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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 that extend Bun with custom loaders. You can explicitly specify which loader to use using the 'loader' import attribute.
https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240index.ts
import my_toml from "./my_file" with { loader: "toml" };

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Built-in loaders

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js

JavaScript loader. 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.

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jsx

JavaScript + JSX loader. 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 for more information.

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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.

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tsx

TypeScript + JSX loader. Default for .tsx. Transpiles both TypeScript and JSX to vanilla JavaScript.

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json

JSON loader. Default for .json. JSON files can be directly imported. 
import pkg from "./package.json";
pkg.name; // => "my-package"
During bundling, the parsed JSON is inlined into the bundle as a JavaScript object. 
const 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 defaults the parsed object. 
{
	"name": "John Doe",
	"age": 35,
	"email": "johndoe@example.com"
}

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toml

TOML loader. Default for .toml. TOML files can be directly imported. Bun will parse them with its fast native TOML parser. 
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. 
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 defaults the parsed object. 
name = "John Doe"
age = 35
email = "johndoe@example.com"

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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. 
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. 
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 defaults the file contents. 
Hello, world!

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napi

Native addon loader. Default for .node. In the runtime, native addons can be directly imported. 
import addon from "./addon.node";
console.log(addon);
In the bundler, .node files are handled using the file loader.

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sqlite

SQLite loader. Requires with { "type": "sqlite" } import attribute. In the runtime and bundler, SQLite databases can be directly imported. This will load the database using bun:sqlite. 
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: 
// embed the database into the bundle
import db from "./my.db" with { type: "sqlite", embed: "true" };
When using a standalone executable, the database is embedded into the single-file executable.Otherwise, the database to embed is copied into the outdir with a hashed filename.

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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:
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>
It will output a new HTML file with the bundled assets:
dist/index.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>
Under the hood, it uses 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]
HTML Loader Behavior in Different ContextsThe html loader behaves differently depending on how it’s used:
  • Static Build: When you run bun build ./index.html, Bun produces a static site with all assets bundled and hashed.
  • 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.
  • 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.

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sh

Bun Shell loader. Default for .sh files. This loader is used to parse Bun Shell scripts. It’s only supported when starting Bun itself, so it’s not available in the bundler or in the runtime. 
bun run ./script.sh

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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. 
// 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. 
bun run logo.ts
# Output: /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. 
// 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.
Public pathResolved import
"" (default)/logo.svg
"/assets"/assets/logo.svg
"https://cdn.example.com/"https://cdn.example.com/logo.svg
The location and file name of the copied file is determined by the value of naming.asset.This loader is copied into the outdir as-is. The name of the copied file is determined using the value of naming.asset.