Google Closure Compiler

The Closure Compiler is a
tool for making JavaScript download and run faster. It is a true compiler for
JavaScript. Instead of compiling from a source language to machine code, it
compiles from JavaScript to better JavaScript. It parses your JavaScript,
analyzes it, removes dead code and rewrites and minimizes what’s left. It also
checks syntax, variable references, and types, and warns about common JavaScript
pitfalls.

Important Caveats

1. Compilation modes other than ADVANCED were always an afterthought and we
   have deprecated those modes. We believe that other tools perform comparably
   for non-ADVANCED modes and are better integrated into the broader JS
   ecosystem.

2. Closure Compiler is not suitable for arbitrary JavaScript. For ADVANCED
   mode to generate working JavaScript, the input JS code must be written with
   closure-compiler in mind.

3. Closure Compiler is a “whole world” optimizer. It expects to directly see or
   at least receive information about every possible use of every global or
   exported variable and every property name.

   It will aggressively remove and rename variables and properties in order to
   make the output code as small as possible. This will result in broken output
   JS, if uses of global variables or properties are hidden from it.

   Although one can write custom externs files to tell the compiler to leave
   some names unchanged so they can safely be accessed by code that is not part
   of the compilation, this is often tedious to maintain.

4. Closure Compiler property renaming requires you to consistently access a
   property with either obj[p] or obj.propName, but not both.

   When you access a property with square brackets (e.g. obj[p]) or using some
   other indirect method like let {p} = obj; this hides the literal name of
   the property being referenced from the compiler. It cannot know if
   obj.propName is referring to the same property as obj[p]. In some cases
   it will notice this problem and stop the compilation with an error. In other
   cases it will rename propName to something shorter, without noticing this
   problem, resulting in broken output JS code.

5. Closure Compiler aggressively inlines global variables and flattens chains
   of property names on global variables (e.g. myFoo.some.sub.property ->
   myFoo$some$sub$property), to make reasoning about them easier for detecting
   unused code.

   It tries to either back off from doing this or halt with an error when
   doing it will generate broken JS output, but there are cases where it will
   fail to recognize the problem and simply generate broken JS without warning.
   This is much more likely to happen in code that was not explicitly written
   with Closure Compiler in mind.

6. Closure compiler and the externs it uses by default assume that the target
   environment is a web browser window.

   WebWorkers are supported also, but the compiler will likely fail to warn
   you if you try to use features that aren’t actually available to a WebWorker.

   Some externs files and features have been added to Closure Compiler to
   support the NodeJS environment, but they are not actively supported and
   never worked very well.

7. JavaScript that does not use the goog.module() and goog.require() from
   base.js to declare and use modules is not well supported.

   The ECMAScript import and export syntax did not exist until 2015.
   Closure compiler and closure-library developed their own means for
   declaring and using modules, and this remains the only well supported
   way of defining modules.

   The compiler does implement some understanding of ECMAScript modules,
   but changing Google’s projects to use the newer syntax has never offered
   a benefit that was worth the cost of the change. Google’s TypeScript code
   uses ECMAScript modules, but they are converted to goog.module() syntax
   before closure-compiler sees them. So, effectively the ECMAScript modules
   support is unused within Google. This means we are unlikely to notice
   or fix bugs in the support for ECMAScript modules.

   Support for CommonJS modules as input was added in the past, but is not
   used within Google, and is likely to be entirely removed sometime in 2024.

Supported uses

Closure Compiler is used by Google projects to:

• Drastically reduce the code size of very large JavaScript applications

• Check the JS code for errors and for conformance to general and/or
  project-specific best practices.

• Define user-visible messages in a way that makes it possible to replace
  them with translated versions to create localized versions of an
  application.

• Transpile newer JS features into a form that will run on browsers that
  lack support for those features.

• Break the output application into chunks that may be individually loaded
  as needed.

  NOTE: These chunks are plain JavaScript scripts. They do not use the
  ECMAScript import and export syntax.

To achieve these goals closure compiler places many restrictions on its input:

• Use goog.module() and goog.require() to declare and use modules.

  Support for the import and export syntax added in ES6 is not actively
  maintained.

