esbuild

README

esbuild

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

Documentation

• JavaScript API documentation
• Go API documentation
• Architecture documentation
• 中文文档

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	3x	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

Status

Similar tools:

• Webpack, Rollup, or Parcel for bundling
• Babel or TypeScript for transpiling
• Terser or UglifyJS for minifying

Used by:

• Vite
• Snowpack
• HUGO

Currently supported:

Here are some of the main features that esbuild offers (a non-exhaustive list):

• Loaders:
  • JavaScript/JSX (more details)
  • TypeScript/TSX (more details)
  • JSON
  • Data (text, base64, binary, data URL, file)
• Minification
  • Remove whitespace
  • Shorten identifiers
  • Compress syntax
• Bundling
  • Scope hoisting for ES6 modules
  • Respects browser overrides in package.json
• Tree shaking
  • Respects sideEffects in package.json
  • Respects /* @__PURE__ */ annotations
• Output formats
  • CommonJS
  • IIFE
  • ES6 module (supports code splitting)
• Source map generation
• Transpilation of JSX and newer JS syntax down to ES6
• Compile-time name substitution with --define
• Bundle metadata in JSON format for analysis

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. You can also set --target to a comma-separated list of browsers and versions (e.g. --target=chrome58,firefox57,safari11,edge16). Note that if you use a syntax feature that esbuild doesn't yet have support for transforming to your current language target, esbuild will generate an error where the unsupported syntax is used.

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
Async functions	es2017	async () => {}
Spread properties	es2018	let x = {...y}
Rest properties	es2018	let {...x} = y
Optional catch binding	es2019	try {} catch {}
Optional chaining	es2020	a?.b
Nullish coalescing	es2020	a ?? b
import.meta	es2020	import.meta
Class instance fields	esnext	class { x }
Static class fields	esnext	class { static x }
Private instance methods	esnext	class { #x() {} }
Private instance fields	esnext	class { #x }
Private static methods	esnext	class { static #x() {} }
Private static fields	esnext	class { static #x }
Logical assignment operators	esnext	a ??= b

Syntax transform caveats (click to expand)

• Nullish coalescing correctness

  By default a ?? b is transformed into a != null ? a : b, which works because a != null is only false if a is null or undefined. However, there's exactly one obscure edge case where this doesn't work. For legacy reasons, the value of document.all is special-cased such that document.all != null is false. If you need to use this value with the nullish coalescing operator, you should enable --strict:nullish-coalescing transforms so a ?? b becomes a !== null && a !== void 0 ? a : b instead, which works correctly with document.all. The strict transform isn't done by default because it causes code bloat for an obscure edge case that shouldn't matter in modern code.

• Optional chaining correctness

  By default a?.b is transformed into a == null ? void 0 : a.b, which works because a == null is only true if a is neither null nor undefined. However, there's exactly one obscure edge case where this doesn't work. For legacy reasons, the value of document.all is special-cased such that document.all == null is true. If you need to use this value with optional chaining, you should enable --strict:optional-chaining transforms so a?.b becomes a === null || a === void 0 ? void 0 : a.b instead, which works correctly with document.all. The strict transform isn't done by default because it causes code bloat for an obscure edge case that shouldn't matter in modern code.

• Class field correctness

  Class fields look like this:

  class Foo {
    foo = 123
  }
  

  By default, the transform for class fields uses a normal assignment for initialization. That looks like this:

  class Foo {
    constructor() {
      this.foo = 123;
    }
  }
  

  This doesn't increase code size by much and stays on the heavily-optimized path for modern JavaScript JITs. It also matches the default behavior of the TypeScript compiler. However, this doesn't exactly follow the initialization behavior in the JavaScript specification. For example, it can cause a setter to be called if one exists with that property name, which isn't supposed to happen. A more accurate transformation would be to use Object.defineProperty() instead like this:

  class Foo {
    constructor() {
      Object.defineProperty(this, "foo", {
        enumerable: true,
        configurable: true,
        writable: true,
        value: 123
      });
    }
  }
  

  This increases code size and decreases performance, but follows the JavaScript specification more accurately. If you need this accuracy, you should either enable the --strict:class-fields option or add the useDefineForClassFields flag to your tsconfig.json file.

• Private member performance

  This transform uses WeakMap and WeakSet to preserve the privacy properties of this feature, similar to the corresponding transforms in the Babel and TypeScript compilers. Most modern JavaScript engines (V8, JavaScriptCore, and SpiderMonkey but not ChakraCore) may not have good performance characteristics for large WeakMap and WeakSet objects. Creating many instances of classes with private fields or private methods with this syntax transform active may cause a lot of overhead for the garbage collector. This is because modern engines (other than ChakraCore) store weak values in an actual map object instead of as hidden properties on the keys themselves, and large map objects can cause performance issues with garbage collection. See this reference for more information.

