JS for Speed & Efficiency: 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.

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:


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
--compilation_level (-O)Specifies the compilation level to use. Options: BUNDLE, WHITESPACE_ONLY, SIMPLE (default), ADVANCED
--envDetermines the set of builtin externs to load. Options: BROWSER, CUSTOM. Defaults to BROWSER.
--externsThe file containing JavaScript externs. You may specify multiple
--jsThe 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_filePrimary output filename. If not specified, output is written to stdout.
--language_inSets 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_outSets 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.


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 Closure Library. It contains functions for enforcing dependencies between scripts, and Closure Compiler will re-order the inputs automatically.

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:

Java 11 or laterUsed to compile the compiler's source code.
GitUsed by Bazel to download dependencies.
BazeliskUsed 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.


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.


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


Code Pathsrc/com/google/javascript/rhino, test/com/google/javascript/rhino
Version1.5R3, with heavy modifications
LicenseNetscape Public License and MPL / GPL dual license
DescriptionA 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 ModificationsThe 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.


Descriptionargs4j is a small Java class library that makes it easy to parse command line options/arguments in your CUI application.
Local ModificationsNone

Guava Libraries

LicenseApache License 2.0
DescriptionGoogle's core Java libraries.
Local ModificationsNone

JSR 305

LicenseBSD License
DescriptionAnnotations for software defect detection.
Local ModificationsNone


LicenseCommon Public License 1.0
DescriptionA framework for writing and running automated tests in Java.
Local ModificationsNone

Protocol Buffers

LicenseNew BSD License
DescriptionSupporting libraries for protocol buffers, an encoding of structured data.
Local ModificationsNone


LicenseNew BSD License
DescriptionLinear time regular expression matching in Java.
Local ModificationsNone


LicenseApache License 2.0
DescriptionAssertion/Proposition framework for Java unit tests
Local ModificationsNone


LicenseApache License 2.0
DescriptionAnt 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 ModificationsNone


LicenseApache license 2.0
DescriptionA Java library to convert JSON to Java objects and vice-versa
Local ModificationsNone

Node.js Closure Compiler Externs

Code Pathcontrib/nodejs
LicenseApache 2.0 license
DescriptionType contracts for NodeJS APIs
Local ModificationsSubstantial changes to make them compatible with NpmCommandLineRunner.

Download Details:

Author: google

Official Github: https://github.com/google/closure-compiler 

License: Apache-2.0 license

#typescript #javascript 

JS for Speed & Efficiency: Closure Compiler
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