• Use annotations in comments to declare type information and provide
  information the compiler needs to avoid breaking some code patterns
  (e.g. @nocollapse and @noinline).

• Either use only dot-access (e.g. object.property) or only use dynamic
  access (e.g. object[propertyName] or Object.keys(object)) to access
  the properties of a particular object type.

  Mixing these will hide some uses of a property from the compiler, resulting
  in broken output code when it renames the property.

• In general the compiler expects to see an entire application as a single
  compilation. Interfaces must be carefully and explicitly constructed in
  order to allow interoperation with code outside of the compilation unit.

  The compiler assumes it can see all uses of all variables and properties
  and will freely rename them or remove them if they appear unused.

• Use externs files to inform the compiler of any variables or properties
  that it must not remove or rename.

  There are default externs files declaring the standard JS and DOM global
  APIs. More externs files are necessary if you are using less common
  APIs or expect some external JavaScript code to access an API in the
  code you are compiling.

Getting Started

The easiest way to install the compiler is with NPM or
Yarn:

yarn global add google-closure-compiler
# OR
npm i -g google-closure-compiler

The package manager will link the binary for you, and you can access the
compiler with:

google-closure-compiler

This starts the compiler in interactive mode. Type:

var x = 17 + 25;

Hit Enter, then Ctrl+Z (on Windows) or Ctrl+D (on Mac/Linux), then Enter
again. The Compiler will respond with the compiled output (using SIMPLE mode
by default):

var x=42;

Downloading from Maven Repository

A pre-compiled release of the compiler is also available via
Maven.

Basic usage

The Closure Compiler has many options for reading input from a file, writing
output to a file, checking your code, and running optimizations. Here is a
simple example of compressing a JS program:

google-closure-compiler --js file.js --js_output_file file.out.js

We get the most benefit from the compiler if we give it all of our source
code (see Compiling Multiple Scripts), which
allows us to use ADVANCED optimizations:

google-closure-compiler -O ADVANCED rollup.js --js_output_file rollup.min.js

NOTE: The output below is just an example and not kept up-to-date. The
Flags and Options wiki page
is updated during each release.

To see all of the compiler’s options, type:

google-closure-compiler --help

--flag	Description
--compilation_level (-O)	Specifies the compilation level to use. Options: BUNDLE, WHITESPACE_ONLY, SIMPLE (default), ADVANCED
--env	Determines the set of builtin externs to load. Options: BROWSER, CUSTOM. Defaults to BROWSER.
--externs	The file containing JavaScript externs. You may specify multiple
--js	The JavaScript filename. You may specify multiple. The flag name is optional, because args are interpreted as files by default. You may also use minimatch-style glob patterns. For example, use --js='**.js' --js='!**_test.js' to recursively include all js files that do not end in _test.js
--js_output_file	Primary output filename. If not specified, output is written to stdout.
--language_in	Sets the language spec to which input sources should conform. Options: ECMASCRIPT3, ECMASCRIPT5, ECMASCRIPT5_STRICT, ECMASCRIPT_2015, ECMASCRIPT_2016, ECMASCRIPT_2017, ECMASCRIPT_2018, ECMASCRIPT_2019, STABLE, ECMASCRIPT_NEXT
--language_out	Sets the language spec to which output should conform. Options: ECMASCRIPT3, ECMASCRIPT5, ECMASCRIPT5_STRICT, ECMASCRIPT_2015, ECMASCRIPT_2016, ECMASCRIPT_2017, ECMASCRIPT_2018, ECMASCRIPT_2019, STABLE
--warning_level (-W)	Specifies the warning level to use. Options: QUIET, DEFAULT, VERBOSE

See the Google Developers Site for documentation including instructions for running the compiler from the command line.

NodeJS API

You can access the compiler in a JS program by importing
google-closure-compiler:

import closureCompiler from 'google-closure-compiler';
const { compiler } = closureCompiler;

new compiler({
  js: 'file-one.js',
  compilation_level: 'ADVANCED'
});

This package will provide programmatic access to the native Graal binary in most
cases, and will fall back to the Java version otherwise.

