Frequently Asked Questions
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-
What is the Closure Compiler? Why should I use it?
-
The Closure Compiler is a tool for making JavaScript download and
run faster. You can use the Closure Compiler to reduce the size of
your JavaScript files and to make them more efficient.
-
How is the Closure Compiler different from the other JavaScript
compressors that are out there?
-
Generally, the Closure Compiler should meet or exceed the compression
of other code minification tools, and improve download times for your
web application. In addition, the Closure Compiler can help you
find syntax errors during development (rather than during testing)
and identify potentially buggy code patterns.
In Simple mode, Closure Compiler should be able to do better than
other tools because it uses compiler-like analysis to find additional
ways to minimize code size. For example, the Closure Compiler can
inline functions that are used in only a few occurrences, reuse
variable names, and pre-compute constant expressions.
In Advanced mode, Closure Compiler can also use your added type
annotations to find hard-to-spot bugs.
-
Can the Closure Compiler compile JavaScript that's embedded in HTML?
-
No. The Closure Compiler works only on files that contain only JavaScript.
-
Can I use the Closure Compiler together with other JavaScript
minifiers?
-
Yes. Closure Compiler reads in any valid JavaScript and generates
valid JavaScript, so you can apply the Closure Compiler to a
JavaScript file either before or after you run the file through a
different minifier.
Remember that Closure Compiler and other minifiers might have
expectations about the incoming code. A minifier that strips
comments may remove licenses or annotation information needed by
another tool, for example.
-
How can I debug the JavaScript that the Closure Compiler produces?
-
If your compiled code is throwing errors or exhibiting unexpected behavior,
you can use Source Maps to debug the issue. A source map provides a mapping
between the compiled code and the original source code so that the browser's
developer tools can show you your original source code instead of the compiled code.
To make the Closure Compiler produce a source map, pass the
--create_source_map
flag on the command line. For example:
$ java -jar compiler.jar --js example.js --create_source_map ./example-map --js_output_file example-compiled.js
Then, if you're using a browser that supports Source Maps (such as Chrome
or Firefox), you can set breakpoints just as you would for uncompiled code,
and the browser's developer tools will show the corresponding line of code
in the original source. For more information about Chrome's developer tools, including
information about source maps, see
Debugging JavaScript.
-
Does the compiler make any trade-off between my application's
execution speed and download code size?
-
Yes. Any optimizing compiler makes trade-offs. Some size
optimizations do introduce small speed overheads. However, the
Closure Compiler's developers have been careful not to introduce
significant additional runtime. Some of the compiler's
optimizations even decrease runtime (see next question).
-
Does the compiler optimize for speed?
-
In most cases smaller code is faster code, since download time is usually the most important speed factor in web applications. Optimizations that
reduce redundancies speed up the run time of code as well.
-
Are there any restrictions on the size of the files that can be compiled?
-
The compilation web service has a maximum file size, but the
standalone compiler application does not.
-
Is the Closure Compiler avalable for all platforms?
-
The compiler is written in Java, so it can run anywhere Java runs.
-
Can the Compiler process any legal JavaScript?
-
Mostly. Some JavaScript constructs, including
eval()
and with()
, can invalidate assumptions on which the
compiler's transformations are based.
-
How much do I need to know about web development to use the
Closure Compiler?
-
The Closure Compiler is a tool for JavaScript development, so you
do need to know how to program in JavaScript to use the
compiler. But anyone who uses JavaScript can benefit from using
the Closure Compiler.
-
How does the Closure Compiler work with the Closure Library?
-
The Closure Compiler provides special checks and optimizations for
code that uses the Closure Library. In addition, the Closure
Compiler service can automatically include Closure Library
files. Finding
Your Way around Closure describes the syntax for declaring the
parts of Closure that you need. See
the API
reference for information on using the Closure Library with the
API. To use the Closure Library with the Closure Compiler
application you must
first download
the Closure Library. Support for the Closure Library is
enabled in the compiler application by default.
-
My code stops working or the Compiler produces errors when I
compile with
ADVANCED_OPTIMIZATIONS
. Why?
-
Using Advanced mode usually requires some preparation and code
changes. Advanced Compilation
and Externs explains how to make sure your code works
with
ADVANCED_OPTIMIZATIONS
.
-
Why are there random line feeds in compiled scripts?
