.txt` file is passed as an entrypoint, it will be converted to a `.js` module that `export default`s the file contents. {% codetabs %} ```txt#Input Hello, world! ``` ```js#Output export default "Hello, world!"; ``` {% /codetabs %} ### `napi` **Native addon loader**. Default for `.node`. In the runtime, native addons can be directly imported. ```ts import addon from "./addon.node"; console.log(addon); ``` In the bundler, `.node` files are handled using the [`file`](#file) loader. ### `sqlite` **SQLite loader**. `with { "type": "sqlite" }` import attribute In the runtime and bundler, SQLite databases can be directly imported. This will load the database using [`bun:sqlite`](https://bun.sh/docs/api/sqlite). ```ts import db from "./my.db" with { type: "sqlite" }; ``` This is only supported when the `target` is `bun`. By default, the database is external to the bundle (so that you can potentially use a database loaded elsewhere), so the database file on-disk won't be bundled into the final output. You can change this behavior with the `"embed"` attribute: ```ts // embed the database into the bundle import db from "./my.db" with { type: "sqlite", embed: "true" }; ``` When using a [standalone executable](https://bun.sh/docs/bundler/executables), the database is embedded into the single-file executable. Otherwise, the database to embed is copied into the `outdir` with a hashed filename. ### `html` The html loader processes HTML files and bundles any referenced assets. It will: - Bundle and hash referenced JavaScript files (`<script src="...">`) - Bundle and hash referenced CSS files (`<link rel="stylesheet" href="...">`) - Hash referenced images (`<img src="...">`) - Preserve external URLs (by default, anything starting with `http://` or `https://`) For example, given this HTML file: {% codetabs %} ```html#src/index.html <!DOCTYPE html> <html> <body> <img src="./image.jpg" alt="Local image"> <img src="https://example.com/image.jpg" alt="External image"> <script type="module" src="./script.js"></script> </body> </html> ``` {% /codetabs %} It will output a new HTML file with the bundled assets: {% codetabs %} ```html#dist/output.html <!DOCTYPE html> <html> <body> <img src="./image-HASHED.jpg" alt="Local image"> <img src="https://example.com/image.jpg" alt="External image"> <script type="module" src="./output-ALSO-HASHED.js"></script> </body> </html> ``` {% /codetabs %} Under the hood, it uses [`lol-html`](https://github.com/cloudflare/lol-html) to extract script and link tags as entrypoints, and other assets as external. Currently, the list of selectors is: - `audio[src]` - `iframe[src]` - `img[src]` - `img[srcset]` - `link:not([rel~='stylesheet']):not([rel~='modulepreload']):not([rel~='manifest']):not([rel~='icon']):not([rel~='apple-touch-icon'])[href]` - `link[as='font'][href], link[type^='font/'][href]` - `link[as='image'][href]` - `link[as='style'][href]` - `link[as='video'][href], link[as='audio'][href]` - `link[as='worker'][href]` - `link[rel='icon'][href], link[rel='apple-touch-icon'][href]` - `link[rel='manifest'][href]` - `link[rel='stylesheet'][href]` - `script[src]` - `source[src]` - `source[srcset]` - `video[poster]` - `video[src]` {% callout %} **HTML Loader Behavior in Different Contexts** The `html` loader behaves differently depending on how it's used: 1. **Static Build:** When you run `bun build ./index.html`, Bun produces a static site with all assets bundled and hashed. 2. **Runtime:** When you run `bun run server.ts` (where `server.ts` imports an HTML file), Bun bundles assets on-the-fly during development, enabling features like hot module replacement. 3. **Full-stack Build:** When you run `bun build --target=bun server.ts` (where `server.ts` imports an HTML file), the import resolves to a manifest object that `Bun.serve` uses to efficiently serve pre-bundled assets in production. {% /callout %} ### `sh` loader **Bun Shell loader**. Default for `.sh` files This loader is used to parse [Bun Shell](https://bun.sh/docs/runtime/shell) scripts. It's only supported when starting Bun itself, so it's not available in the bundler or in the runtime. ```sh $ bun run ./script.sh ``` ### `file` **File loader**. Default for all unrecognized file types. The file loader resolves the import as a _path/URL_ to the imported file. It's commonly used for referencing media or font assets. ```ts#logo.ts import logo from "./logo.svg"; console.log(logo); ``` _In the runtime_, Bun checks that the `logo.svg` file exists and converts it to an absolute path to the location of `logo.svg` on disk. ```bash $ bun run logo.ts /path/to/project/logo.svg ``` _In the bundler_, things are slightly different. The file is copied into `outdir` as-is, and the import is resolved as a relative path pointing to the copied file. ```ts#Output var logo = "./logo.svg"; console.log(logo); ``` If a value is specified for `publicPath`, the import will use value as a prefix to construct an absolute path/URL. {% table %} - Public path - Resolved import --- - `""` (default) - `/logo.svg` --- - `"/assets"` - `/assets/logo.svg` --- - `"https://cdn.example.com/"` - `https://cdn.example.com/logo.svg` {% /table %} {% callout %} The location and file name of the copied file is determined by the value of [`naming.asset`](https://bun.sh/docs/bundler#naming). {% /callout %} This loader is copied into the `outdir` as-is. The name of the copied file is determined using the value of `naming.asset`. {% details summary="Fixing TypeScript import errors" %} If you're using TypeScript, you may get an error like this: ```ts // TypeScript error // Cannot find module './logo.svg' or its corresponding type declarations. ``` This can be fixed by creating `*.d.ts` file anywhere in your project (any name will work) with the following contents: ```ts declare module "*.svg" { const content: string; export default content; } ``` This tells TypeScript that any default imports from `.svg` should be treated as a string. {% /details %}