Please see the closure-compiler-npm repository for documentation on accessing the compiler in JS.

Compiling Multiple Scripts

If you have multiple scripts, you should compile them all together with one
compile command.

google-closure-compiler in1.js in2.js in3.js --js_output_file out.js

You can also use minimatch-style globs.

# Recursively include all js files in subdirs
google-closure-compiler 'src/**.js' --js_output_file out.js

# Recursively include all js files in subdirs, excluding test files.
# Use single-quotes, so that bash doesn't try to expand the '!'
google-closure-compiler 'src/**.js' '!**_test.js' --js_output_file out.js

The Closure Compiler will concatenate the files in the order they’re passed at
the command line.

If you’re using globs or many files, you may start to run into problems with
managing dependencies between scripts. In this case, you should use the
included lib/base.js that provides functions for enforcing
dependencies between scripts (namely goog.module and goog.require). Closure
Compiler will re-order the inputs automatically.

Closure JavaScript Library

The Closure Compiler releases with lib/base.js that provides
JavaScript functions and variables that serve as primitives enabling certain
features of the Closure Compiler. This file is a derivative of the
identically named base.js
in the
soon-to-be deprecated
Closure Library. This base.js will be supported by Closure Compiler going
forward and may receive new features. It was designed to only retain its
perceived core parts.

Getting Help

1. Post in the
   Closure Compiler Discuss Group.
2. Ask a question on
   Stack Overflow.
3. Consult the FAQ.

Building the Compiler

To build the compiler yourself, you will need the following:

Prerequisite	Description
Java 11 or later	Used to compile the compiler’s source code.
Git	Used by Bazel to download dependencies.
Bazelisk	Used to build the various compiler targets.

Installing Bazelisk

Bazelisk is a wrapper around Bazel that dynamically loads the appropriate
version of Bazel for a given repository. Using it prevents spurious errors that
result from using the wrong version of Bazel to build the compiler, as well as
makes it easy to use different Bazel versions for other projects.

Bazelisk is available through many package managers. Feel free to use whichever
you’re most comfortable with.

Instructions for installing Bazelisk.

Building from a terminal

$ bazelisk build //:compiler_uberjar_deploy.jar
# OR to build everything
$ bazelisk build //:all

Testing from a terminal

Tests can be executed in a similar way. The following command will run all tests
in the repo.

$ bazelisk test //:all

There are hundreds of individual test targets, so it will take a few
minutes to run all of them. While developing, it’s usually better to specify
the exact tests you’re interested in.

bazelisk test //:$path_to_test_file

Building from an IDE

See Bazel IDE Integrations.

Running

Once the compiler has been built, the compiled JAR will be in the bazel-bin/
directory. You can access it with a call to java -jar ... or by using the
package.json script:

# java -jar bazel-bin/compiler_uberjar_deploy.jar [...args]
yarn compile [...args]

Running using Eclipse

1. Open the class src/com/google/javascript/jscomp/CommandLineRunner.java or
   create your own extended version of the class.
2. Run the class in Eclipse.
3. See the instructions above on how to use the interactive mode - but beware
   of the
   bug
   regarding passing “End of Transmission” in the Eclipse console.

Contributing

Contributor code of conduct

However you choose to contribute, please abide by our
code of conduct to
keep our community a healthy and welcoming place.

Reporting a bug

1. First make sure that it is really a bug and not simply the way that Closure
   Compiler works (especially true for ADVANCED_OPTIMIZATIONS).
   • Check the
     official documentation
   • Consult the FAQ
   • Search on
     Stack Overflow
     and in the
     Closure Compiler Discuss Group
   • Look through the list of
     compiler assumptions.
2. If you still think you have found a bug, make sure someone hasn’t already
   reported it. See the list of
   known issues.
3. If it hasn’t been reported yet, post a new issue. Make sure to add enough
   detail so that the bug can be recreated. The smaller the reproduction code,
   the better.

Suggesting a feature

1. Consult the FAQ to
   make sure that the behaviour you would like isn’t specifically excluded
   (such as string inlining).
2. Make sure someone hasn’t requested the same thing. See the list of
   known issues.
3. Read up on
   what type of feature requests are accepted.
4. Submit your request as an issue.