-
The Closure Compiler intentionally adds line breaks every 500
characters or so. Firewalls and proxies sometimes corrupt or
ignore large JavaScript files with very long lines. Adding line
breaks every 500 characters prevents this problem. Removing the
line breaks has no effect on a script's semantics. The impact on code size is small, and the Compiler
optimizes line break placement so that the code size penalty is even smaller when files are gzipped.
-
I have copyright notices or open source
license text that must appear in my source code. How do I keep the
Closure Compiler from stripping this text out?
-
Closure Compiler supports the JSDoc
@license
tag. Add the
@license
tag to any JSDoc comment to preserve the
comment in the compiler
output. See Annotating
JavaScript for the Closure Compiler for more information.
Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.
Last updated 2023-10-25 UTC.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2023-10-25 UTC."],[[["\u003cp\u003eThe Closure Compiler is a tool for making JavaScript download and run faster by reducing file size and improving efficiency.\u003c/p\u003e\n"],["\u003cp\u003eIt offers greater compression than standard minifiers and helps identify syntax errors and potential bugs during development.\u003c/p\u003e\n"],["\u003cp\u003eThe Closure Compiler works with any valid JavaScript and can be used alongside other minifiers for enhanced optimization.\u003c/p\u003e\n"],["\u003cp\u003eSource maps enable debugging of compiled code by providing a mapping back to the original source.\u003c/p\u003e\n"],["\u003cp\u003eWhile prioritizing download speed, the compiler also performs optimizations to enhance execution speed without significant runtime overhead.\u003c/p\u003e\n"]]],[],null,["# Frequently Asked Questions\n\n- [What is the Closure Compiler? Why should I use it?](#what)\n- [How is the Closure Compiler different\n from the other JavaScript compressors that are out there?](#how-different)\n- [Can the Closure Compiler compile JavaScript that's embedded in HTML?](#tags)\n- [Can I use the Closure Compiler together with other JavaScript\n minifiers?](#other-minifiers)\n- [How can I debug the JavaScript that the Closure Compiler produces?](#sourcemaps)\n- [Does the compiler make any trade-off between my application's\n execution speed and download code size?](#tradeoffs)\n- [Does the compiler optimize for speed?](#speed)\n- [Are there any restrictions on the size of the files that can be compiled?](#size-restrictions)\n- [Is the Closure Compiler available for all platforms?](#platforms)\n- [Can the Compiler process any legal JavaScript?](#restrictions)\n- [How does the Closure Compiler work with\n the Closure Library?](#how-closure)\n- [My code stops working or the Compiler produces\n errors when I compile\n with `ADVANCED_OPTIMIZATIONS`. Why?](#advanced)\n- [Why are there random line feeds in\n compiled scripts?](#linefeeds)\n- [I have copyright notices or open source license\n text that must appear in my source code. How do I keep the Closure\n Compiler from stripping this text out?](#license)\n\n\u003cbr /\u003e\n\n\u003cbr /\u003e\n\n\nWhat is the Closure Compiler? Why should I use it?\n:\n The Closure Compiler is a tool for making JavaScript download and\n run faster. You can use the Closure Compiler to reduce the size of\n your JavaScript files and to make them more efficient.\n\n\nHow is the Closure Compiler different from the other JavaScript\ncompressors that are out there?\n\n: Generally, the Closure Compiler should meet or exceed the compression\n of other code minification tools, and improve download times for your\n web application. In addition, the Closure Compiler can help you\n find syntax errors during development (rather than during testing)\n and identify potentially buggy code patterns.\n\n In Simple mode, Closure Compiler should be able to do better than\n other tools because it uses compiler-like analysis to find additional\n ways to minimize code size. For example, the Closure Compiler can\n inline functions that are used in only a few occurrences, reuse\n variable names, and pre-compute constant expressions.\n\n In Advanced mode, Closure Compiler can also use your added type\n annotations to find hard-to-spot bugs.\n\n\nCan the Closure Compiler compile JavaScript that's embedded in HTML?\n:\n No. The Closure Compiler works only on files that contain only JavaScript.\n\n\nCan I use the Closure Compiler together with other JavaScript\nminifiers?\n\n: Yes. Closure Compiler reads in any valid JavaScript and generates\n valid JavaScript, so you can apply the Closure Compiler to a\n JavaScript file either before or after you run the file through a\n different minifier.