Note that if you want to enable strictness for all transforms, you can just pass --strict instead of using --strict:... for each transform.

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

Syntax transform	Unsupported when --target is below	Example
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
Top-level await	esnext	await import(x)

Note that while transforming code containing top-level await is supported, bundling code containing top-level await is not yet supported.

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

Transforming ES6+ syntax to ES5 is not supported yet. However, if you're using esbuild to transform ES5 code, you should still set --target=es5. This prevents esbuild from introducing ES6 syntax into your ES5 code. For example, without this flag the object literal {x: x} will become {x} and the string "a\nb" will become a multi-line template literal when minifying. Both of these substitutions are done because the resulting code is shorter, but the substitutions will not be performed if --target=es5 is present.

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
Experimental decorators	@sealed class Foo {}	The emitDecoratorMetadata flag is not supported

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'

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 and a Go API. This 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 and Snowpack 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.

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.

Other installation methods (click to expand)

The binary is only hosted on npm for convenience. Since esbuild is a native binary, it's not necessary to install npm to use esbuild.

Download using HTTP

The binary can be downloaded from the npm registry directly over HTTP without needing to install npm first.

• For Linux (the binary will be ./package/bin/esbuild):

  curl -O https://registry.npmjs.org/esbuild-linux-64/-/esbuild-linux-64-0.0.0.tgz
  tar xf ./esbuild-linux-64-0.0.0.tgz
  

• For macOS (the binary will be ./package/bin/esbuild):

  curl -O https://registry.npmjs.org/esbuild-darwin-64/-/esbuild-darwin-64-0.0.0.tgz
  tar xf ./esbuild-darwin-64-0.0.0.tgz
  

• For Windows (the binary will be ./package/esbuild.exe):

  curl -O https://registry.npmjs.org/esbuild-windows-64/-/esbuild-windows-64-0.0.0.tgz
  tar xf ./esbuild-windows-64-0.0.0.tgz
  

Substitute 0.0.0 for the version of esbuild that you want to download.

Install using Go

If you have the Go compiler toolchain installed, you can use it to install the esbuild command globally:

GO111MODULE=on go get github.com/evanw/esbuild/cmd/esbuild@v0.0.0

The binary will be placed in Go's global binary directory (the directory called bin located inside the directory returned by go env GOPATH). You may need to add that bin directory to your PATH. Substitute v0.0.0 for the version of esbuild that you want to build.

Build from source

You can also build esbuild from source:

git clone --depth 1 --branch v0.0.0 https://github.com/evanw/esbuild.git
cd esbuild
go build ./cmd/esbuild

This will create a binary called esbuild (esbuild.exe on Windows) in the current directory. Substitute v0.0.0 for the version of esbuild that you want to build.

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:
  --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=...          Environment target (e.g. es2017, chrome58, firefox57,
                        safari11, edge16, node10, default esnext)
  --platform=...        Platform target (browser | node, default browser)
  --external:M          Exclude module M from the bundle
  --format=...          Output format (iife | cjs | esm, no default when not
                        bundling, otherwise default is iife when platform
                        is browser and cjs when platform is node)
  --splitting           Enable code splitting (currently only for esm)
  --global-name=...     The name of the global for the IIFE format

  --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 |
                        file | dataurl | binary

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 | silent,
                            default info)
  --resolve-extensions=...  A comma-separated list of implicit extensions
                            (default ".tsx,.ts,.jsx,.mjs,.cjs,.js,.json")
  --metafile=...            Write metadata about the build to a JSON file
  --strict                  Transforms handle edge cases but have more overhead
                            (enable individually using --strict:X where X is
                            one of: nullish-coalescing | optional-chaining |
                            class-fields)
  --pure:N                  Mark the name N as a pure function for tree shaking
  --tsconfig=...            Use this tsconfig.json file instead of other ones
  --out-extension:.js=.mjs  Use a custom output extension instead of ".js"
  --main-fields=...         Override the main file order in package.json
                            (default "browser,module,main" when platform is
                            browser and "main,module" when platform is node)
  --color=...               Force use of color terminal escapes (true | false)

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

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 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 need to configure the JSX factory. You can either pass --jsx-factory=h --jsx-fragment=Fragment to esbuild on the command line, or add "jsxFactory": "h", "jsxFragmentFactory": "Fragment" to your tsconfig.json file. You will also have to add import {h, Fragment} from 'preact' in files containing JSX syntax.

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