esbuild
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.
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.
# 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.esbuild --define:foo=barbun build --define foo=bar |
--external:<pkg> |
--external <pkg> |
Small syntax difference; no colon.esbuild --external:reactbun 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=textbun 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.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.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.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. |
…(truncated)
1---2name: bun-esbuild3description: Migration guide from esbuild to Bun's bundler4---56# esbuild78> Migration guide from esbuild to Bun's bundler910Bun'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.1112There are a few behavioral differences to note.1314<Note>15 **Bundling by default.** Unlike esbuild, Bun always bundles by default. This is why the `--bundle` flag isn't16 necessary in the Bun example. To transpile each file individually, use `Bun.Transpiler`.17</Note>1819<Note>20 **It's just a bundler.** Unlike esbuild, Bun's bundler does not include a built-in development server or file watcher.21 It's just a bundler. The bundler is intended for use in conjunction with `Bun.serve` and other runtime APIs to achieve22 the same effect. As such, all options relating to HTTP/file watching are not applicable.23</Note>2425## Performance2627With 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.2829<Info>Bundling 10 copies of three.js from scratch, with sourcemaps and minification</Info>3031## CLI API3233Bun and esbuild both provide a command-line interface.3435```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}}36# esbuild37esbuild <entrypoint> --outdir=out --bundle3839# bun40bun build <entrypoint> --outdir=out41```4243In 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`.4445| esbuild | bun build | Notes |46| ---------------------- | -------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |47| `--bundle` | n/a | Bun always bundles, use `--no-bundle` to disable this behavior. |48| `--define:K=V` | `--define K=V` | Small syntax difference; no colon.<br />`esbuild --define:foo=bar`<br />`bun build --define foo=bar` |49| `--external:<pkg>` | `--external <pkg>` | Small syntax difference; no colon.<br />`esbuild --external:react`<br />`bun build --external react` |50| `--format` | `--format` | Bun supports `"esm"` and `"cjs"` currently, but more module formats are planned. esbuild defaults to `"iife"`. |51| `--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` |52| `--minify` | `--minify` | No differences |53| `--outdir` | `--outdir` | No differences |54| `--outfile` | `--outfile` | No differences |55| `--packages` | `--packages` | No differences |56| `--platform` | `--target` | Renamed to `--target` for consistency with tsconfig. Does not support `neutral`. |57| `--serve` | n/a | Not applicable |58| `--sourcemap` | `--sourcemap` | No differences |59| `--splitting` | `--splitting` | No differences |60| `--target` | n/a | Not supported. Bun's bundler performs no syntactic down-leveling at this time. |61| `--watch` | `--watch` | No differences |62| `--allow-overwrite` | n/a | Overwriting is never allowed |63| `--analyze` | n/a | Not supported |64| `--asset-names` | `--asset-naming` | Renamed for consistency with naming in JS API |65| `--banner` | `--banner` | Only applies to js bundles |66| `--footer` | `--footer` | Only applies to js bundles |67| `--certfile` | n/a | Not applicable |68| `--charset=utf8` | n/a | Not supported |69| `--chunk-names` | `--chunk-naming` | Renamed for consistency with naming in JS API |70| `--color` | n/a | Always enabled |71| `--drop` | `--drop` | |72| n/a | `--feature` | Bun-specific. Enable feature flags for compile-time dead-code elimination via `import { feature } from "bun:bundle"` |73| `--entry-names` | `--entry-naming` | Renamed for consistency with naming in JS API |74| `--global-name` | n/a | Not applicable, Bun does not support `iife` output at this time |75| `--ignore-annotations` | `--ignore-dce-annotations` | |76| `--inject` | n/a | Not supported |77| `--jsx` | `--jsx-runtime <runtime>` | Supports `"automatic"` (uses jsx transform) and `"classic"` (uses `React.createElement`) |78| `--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`. |79| `--jsx-factory` | `--jsx-factory` | |80| `--jsx-fragment` | `--jsx-fragment` | |81| `--jsx-import-source` | `--jsx-import-source` | |82| `--jsx-side-effects` | n/a | JSX is always assumed to be side-effect-free |83| `--keep-names` | n/a | Not supported |84| `--keyfile` | n/a | Not applicable |85| `--legal-comments` | n/a | Not supported |86| `--log-level` | n/a | Not supported. This can be set in `bunfig.toml` as `logLevel`. |87| `--log-limit` | n/a | Not supported |88| `--log-override:X=Y` | n/a | Not supported |89| `--main-fields` | n/a | Not supported |90| `--mangle-cache` | n/a | Not supported |91| `--mangle-props` | n/a | Not supported |92| `--mangle-quoted` | n/a | Not supported |93| `--metafile` | n/a | Not supported |94| `--minify-whitespace` | `--minify-whitespace` | |95| `--minify-identifiers` | `--minify-identifiers` | |96| `--minify-syntax` | `--minify-syntax` | |97| `--out-extension` | n/a | Not supported |98| `--outbase` | `--root` | |99| `--preserve-symlinks` | n/a | Not supported |100| `--public-path` | `--public-path` | |101| `--pure` | n/a | Not supported |102| `--reserve-props` | n/a | Not supported |103| `--resolve-extensions` | n/a | Not supported |104| `--servedir` | n/a | Not applicable |105| `--source-root` | n/a | Not supported |106| `--sourcefile` | n/a | Not supported. Bun does not support stdin input yet. |107| `--sourcemap` | `--sourcemap` | No differences |108| `--sources-content` | n/a | Not supported |109| `--supported` | n/a | Not supported |110| `--tree-shaking` | n/a | Always true |111| `--tsconfig` | `--tsconfig-override` | |112| `--version` | n/a | Run `bun --version` to see the version of Bun. |113114## JavaScript API115116| esbuild.build() | Bun.build() | Notes |117| ------------------- | -------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |118| `absWorkingDir` | n/a | Always set to `process.cwd()` |119| `alias` | n/a | Not supported |120| `allowOverwrite` | n/a | Always false |121| `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/>` |122| `banner` | n/a | Not supported |123| `bundle` | n/a | Always true. Use `Bun.Transpiler` to transpile without bundling. |124| `charset` | n/a | Not supported |125| `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/>` |126| `color` | n/a | Bun returns logs in the `logs` property of the build result. |127| `conditions` | n/a | Not supported. Export conditions priority is determined by `target`. |128| `define` | `define` | |129| `drop` | n/a | Not supported |130| `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/>` |131| `entryPoints` | `entrypoints` | Capitalization difference |132| `external` | `external` | No differences |133| `footer` | n/a | Not supported |134| `format` | `format` | Only supports `"esm"` currently. Support for `"cjs"` and `"iife"` is planned. 135136…(truncated)
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