bun.sh/docs/bundler/vs-esbuild.md

Bun's bundler API is inspired heavily by esbuild. Migrating to Bun's bundler from esbuild should be relatively painless. This guide will briefly explain why you might consider migrating to Bun's bundler and provide a side-by-side API comparison reference for those who are already familiar with esbuild's API.

There are a few behavioral differences to note.

• Bundling by default. Unlike esbuild, Bun always bundles by default. This is why the --bundle flag isn't necessary in the Bun example. To transpile each file individually, use Bun.Transpiler.
• It's just a bundler. Unlike esbuild, Bun's bundler does not include a built-in development server or file watcher. It's just a bundler. The bundler is intended for use in conjunction with Bun.serve and other runtime APIs to achieve the same effect. As such, all options relating to HTTP/file watching are not applicable.

Performance

With a performance-minded API coupled with the extensively optimized Zig-based JS/TS parser, Bun's bundler is 1.75x faster than esbuild on esbuild's three.js benchmark.

{% image src="/images/bundler-speed.png" caption="Bundling 10 copies of three.js from scratch, with sourcemaps and minification" /%}

CLI API

Bun and esbuild both provide a command-line interface.

$ esbuild <entrypoint> --outdir=out --bundle
$ bun build <entrypoint> --outdir=out

In Bun's CLI, simple boolean flags like --minify do not accept an argument. Other flags like --outdir <path> do accept an argument; these flags can be written as --outdir out or --outdir=out. Some flags like --define can be specified several times: --define foo=bar --define bar=baz.

{% table %}

• esbuild
• bun build

---

• --bundle
• n/a
• Bun always bundles, use --no-bundle to disable this behavior.

---

• --define:K=V

• --define K=V

• Small syntax difference; no colon.

  $ esbuild --define:foo=bar
  $ bun build --define foo=bar
  

---

• --external:<pkg>

• --external <pkg>

• Small syntax difference; no colon.

  $ esbuild --external:react
  $ bun build --external react
  

---

• --format
• --format
• Bun supports "esm" and "cjs" currently, but more module formats are planned. esbuild defaults to "iife".

---

• --loader:.ext=loader

• --loader .ext:loader

• Bun supports a different set of built-in loaders than esbuild; see Bundler > Loaders for a complete reference. The esbuild loaders dataurl, binary, base64, copy, and empty are not yet implemented.

  The syntax for --loader is slightly different.

  $ esbuild app.ts --bundle --loader:.svg=text
  $ bun build app.ts --loader .svg:text
  

---

• --minify
• --minify
• No differences

---

• --outdir
• --outdir
• No differences

---

• --outfile
• --outfile

---

• --packages
• --packages
• No differences

---

• --platform
• --target
• Renamed to --target for consistency with tsconfig. Does not support neutral.

---

• --serve
• n/a
• Not applicable

---

• --sourcemap
• --sourcemap
• No differences

---

• --splitting
• --splitting
• No differences

---

• --target
• n/a
• Not supported. Bun's bundler performs no syntactic down-leveling at this time.

---

• --watch
• --watch
• No differences

---

• --allow-overwrite
• n/a
• Overwriting is never allowed

---

• --analyze
• n/a
• Not supported

---

• --asset-names
• --asset-naming
• Renamed for consistency with naming in JS API

---

• --banner
• --banner
• Only applies to js bundles

---

• --footer
• --footer
• Only applies to js bundles

---

• --certfile
• n/a
• Not applicable

---

• --charset=utf8
• n/a
• Not supported

---

• --chunk-names
• --chunk-naming
• Renamed for consistency with naming in JS API

---

• --color
• n/a
• Always enabled

---

• --drop
• --drop

---

• --entry-names
• --entry-naming
• Renamed for consistency with naming in JS API

---

• --global-name
• n/a
• Not applicable, Bun does not support iife output at this time

---

• --ignore-annotations
• --ignore-dce-annotations

---

• --inject
• n/a
• Not supported

---

• --jsx
• --jsx-runtime <runtime>
• Supports "automatic" (uses jsx transform) and "classic" (uses React.createElement)

---

• --jsx-dev
• n/a
• Bun reads compilerOptions.jsx from tsconfig.json to determine a default. If compilerOptions.jsx is "react-jsx", or if NODE_ENV=production, Bun will use the jsx transform. Otherwise, it uses jsxDEV. For any to Bun uses jsxDEV. The bundler does not support preserve.

---

• --jsx-factory
• --jsx-factory

---

• --jsx-fragment
• --jsx-fragment

---

• --jsx-import-source
• --jsx-import-source

---

• --jsx-side-effects
• n/a
• JSX is always assumed to be side-effect-free

---

• --keep-names
• n/a
• Not supported

---

• --keyfile
• n/a
• Not applicable

---

• --legal-comments
• n/a
• Not supported

---

• --log-level
• n/a
• Not supported. This can be set in bunfig.toml as logLevel.

