--- title: esbuild description: Migration guide from esbuild to Bun's bundler --- 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. Bundling 10 copies of three.js from scratch, with sourcemaps and minification ## CLI API Bun and esbuild both provide a command-line interface. ```bash terminal icon="terminal" # esbuild esbuild --outdir=out --bundle # bun bun build --outdir=out ``` In Bun's CLI, simple boolean flags like `--minify` do not accept an argument. Other flags like `--outdir ` 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`. | esbuild | bun build | Notes | | ---------------------- | -------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `--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:` | `--external ` | 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` | No differences | | `--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` | | | n/a | `--feature` | Bun-specific. Enable feature flags for compile-time dead-code elimination via `import { feature } from "bun:bundle"` | | `--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 ` | 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`. 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. | ## JavaScript API | esbuild.build() | Bun.build() | Notes | | ------------------- | -------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `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.

`ts
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.

`ts
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.

`ts
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.

`ts
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 | ## 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. 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. Plugins in Bun and esbuild are defined with a builder object. ```ts title="myPlugin.ts" icon="/icons/typescript.svg" 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 `onStart`, `onEnd`, `onResolve`, and `onLoad`. It does not yet implement the esbuild hooks `onDispose` and `resolve`. `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. ```ts title="myPlugin.ts" icon="/icons/typescript.svg" import type { BunPlugin } from "bun"; const myPlugin: BunPlugin = { name: "my-plugin", setup(builder) { builder.onStart(() => { /* called when the bundle starts */ }); builder.onResolve( { /* onResolve.options */ }, args => { return { /* onResolve.results */ }; }, ); builder.onLoad( { /* onLoad.options */ }, args => { return { /* onLoad.results */ }; }, ); builder.onEnd(result => { /* called when the bundle is complete */ }); }, }; ``` ### onResolve - 🟢 `filter` - 🟢 `namespace` - 🟢 `path` - 🟢 `importer` - 🔴 `namespace` - 🔴 `resolveDir` - 🔴 `kind` - 🔴 `pluginData` - 🟢 `namespace` - 🟢 `path` - 🔴 `errors` - 🔴 `external` - 🔴 `pluginData` - 🔴 `pluginName` - 🔴 `sideEffects` - 🔴 `suffix` - 🔴 `warnings` - 🔴 `watchDirs` - 🔴 `watchFiles` ### onLoad - 🟢 `filter` - 🟢 `namespace` - 🟢 `path` - 🔴 `namespace` - 🔴 `suffix` - 🔴 `pluginData` - 🟢 `contents` - 🟢 `loader` - 🔴 `errors` - 🔴 `pluginData` - 🔴 `pluginName` - 🔴 `resolveDir` - 🔴 `warnings` - 🔴 `watchDirs` - 🔴 `watchFiles`