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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

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

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

NOTE: Maven releases were put on hold in early 2024 and are not likely to resume until early 2025.

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

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:

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

Args4j

Guava Libraries

JSR 305

JUnit

Protocol Buffers

RE2/J

Truth

Ant

GSON

Node.js Closure Compiler Externs