Submitting patches

1. All contributors must sign a contributor license agreement (CLA). A CLA
   basically says that you own the rights to any code you contribute, and that
   you give us permission to use that code in Closure Compiler. You maintain
   the copyright on that code. If you own all the rights to your code, you can
   fill out an
   individual CLA. If
   your employer has any rights to your code, then they also need to fill out a
   corporate CLA. If
   you don’t know if your employer has any rights to your code, you should ask
   before signing anything. By default, anyone with an @google.com email
   address already has a CLA signed for them.
2. To make sure your changes are of the type that will be accepted, ask about
   your patch on the
   Closure Compiler Discuss Group
3. Fork the repository.
4. Make your changes. Check out our
   coding conventions
   for details on making sure your code is in correct style.
5. Submit a pull request for your changes. A project developer will review your
   work and then merge your request into the project.

Closure Compiler License

Copyright 2009 The Closure Compiler Authors.

Licensed under the Apache License, Version 2.0 (the “License”); you may not use
this file except in compliance with the License. You may obtain a copy of the
License at http://www.apache.org/licenses/LICENSE-2.0.

Unless required by applicable law or agreed to in writing, software distributed
under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR
CONDITIONS OF ANY KIND, either express or implied. See the License for the
specific language governing permissions and limitations under the License.

Dependency Licenses

Rhino

Code Path	src/com/google/javascript/rhino, test/com/google/javascript/rhino
URL	https://developer.mozilla.org/en-US/docs/Mozilla/Projects/Rhino
Version	1.5R3, with heavy modifications
License	Netscape Public License and MPL / GPL dual license
Description	A partial copy of Mozilla Rhino. Mozilla Rhino is an implementation of JavaScript for the JVM. The JavaScript parse tree data structures were extracted and modified significantly for use by Google's JavaScript compiler.
Local Modifications	The packages have been renamespaced. All code not relevant to the parse tree has been removed. A JsDoc parser and static typing system have been added.

Args4j

URL	http://args4j.kohsuke.org/
Version	2.33
License	MIT
Description	args4j is a small Java class library that makes it easy to parse command line options/arguments in your CUI application.
Local Modifications	None

Guava Libraries

URL	https://github.com/google/guava
Version	31.0.1
License	Apache License 2.0
Description	Google's core Java libraries.
Local Modifications	None

JSR 305

URL	https://github.com/findbugsproject/findbugs
Version	3.0.1
License	BSD License
Description	Annotations for software defect detection.
Local Modifications	None

JUnit

URL	http://junit.org/junit4/
Version	4.13
License	Common Public License 1.0
Description	A framework for writing and running automated tests in Java.
Local Modifications	None

Protocol Buffers

URL	https://github.com/google/protobuf
Version	3.0.2
License	New BSD License
Description	Supporting libraries for protocol buffers, an encoding of structured data.
Local Modifications	None

RE2/J

URL	https://github.com/google/re2j
Version	1.3
License	New BSD License
Description	Linear time regular expression matching in Java.
Local Modifications	None

Truth

URL	https://github.com/google/truth
Version	1.1
License	Apache License 2.0
Description	Assertion/Proposition framework for Java unit tests
Local Modifications	None

Ant

URL	https://ant.apache.org/bindownload.cgi
Version	1.10.11
License	Apache License 2.0
Description	Ant is a Java based build tool. In theory it is kind of like "make" without make's wrinkles and with the full portability of pure java code.
Local Modifications	None

GSON

URL	https://github.com/google/gson
Version	2.9.1
License	Apache license 2.0
Description	A Java library to convert JSON to Java objects and vice-versa
Local Modifications	None

Node.js Closure Compiler Externs

Code Path	contrib/nodejs
URL	https://github.com/dcodeIO/node.js-closure-compiler-externs
Version	e891b4fbcf5f466cc4307b0fa842a7d8163a073a
License	Apache 2.0 license
Description	Type contracts for NodeJS APIs
Local Modifications	Substantial changes to make them compatible with NpmCommandLineRunner.