esbuild

README

esbuild

This is a JavaScript bundler and minifier. It packages up JavaScript and TypeScript code for distribution on the web.

Why?

Why build another JavaScript build tool? The current build tools for the web are at least an order of magnitude slower than they should be. I'm hoping that this project serves as an "existence proof" that our JavaScript tooling can be much, much faster.

Benchmarks

The use case I have in mind is packaging a large codebase for production. This includes minifying the code, which reduces network transfer time, and producing source maps, which are important for debugging errors in production. Ideally the build tool should also build quickly without having to warm up a cache first.

I currently have two benchmarks that I'm using to measure esbuild performance. For these benchmarks, esbuild is at least 100x faster than the other JavaScript bundlers I tested:

Here are the details about each benchmark:

• JavaScript benchmark

  This benchmark approximates a large JavaScript codebase by duplicating the three.js library 10 times and building a single bundle from scratch, without any caches. The benchmark can be run with make bench-three.

  Bundler	Time	Relative slowdown	Absolute speed	Output size
  esbuild	0.36s	1x	1520.7 kloc/s	5.82mb
  esbuild (1 thread)	1.25s	4x	438.0 kloc/s	5.82mb
  rollup + terser	36.06s	100x	15.2 kloc/s	5.80mb
  webpack	44.74s	124x	12.2 kloc/s	5.97mb
  fuse-box@next	59.52s	165x	9.2 kloc/s	6.34mb
  parcel	121.18s	337x	4.5 kloc/s	5.89mb

  Each time reported is the best of three runs. I'm running esbuild with --bundle --minify --sourcemap (the single-threaded version uses GOMAXPROCS=1). I used the rollup-plugin-terser plugin because Rollup itself doesn't support minification. Webpack uses --mode=production --devtool=sourcemap. Parcel uses the default options. FuseBox is configured with useSingleBundle: true. Absolute speed is based on the total line count including comments and blank lines, which is currently 547,441. The tests were done on a 6-core 2019 MacBook Pro with 16gb of RAM.

  Caveats:

  • Parcel: The bundle crashes at run time with TypeError: Cannot redefine property: dynamic
  • FuseBox: The line numbers in source maps appear to be off by one

• TypeScript benchmark

  This benchmark uses the Rome build tool to approximate a large TypeScript codebase. All code must be combined into a single minified bundle with source maps and the resulting bundle must work correctly. The benchmark can be run with make bench-rome.

  Bundler	Time	Relative slowdown	Absolute speed	Output size
  esbuild	0.10s	1x	1287.5 kloc/s	0.98mb
  esbuild (1 thread)	0.32s	2x	412.0 kloc/s	0.98mb
  parcel	16.77s	168x	7.9 kloc/s	1.55mb
  webpack	18.67s	187x	7.1 kloc/s	1.26mb

  Each time reported is the best of three runs. I'm running esbuild with --bundle --minify --sourcemap --platform=node (the single-threaded version uses GOMAXPROCS=1). Webpack uses ts-loader with transpileOnly: true and --mode=production --devtool=sourcemap. Parcel uses --target node --bundle-node-modules. Absolute speed is based on the total line count including comments and blank lines, which is currently 131,836. The tests were done on a 6-core 2019 MacBook Pro with 16gb of RAM.

  The results don't include Rollup because I couldn't get it to work. I tried rollup-plugin-typescript, @rollup/plugin-typescript, and @rollup/plugin-sucrase and they all didn't work for different reasons relating to TypeScript compilation. And I'm not familiar with FuseBox so I'm not sure how work around build failures due to the use of builtin node modules.

Why is it fast?