\n\n Remember that Closure Compiler and other minifiers might have\n expectations about the incoming code. A minifier that strips\n comments may remove licenses or annotation information needed by\n another tool, for example.\n\n\nHow can I debug the JavaScript that the Closure Compiler produces?\n:\n If your compiled code is throwing errors or exhibiting unexpected behavior,\n you can use Source Maps to debug the issue. A source map provides a mapping\n between the compiled code and the original source code so that the browser's\n developer tools can show you your original source code instead of the compiled code.\n To make the Closure Compiler produce a source map, pass the\n `--create_source_map` flag on the command line. For example: \n\n ```\n $ java -jar compiler.jar --js example.js --create_source_map ./example-map --js_output_file example-compiled.js\n ```\n\n Then, if you're using a browser that supports Source Maps (such as Chrome\n or Firefox), you can set breakpoints just as you would for uncompiled code,\n and the browser's developer tools will show the corresponding line of code\n in the original source. For more information about Chrome's developer tools, including\n information about source maps, see\n [Debugging JavaScript](https://developers.google.com/chrome-developer-tools/docs/javascript-debugging).\n\n\nDoes the compiler make any trade-off between my application's\nexecution speed and download code size?\n:\n Yes. Any optimizing compiler makes trade-offs. Some size\n optimizations do introduce small speed overheads. However, the\n Closure Compiler's developers have been careful not to introduce\n significant additional runtime. Some of the compiler's\n optimizations even decrease runtime (see next question).\n\n\nDoes the compiler optimize for speed?\n:\n In most cases smaller code is faster code, since download time is usually the most important speed factor in web applications. Optimizations that\n reduce redundancies speed up the run time of code as well.\n\n\nAre there any restrictions on the size of the files that can be compiled?\n:\n The compilation web service has a maximum file size, but the\n standalone compiler application does not.\n\n\nIs the Closure Compiler avalable for all platforms?\n:\n The compiler is written in Java, so it can run anywhere Java runs.\n\n\nCan the Compiler process any legal JavaScript?\n:\n Mostly. Some JavaScript constructs, including `eval()`\n and `with()`, can invalidate assumptions on which the\n compiler's transformations are based.\n\n\nHow much do I need to know about web development to use the\nClosure Compiler?\n:\n The Closure Compiler is a tool for JavaScript development, so you\n do need to know how to program in JavaScript to use the\n compiler. But anyone who uses JavaScript can benefit from using\n the Closure Compiler.\n\n\nHow does the Closure Compiler work with the Closure Library?\n:\n The Closure Compiler provides special checks and optimizations for\n code that uses the Closure Library. In addition, the Closure\n Compiler service can automatically include Closure Library\n files. [Finding\n Your Way around Closure](/closure/library/docs/introduction#deps) describes the syntax for declaring the\n parts of Closure that you need. See\n the [API\n reference](/closure/compiler/docs/api-ref#closure) for information on using the Closure Library with the\n API. To use the Closure Library with the Closure Compiler\n application you must\n first [download\n the Closure Library](https://github.com/google/closure-library). Support for the Closure Library is\n enabled in the compiler application by default.\n\n\nMy code stops working or the Compiler produces errors when I\ncompile with `ADVANCED_OPTIMIZATIONS`. Why?\n:\n Using Advanced mode usually requires some preparation and code\n changes. [Advanced Compilation\n and Externs](/closure/compiler/docs/api-tutorial3) explains how to make sure your code works\n with `ADVANCED_OPTIMIZATIONS`.\n\n\nWhy are there random line feeds in compiled scripts?\n:\n The Closure Compiler intentionally adds line breaks every 500\n characters or so. Firewalls and proxies sometimes corrupt or\n ignore large JavaScript files with very long lines. Adding line\n breaks every 500 characters prevents this problem. Removing the\n line breaks has no effect on a script's semantics. The impact on code size is small, and the Compiler\n optimizes line break placement so that the code size penalty is even smaller when files are gzipped.\n\n\nI have copyright notices or open source\nlicense text that must appear in my source code. How do I keep the\nClosure Compiler from stripping this text out?\n:\n Closure Compiler supports the JSDoc `@license` tag. Add the\n `@license` tag to any JSDoc comment to preserve the\n comment in the compiler\n output. See [Annotating\n JavaScript for the Closure Compiler](/closure/compiler/docs/js-for-compiler#tag-license) for more information."]]