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TheClosure Compiler is atool for making JavaScript download and run faster. It is a true compiler forJavaScript. Instead of compiling from a source language to machine code, itcompiles from JavaScript to better JavaScript. It parses your JavaScript,analyzes it, removes dead code and rewrites and minimizes what's left. It alsochecks syntax, variable references, and types, and warns about common JavaScriptpitfalls.

1. Compilation modes other thanADVANCED were always an afterthought and wehave deprecated those modes. We believe that other tools perform comparablyfor non-ADVANCED modes and are better integrated into the broader JSecosystem.

2. Closure Compiler is not suitable for arbitrary JavaScript. ForADVANCEDmode to generate working JavaScript, the input JS code must be written withclosure-compiler in mind.

3. Closure Compiler is a "whole world" optimizer. It expects to directly see orat least receive information about every possible use of every global orexported variable and every property name.

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

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

4. Closure Compiler property renaming requires you to consistently access aproperty with eitherobj[p] orobj.propName, but not both.

   When you access a property with square brackets (e.g.obj[p]) or using someother indirect method likelet {p} = obj; this hides the literal name ofthe property being referenced from the compiler. It cannot know ifobj.propName is referring to the same property asobj[p]. In some casesit will notice this problem and stop the compilation with an error. In othercases it will renamepropName to something shorter, without noticing thisproblem, resulting in broken output JS code.

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

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

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

   WebWorkers are supported also, but the compiler will likely fail to warnyou 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 tosupport the NodeJS environment, but they are not actively supported andnever worked very well.

7. JavaScript that does not use thegoog.module() andgoog.require() frombase.js to declare and use modules is not well supported.

   The ECMAScriptimport andexport syntax did not exist until 2015.Closure compiler andclosure-library developed their own means fordeclaring and using modules, and this remains the only well supportedway of defining modules.

   The compiler does implement some understanding of ECMAScript modules,but changing Google's projects to use the newer syntax has never offereda benefit that was worth the cost of the change. Google's TypeScript codeuses ECMAScript modules, but they are converted togoog.module() syntaxbefore closure-compiler sees them. So, effectively the ECMAScript modulessupport is unused within Google. This means we are unlikely to noticeor fix bugs in the support for ECMAScript modules.

   Support for CommonJS modules as input was added in the past, but is notused within Google, and is likely to be entirely removed sometime in 2024.

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/orproject-specific best practices.

• Define user-visible messages in a way that makes it possible to replacethem with translated versions to create localized versions of anapplication.

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

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

  NOTE: These chunks are plain JavaScript scripts. They do not use theECMAScriptimport andexport syntax.

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

• Usegoog.module() andgoog.require() to declare and use modules.

  Support for theimport andexport syntax added in ES6 is not activelymaintained.

• Use annotations in comments to declare type information and provideinformation 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 dynamicaccess (e.g.object[propertyName] orObject.keys(object)) to accessthe properties of a particular object type.

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

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

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

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

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

NOTE: NPM releases were put on hold in early 2024 and are not likely to resumeuntil early 2025.

The easiest way to install the compiler is withNPM orYarn:

yarn global add google-closure-compiler# ORnpm i -g google-closure-compiler

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

google-closure-compiler

This starts the compiler in interactive mode. Type:

varx=17+25;

HitEnter, thenCtrl+Z (on Windows) orCtrl+D (on Mac/Linux), thenEnteragain. The Compiler will respond with the compiled output (usingSIMPLE modeby default):

varx=42;

NOTE: Maven releases were put on hold in early 2024 and are not likely to resumeuntil early 2025. See#4220.

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

https://jscompressor.treblereel.dev/ is a web-based UI and REST API for ClosureCompiler, developed and maintained by athttps://github.com/treblereel/jscompressor.

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

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

We get themost benefit from the compiler if we give itall of our sourcecode (seeCompiling Multiple Scripts), whichallows us to useADVANCED 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. TheFlags and Options wiki pageis updated during each release.

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

google-closure-compiler --help

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

importclosureCompilerfrom'google-closure-compiler';const{ compiler}=closureCompiler;newcompiler({js:'file-one.js',compilation_level:'ADVANCED'});

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

If you have multiple scripts, you should compile them all together with onecompile 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 subdirsgoogle-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 atthe command line.

If you're using globs or many files, you may start to run into problems withmanaging dependencies between scripts. In this case, you should use theincludedlib/base.js that provides functions for enforcingdependencies between scripts (namelygoog.module andgoog.require). ClosureCompiler will re-order the inputs automatically.

The Closure Compiler releases withlib/base.js that providesJavaScript functions and variables that serve as primitives enabling certainfeatures of the Closure Compiler. This file is a derivative of theidentically named base.jsin thesoon-to-be deprecatedClosure Library. Thisbase.js will be supported by Closure Compiler goingforward and may receive new features. It was designed to only retain itsperceived core parts.

1. Post in theClosure Compiler Discuss Group.
2. Ask a question onStack Overflow.
3. Consult theFAQ.

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

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

Bazelisk is available through many package managers. Feel free to use whicheveryou're most comfortable with.

Instructions for installing Bazelisk.

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

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

$ bazelisktest //:all

There are hundreds of individual test targets, so it will take a fewminutes to run all of them. While developing, it's usually better to specifythe exact tests you're interested in.

bazelisktest //:$path_to_test_file

SeeBazel IDE Integrations.

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

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

1. Open the classsrc/com/google/javascript/jscomp/CommandLineRunner.java orcreate 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 bewareof thebugregarding passing "End of Transmission" in the Eclipse console.

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

1. First make sure that it is really a bug and not simply the way that ClosureCompiler works (especially true for ADVANCED_OPTIMIZATIONS).
   • Check theofficial documentation
   • Consult theFAQ
   • Search onStack Overflowand in theClosure Compiler Discuss Group
   • Look through the list ofcompiler assumptions.
2. If you still think you have found a bug, make sure someone hasn't alreadyreported it. See the list ofknown issues.
3. If it hasn't been reported yet, post a new issue. Make sure to add enoughdetail so that the bug can be recreated. The smaller the reproduction code,the better.

1. Consult theFAQ tomake 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 ofknown issues.
3. Read up onwhat type of feature requests are accepted.
4. Submit your request as an issue.

1. All contributors must sign a contributor license agreement (CLA). A CLAbasically says that you own the rights to any code you contribute, and thatyou give us permission to use that code in Closure Compiler. You maintainthe copyright on that code. If you own all the rights to your code, you canfill out anindividual CLA. Ifyour employer has any rights to your code, then they also need to fill out acorporate CLA. Ifyou don't know if your employer has any rights to your code, you should askbefore signing anything. By default, anyone with an @google.com emailaddress already has a CLA signed for them.
2. To make sure your changes are of the type that will be accepted, ask aboutyour patch on theClosure Compiler Discuss Group
3. Fork the repository.
4. Make your changes. Check out ourcoding conventionsfor details on making sure your code is in correct style.
5. Submit a pull request for your changes. A project developer will review yourwork and then merge your request into the project.

Copyright 2009 The Closure Compiler Authors.

Licensed under the Apache License, Version 2.0 (the "License"); you may not usethis file except in compliance with the License. You may obtain a copy of theLicense athttp://www.apache.org/licenses/LICENSE-2.0.

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