---

• --log-limit
• n/a
• Not supported

---

• --log-override:X=Y
• n/a
• Not supported

---

• --main-fields
• n/a
• Not supported

---

• --mangle-cache
• n/a
• Not supported

---

• --mangle-props
• n/a
• Not supported

---

• --mangle-quoted
• n/a
• Not supported

---

• --metafile
• n/a
• Not supported

---

• --minify-whitespace
• --minify-whitespace

---

• --minify-identifiers
• --minify-identifiers

---

• --minify-syntax
• --minify-syntax

---

• --out-extension
• n/a
• Not supported

---

• --outbase
• --root

---

• --preserve-symlinks
• n/a
• Not supported

---

• --public-path
• --public-path

---

• --pure
• n/a
• Not supported

---

• --reserve-props
• n/a
• Not supported

---

• --resolve-extensions
• n/a
• Not supported

---

• --servedir
• n/a
• Not applicable

---

• --source-root
• n/a
• Not supported

---

• --sourcefile
• n/a
• Not supported. Bun does not support stdin input yet.

---

• --sourcemap
• --sourcemap
• No differences

---

• --sources-content
• n/a
• Not supported

---

• --supported
• n/a
• Not supported

---

• --tree-shaking
• n/a
• Always true

---

• --tsconfig
• --tsconfig-override

---

• --version
• n/a
• Run bun --version to see the version of Bun.

{% /table %}

JavaScript API

{% table %}

• esbuild.build()
• Bun.build()

---

• absWorkingDir
• n/a
• Always set to process.cwd()

---

• alias
• n/a
• Not supported

---

• allowOverwrite
• n/a
• Always false

---

• assetNames

• naming.asset

• Uses same templating syntax as esbuild, but [ext] must be included explicitly.

  Bun.build({
    entrypoints: ["./index.tsx"],
    naming: {
      asset: "[name].[ext]",
    },
  });
  

---

• banner
• n/a
• Not supported

---

• bundle
• n/a
• Always true. Use Bun.Transpiler to transpile without bundling.

---

• charset
• n/a
• Not supported

---

• chunkNames

• naming.chunk

• Uses same templating syntax as esbuild, but [ext] must be included explicitly.

  Bun.build({
    entrypoints: ["./index.tsx"],
    naming: {
      chunk: "[name].[ext]",
    },
  });
  

---

• color
• n/a
• Bun returns logs in the logs property of the build result.

---

• conditions
• n/a
• Not supported. Export conditions priority is determined by target.

---

• define
• define

---

• drop
• n/a
• Not supported

---

• entryNames

• naming or naming.entry

• Bun supports a naming key that can either be a string or an object. Uses same templating syntax as esbuild, but [ext] must be included explicitly.

  Bun.build({
    entrypoints: ["./index.tsx"],
    // when string, this is equivalent to entryNames
    naming: "[name].[ext]",
  
    // granular naming options
    naming: {
      entry: "[name].[ext]",
      asset: "[name].[ext]",
      chunk: "[name].[ext]",
    },
  });
  

---

• entryPoints
• entrypoints
• Capitalization difference

---

• external
• external
• No differences

---

• footer
• n/a
• Not supported

---

• format
• format
• Only supports "esm" currently. Support for "cjs" and "iife" is planned.

---

• globalName
• n/a
• Not supported

---

• ignoreAnnotations
• n/a
• Not supported

---

• inject
• n/a
• Not supported

---

• jsx
• jsx
• Not supported in JS API, configure in tsconfig.json

---

• jsxDev
• jsxDev
• Not supported in JS API, configure in tsconfig.json

---

• jsxFactory
• jsxFactory
• Not supported in JS API, configure in tsconfig.json

---

• jsxFragment
• jsxFragment
• Not supported in JS API, configure in tsconfig.json

---

• jsxImportSource
• jsxImportSource
• Not supported in JS API, configure in tsconfig.json

---

• jsxSideEffects
• jsxSideEffects
• Not supported in JS API, configure in tsconfig.json

---

• keepNames
• n/a
• Not supported

---

• legalComments
• n/a
• Not supported

---

• loader
• loader
• Bun supports a different set of built-in loaders than esbuild; see Bundler > Loaders for a complete reference. The esbuild loaders dataurl, binary, base64, copy, and empty are not yet implemented.

