---
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.

<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`.
</Note>

<Note>
  **Bundler only.** Unlike esbuild, Bun's bundler does not include a built-in development server or file watcher. 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.
</Note>

## 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.

<Info>Bundling 10 copies of three.js from scratch, with sourcemaps and minification</Info>

## CLI API

Bun and esbuild both provide a command-line interface.

```bash terminal icon="terminal"
# esbuild
esbuild <entrypoint> --outdir=out --bundle

# bun
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`.

| 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.<br/>`esbuild --define:foo=bar`<br/>`bun build --define foo=bar`                                                                                                                                                                                                                                                                  |
| `--external:<pkg>`     | `--external <pkg>`         | Small syntax difference; no colon.<br/>`esbuild --external:react`<br/>`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.<br/><br/>The syntax for `--loader` is slightly different.<br/>`esbuild app.ts --bundle --loader:.svg=text`<br/>`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 <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.<br/><br/>`ts<br/>Bun.build({<br/>  entrypoints: ["./index.tsx"],<br/>  naming: {<br/>    asset: "[name].[ext]",<br/>  },<br/>});<br/>`                                                                                                                                                                                                                                                                |
| `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.<br/><br/>`ts<br/>Bun.build({<br/>  entrypoints: ["./index.tsx"],<br/>  naming: {<br/>    chunk: "[name].[ext]",<br/>  },<br/>});<br/>`                                                                                                                                                                                                                                                                |
| `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.<br/><br/>`ts<br/>Bun.build({<br/>  entrypoints: ["./index.tsx"],<br/>  // when string, this is equivalent to entryNames<br/>  naming: "[name].[ext]",<br/><br/>  // granular naming options<br/>  naming: {<br/>    entry: "[name].[ext]",<br/>    asset: "[name].[ext]",<br/>    chunk: "[name].[ext]",<br/>  },<br/>});<br/>` |
| `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.<br/><br/>`ts<br/>await Bun.build({<br/>  entrypoints: ['./index.tsx'],<br/>  // enable all minification<br/>  minify: true<br/><br/>  // granular options<br/>  minify: {<br/>    identifiers: true,<br/>    syntax: true,<br/>    whitespace: true<br/>  }<br/>})<br/>`                                                                                                                                                               |
| `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.

<Note>
  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.
</Note>

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

<Tabs>
  <Tab title="options">

    - 🟢 `filter`
    - 🟢 `namespace`

  </Tab>
  <Tab title="arguments">

    - 🟢 `path`
    - 🟢 `importer`
    - 🔴 `namespace`
    - 🔴 `resolveDir`
    - 🔴 `kind`
    - 🔴 `pluginData`

  </Tab>
  <Tab title="results">

    - 🟢 `namespace`
    - 🟢 `path`
    - 🔴 `errors`
    - 🔴 `external`
    - 🔴 `pluginData`
    - 🔴 `pluginName`
    - 🔴 `sideEffects`
    - 🔴 `suffix`
    - 🔴 `warnings`
    - 🔴 `watchDirs`
    - 🔴 `watchFiles`

  </Tab>
</Tabs>

### onLoad

<Tabs>
  <Tab title="options">

    - 🟢 `filter`
    - 🟢 `namespace`

  </Tab>
  <Tab title="arguments">
  
    - 🟢 `path`
    - 🔴 `namespace`
    - 🔴 `suffix`
    - 🔴 `pluginData`

  </Tab>
  <Tab title="results">

    - 🟢 `contents`
    - 🟢 `loader`
    - 🔴 `errors`
    - 🔴 `pluginData`
    - 🔴 `pluginName`
    - 🔴 `resolveDir`
    - 🔴 `warnings`
    - 🔴 `watchDirs`
    - 🔴 `watchFiles`

  </Tab>
</Tabs>