Several reasons:

• It's written in Go, a language that compiles to native code
• Parsing, printing, and source map generation are all fully parallelized
• Everything is done in very few passes without expensive data transformations
• Code is written with speed in mind, and tries to avoid unnecessary allocations

Documentation

• Architecture
• 中文文档

Status

Currently supported:

• JavaScript and TypeScript syntax
• CommonJS and ES6 modules
• JSX-to-JavaScript conversion
• Bundling using --bundle with scope hoisting and tree shaking of ES6 modules
• Full minification with --minify (whitespace, identifiers, and mangling)
• Full source map support when --sourcemap is enabled
• Compile-time identifier substitutions via --define
• Path substitution using the browser field in package.json
• Automatic detection of baseUrl in tsconfig.json

JavaScript syntax support:

Syntax transforms convert newer JavaScript syntax to older JavaScript syntax for use with older browsers. You can set the language target with the --target flag, which goes back as far as ES6. Note that if you use a syntax feature that esbuild doesn't yet have support for transforming to your current language target, esbuild will still build successfully but will generate a warning where the unsupported syntax is used and will pass the syntax through un-transformed.

These syntax features are always transformed for older browsers:

Syntax transform	Language version	Example
Trailing commas in function parameter lists and calls	es2017	foo(a, b, )
Numeric separators	esnext	1_000_000

These syntax features are conditionally transformed for older browsers depending on the configured language target:

Syntax transform	Transformed when --target is below	Example
Exponentiation operator	es2016	a ** b
Spread properties	es2018	let x = {...y}
Optional catch binding	es2019	try {} catch {}
Optional chaining	es2020	a?.b
Nullish coalescing	es2020	a ?? b
Class instance fields	esnext	class { x }
Static class fields	esnext	class { static x }

These syntax features are currently always passed through un-transformed:

Syntax transform	Unsupported when --target is below	Example
Async functions	es2017	async () => {}
Rest properties	es2018	let {...x} = y
Asynchronous iteration	es2018	for await (let x of y) {}
Async generators	es2018	async function* foo() {}
BigInt	es2020	123n
Hashbang grammar	esnext	#!/usr/bin/env node

These syntax features are not yet supported, and currently cannot be parsed:

Syntax transform	Language version	Example
Private instance methods	esnext	class { #x() {} }
Private instance fields	esnext	class { #x }
Private static fields and methods	esnext	class { static #x }
Logical assignment operators	esnext	a ??= b
Top-level await	esnext	await import(x)

See also the list of finished ECMAScript proposals and the list of active ECMAScript proposals.

TypeScript syntax support:

TypeScript files are transformed by removing type annotations and converting TypeScript-only syntax features to JavaScript code. This section documents the support of TypeScript-only syntax features. Please refer to the previous section for support of JavaScript syntax features, which also applies in TypeScript files.

Note that esbuild does not do any type checking. You will still want to run type checking using something like tsc -noEmit.

These TypeScript-only syntax features are supported, and are always converted to JavaScript (a non-exhaustive list):

Syntax feature	Example	Notes
Namespaces	namespace Foo {}
Enums	enum Foo { A, B }
Const enums	const enum Foo { A, B }	Behaves the same as regular enums
Generic type parameters	<T>(a: T): T => a
JSX with types	<Element<T>/>
Type casts	a as B and <B>a
Type imports	import {Type} from 'foo'	Handled by removing all unused imports
Type exports	export {Type} from 'foo'	Handled by ignoring missing exports in TypeScript files

These TypeScript-only syntax features are parsed and ignored (a non-exhaustive list):

Syntax feature	Example
Interface declarations	interface Foo {}
Type declarations	type Foo = number
Function declarations	function foo(): void;
Ambient declarations	declare module 'foo' {}
Type-only imports	import type {Type} from 'foo'
Type-only exports	export type {Type} from 'foo'

These TypeScript-only syntax features are not yet supported, and currently cannot be parsed (an exhaustive list):

Syntax feature	Example	Notes
Experimental decorators	@sealed class Foo {}	This is tracked here

Disclaimers:

• As far as I know, this hasn't yet been used in production by anyone yet. It's still pretty new code and you may run into some bugs.