---

• logLevel
• n/a
• Not supported

---

• logLimit
• n/a
• Not supported

---

• logOverride
• n/a
• Not supported

---

• mainFields
• n/a
• Not supported

---

• mangleCache
• n/a
• Not supported

---

• mangleProps
• n/a
• Not supported

---

• mangleQuoted
• n/a
• Not supported

---

• metafile
• n/a
• Not supported

---

• minify

• minify

• In Bun, minify can be a boolean or an object.

  await Bun.build({
    entrypoints: ['./index.tsx'],
    // enable all minification
    minify: true
  
    // granular options
    minify: {
      identifiers: true,
      syntax: true,
      whitespace: true
    }
  })
  

---

• minifyIdentifiers
• minify.identifiers
• See minify

---

• minifySyntax
• minify.syntax
• See minify

---

• minifyWhitespace
• minify.whitespace
• See minify

---

• nodePaths
• n/a
• Not supported

---

• outExtension
• n/a
• Not supported

---

• outbase
• root
• Different name

---

• outdir
• outdir
• No differences

---

• outfile
• outfile
• No differences

---

• packages
• n/a
• Not supported, use external

---

• platform
• target
• Supports "bun", "node" and "browser" (the default). Does not support "neutral".

---

• plugins
• plugins
• Bun's plugin API is a subset of esbuild's. Some esbuild plugins will work out of the box with Bun.

---

• preserveSymlinks
• n/a
• Not supported

---

• publicPath
• publicPath
• No differences

---

• pure
• n/a
• Not supported

---

• reserveProps
• n/a
• Not supported

---

• resolveExtensions
• n/a
• Not supported

---

• sourceRoot
• n/a
• Not supported

---

• sourcemap
• sourcemap
• Supports "inline", "external", and "none"

---

• sourcesContent
• n/a
• Not supported

---

• splitting
• splitting
• No differences

---

• stdin
• n/a
• Not supported

---

• supported
• n/a
• Not supported

---

• target
• n/a
• No support for syntax downleveling

---

• treeShaking
• n/a
• Always true

---

• tsconfig
• n/a
• Not supported

---

• write
• n/a
• Set to true if outdir/outfile is set, otherwise false

---

{% /table %}

Plugin API

Bun's plugin API is designed to be esbuild compatible. Bun doesn't support esbuild's entire plugin API surface, but the core functionality is implemented. Many third-party esbuild plugins will work out of the box with Bun.

{% callout %} Long term, we aim for feature parity with esbuild's API, so if something doesn't work please file an issue to help us prioritize.

{% /callout %}

Plugins in Bun and esbuild are defined with a builder object.

import type { BunPlugin } from "bun";

const myPlugin: BunPlugin = {
  name: "my-plugin",
  setup(builder) {
    // define plugin
  },
};

The builder object provides some methods for hooking into parts of the bundling process. Bun implements onResolve and onLoad; it does not yet implement the esbuild hooks onStart, onEnd, and onDispose, and resolve utilities. initialOptions is partially implemented, being read-only and only having a subset of esbuild's options; use config (same thing but with Bun's BuildConfig format) instead.

import type { BunPlugin } from "bun";
const myPlugin: BunPlugin = {
  name: "my-plugin",
  setup(builder) {
    builder.onResolve(
      {
        /* onResolve.options */
      },
      args => {
        return {
          /* onResolve.results */
        };
      },
    );
    builder.onLoad(
      {
        /* onLoad.options */
      },
      args => {
        return {
          /* onLoad.results */
        };
      },
    );
  },
};

onResolve

options

{% table %}

• 🟢
• filter

---

• 🟢
• namespace

{% /table %}

arguments

{% table %}

• 🟢
• path

---

• 🟢
• importer

---

• 🔴
• namespace

---

• 🔴
• resolveDir

---

• 🔴
• kind

---

• 🔴
• pluginData

{% /table %}

results

{% table %}

• 🟢
• namespace

---

• 🟢
• path

---

• 🔴
• errors

---

• 🔴
• external

---

• 🔴
• pluginData

---

• 🔴
• pluginName

---

• 🔴
• sideEffects

---

• 🔴
• suffix

---

• 🔴
• warnings

---

• 🔴
• watchDirs

---

• 🔴
• watchFiles

{% /table %}

onLoad

options

{% table %}

---

• 🟢
• filter

---

• 🟢
• namespace

{% /table %}

arguments

{% table %}

---

• 🟢
• path

---

• 🔴
• namespace

---

• 🔴
• suffix

---

• 🔴
• pluginData

{% /table %}

results

{% table %}

---

• 🟢
• contents

---

• 🟢
• loader

---

• 🔴
• errors

---

• 🔴
• pluginData

---

• 🔴
• pluginName

---

• 🔴
• resolveDir

---

• 🔴
• warnings

---

• 🔴
• watchDirs

---

• 🔴
• watchFiles

{% /table %}