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

A JavaScript checker and optimizer.
https://github.com/google/closure-compiler

closure-compiler javascript optimization typechecking

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A JavaScript checker and optimizer.

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README 

        
# [Google Closure Compiler](https://developers.google.com/closure/compiler/)



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The [Closure Compiler](https://developers.google.com/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.



1. 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.



1. 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.



1. 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.



1. 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.



1. 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.



1. 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](https://npmjs.com) or

[Yarn](https://yarnpkg.com):



```bash

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:



```bash

google-closure-compiler

```



This starts the compiler in interactive mode. Type:



```javascript

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):



```javascript

var x=42;

```



#### Downloading from Maven Repository



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

[Maven](https://mvnrepository.com/artifact/com.google.javascript/closure-compiler).



### 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:



```bash

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](#compiling-multiple-scripts)), which

allows us to use `ADVANCED` optimizations:



```bash

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](https://github.com/google/closure-compiler/wiki/Flags-and-Options)

  is updated during each release.



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



```bash

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](https://developers.google.com/closure/compiler/docs/gettingstarted_app) 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`:



```javascript

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](https://github.com/google/closure-compiler-npm/tree/master/packages/google-closure-compiler) 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.



```bash

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

```



You can also use minimatch-style globs.



```bash

# 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](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](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](https://github.com/google/closure-library/blob/7818ff7dc0b53555a7fb3c3427e6761e88bde3a2/closure/goog/base.js)

in the

[soon-to-be deprecated](https://github.com/google/closure-library/issues/1214)

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](https://groups.google.com/forum/#!forum/closure-compiler-discuss).

2.  Ask a question on

    [Stack Overflow](https://stackoverflow.com/questions/tagged/google-closure-compiler).

3.  Consult the [FAQ](https://github.com/google/closure-compiler/wiki/FAQ).



## Building the Compiler



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



Prerequisite                                                               | Description

-------------------------------------------------------------------------- | -----------

[Java 11 or later](https://java.com)                                       | Used to compile the compiler's source code.

[NodeJS](https://nodejs.org)                                               | Used to generate resources used by Java compilation

[Git](https://git-scm.com/)                                                | Used by Bazel to download dependencies.

[Bazelisk](https://bazel.build/install/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](https://bazel.build/install/bazelisk).



### Building from a terminal



```bash

$ 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.



```bash

$ 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.



```bash

bazelisk test //:$path_to_test_file

```



### Building from an IDE



See [Bazel IDE Integrations](https://docs.bazel.build/versions/master/ide.html).



### 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:



```bash

# 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](https://stackoverflow.com/questions/4711098/passing-end-of-transmission-ctrl-d-character-in-eclipse-cdt-console)

    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](https://github.com/google/closure-compiler/blob/master/code_of_conduct.md) 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](https://developers.google.com/closure/compiler/)

    *   Consult the [FAQ](https://github.com/google/closure-compiler/wiki/FAQ)

    *   Search on

        [Stack Overflow](https://stackoverflow.com/questions/tagged/google-closure-compiler)

        and in the

        [Closure Compiler Discuss Group](https://groups.google.com/forum/#!forum/closure-compiler-discuss)

    *   Look through the list of

        [compiler assumptions](https://github.com/google/closure-compiler/wiki/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](https://github.com/google/closure-compiler/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](https://github.com/google/closure-compiler/wiki/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](https://github.com/google/closure-compiler/issues).

3.  Read up on

    [what type of feature requests are accepted](https://github.com/google/closure-compiler/wiki/FAQ#how-do-i-submit-a-feature-request-for-a-new-type-of-optimization).

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](https://code.google.com/legal/individual-cla-v1.0.html). If

    your employer has any rights to your code, then they also need to fill out a

    [corporate CLA](https://code.google.com/legal/corporate-cla-v1.0.html). 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](https://groups.google.com/forum/#!forum/closure-compiler-discuss)

3.  Fork the repository.

4.  Make your changes. Check out our

    [coding conventions](https://github.com/google/closure-compiler/wiki/Contributors#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.