  That said, a lot of effort has gone into handling all of the various edge cases in the JavaScript and TypeScript language specifications and esbuild works well on many real-world code bases. For example, esbuild can bundle rollup, sucrase, and esprima (all TypeScript code bases) and the generated bundles pass all tests. If you find an issue with language support (especially with real-world code), please report it so it can get fixed.

• This project is still pretty early and I'd like to keep the scope relatively focused, at least for now. I'm trying to create a build tool that a) works well for a given sweet spot of use cases and b) resets the expectations of the community for what it means for a JavaScript build tool to be fast. I'm not trying to create an extremely flexible build system that can build anything.

  That said, esbuild now has a JavaScript API that exposes some of its transform code. This means it can be used as a library to minify JavaScript, convert TypeScript/JSX to JavaScript, or convert newer JavaScript to older JavaScript. So even if esbuild doesn't support a particular technology, it's possible that esbuild can still be integrated as a library to help speed it up. For example, Vite recently started using esbuild's transform library to add support for TypeScript (the official TypeScript compiler was too slow).

• I'm mainly looking for feedback at the moment, not contributions. The project is early and I'm still working toward an MVP bundler that can reasonably replace real-world toolchains. There are still major fundamental pieces that haven't been put in place yet (e.g. CSS support, watch mode, code splitting) and they all need to work well together to have best-in-class performance.

• The Go code in this repo isn't intended to be built upon. Go is just an implementation detail of how I built this tool. The stable interfaces for this project are the command-line API and the JavaScript API, not the internal Go code. I'm may change the internals in a backwards-incompatible way at any time to improve performance or introduce new features.

There is now some documentation about the architecture and about certain subtleties in the code here: docs/architecture.md. I hope it will be helpful for those interested in learning more about how the code works.

Install

A prebuilt binary can be installed using npm:

• Local install (recommended)

  This installs the esbuild command locally in your project's package.json file:

  npm install --save-dev esbuild
  

  Invoke it using npx esbuild [arguments]. Note that this uses the npx package runner command, not the npm package manager command.

  This is the recommended project-based workflow because it allows you to have a different version of esbuild for each project and it ensures that everyone working on a given project has the same version of esbuild.

• Global install

  This adds a global command called esbuild to your path:

  npm install --global esbuild
  

  Invoke it using esbuild [arguments].

  A global install can be handy if you want to run esbuild outside of a project context for one-off file manipulation tasks.

The esbuild package should work on 64-bit macOS, Linux, and Windows systems. It contains an install script that downloads the appropriate package for the current platform. If the install script isn't working or you need to run esbuild on an unsupported platform, there is a fallback WebAssembly package called esbuild-wasm that should work on all platforms.

For development, the executable can be built by running make (assuming you have the Go language toolchain installed).

Command-line usage

The command-line interface takes a list of entry points and produces one bundle file per entry point. Here are the available options:

Usage:
  esbuild [options] [entry points]

Options:
  --name=...            The name of the module
  --bundle              Bundle all dependencies into the output files
  --outfile=...         The output file (for one entry point)
  --outdir=...          The output directory (for multiple entry points)
  --sourcemap           Emit a source map
  --target=...          Language target (default esnext)
  --platform=...        Platform target (browser or node, default browser)
  --external:M          Exclude module M from the bundle
  --format=...          Output format (iife, cjs, esm)
  --color=...           Force use of color terminal escapes (true or false)

  --minify              Sets all --minify-* flags
  --minify-whitespace   Remove whitespace
  --minify-identifiers  Shorten identifiers
  --minify-syntax       Use equivalent but shorter syntax

  --define:K=V          Substitute K with V while parsing
  --jsx-factory=...     What to use instead of React.createElement
  --jsx-fragment=...    What to use instead of React.Fragment
  --loader:X=L          Use loader L to load file extension X, where L is
                        one of: js, jsx, ts, tsx, json, text, base64, dataurl

Advanced options:
  --version             Print the current version and exit
  --sourcemap=inline    Emit the source map with an inline data URL
  --sourcemap=external  Do not link to the source map with a comment
  --sourcefile=...      Set the source file for the source map (for stdin)
  --error-limit=...     Maximum error count or 0 to disable (default 10)
  --log-level=...       Disable logging (info, warning, error)

  --trace=...           Write a CPU trace to this file
  --cpuprofile=...      Write a CPU profile to this file

Examples:
  # Produces dist/entry_point.js and dist/entry_point.js.map
  esbuild --bundle entry_point.js --outdir=dist --minify --sourcemap

  # Allow JSX syntax in .js files
  esbuild --bundle entry_point.js --outfile=out.js --loader:.js=jsx

  # Substitute the identifier RELEASE for the literal true
  esbuild example.js --outfile=out.js --define:RELEASE=true

  # Provide input via stdin, get output via stdout
  esbuild --minify --loader=ts < input.ts > output.js

JavaScript API usage

The esbuild npm package also exposes a JavaScript API that can be used to invoke the command-line tool from JavaScript.

Running a build

The build() API is the same as invoking the command-line tool. It reads from files on disk and writes back to files on disk. Using this API can be more convenient than managing a lot of command-line flags and also works on all platforms, unlike shell scripts. This is similar to "config files" from other bundlers.

Example build script:

const { build } = require('esbuild')

build({
  stdio: 'inherit',
  entryPoints: ['./src/main.ts'],
  outfile: './dist/main.js',
  minify: true,
  bundle: true,
}).catch(() => process.exit(1))

See the TypeScript type definitions for the complete set of options.

Transforming a file

The transform() API transforms a single file in memory. It can be used to minify JavaScript, convert TypeScript/JSX to JavaScript, or convert newer JavaScript to older JavaScript. It's roughly equivalent to running build() on a single file with bundle: false.

To access this API you need to start a service, which is a long-lived esbuild child process that is then reused. You can use the service to transform many files without the overhead of starting up a new child process each time.

Example usage:

(async () => {
  const jsx = `
    import * as React from 'react'
    import * as ReactDOM from 'react-dom'

    ReactDOM.render(
      <h1>Hello, world!</h1>,
      document.getElementById('root')
    );
  `

  // Start the esbuild child process once
  const esbuild = require('esbuild')
  const service = await esbuild.startService()

  // This can be called many times without the overhead of starting a service
  const { js } = await service.transform(jsx, { loader: 'jsx' })
  console.log(js)

  // The child process can be explicitly killed when it's no longer needed
  service.stop()
})()

See the TypeScript type definitions for the complete set of options.

Using with React

To use esbuild with React:

• Either put all JSX syntax in .jsx files instead of .js files, or use --loader:.js=jsx to use the JSX loader for .js files.

• If you're using TypeScript, pass esbuild your .tsx file as the entry point. There should be no need to convert TypeScript files to JavaScript first because because esbuild parses TypeScript syntax itself.

  Note that esbuild does not do any type checking, so you'll want to run tsc -noEmit in parallel to check types.

• If you're using esbuild to bundle React yourself instead of including it with a <script> tag in your HTML, you'll need to pass '--define:process.env.NODE_ENV="development"' or '--define:process.env.NODE_ENV="production"' to esbuild on the command line.

  Note that the double quotes around "production" are important because the replacement should be a string, not an identifier. The outer single quotes are for escaping the double quotes in Bash but may not be necessary in other shells.

• If you're using Preact instead of React, you'll also need to pass --jsx-factory=preact.h --jsx-fragment=preact.Fragment to esbuild on the command line.

For example, if you have a file called example.tsx with the following contents:

import * as React from 'react'
import * as ReactDOM from 'react-dom'

ReactDOM.render(
  <h1>Hello, world!</h1>,
  document.getElementById('root')
);

Use this for a development build:

esbuild example.tsx --bundle '--define:process.env.NODE_ENV="development"' --outfile=out.js

Use this for a production build:

esbuild example.tsx --bundle '--define:process.env.NODE_ENV="production"' --minify --outfile=out.js