Advanced Compilation and Externs


Using the Closure Compiler with a compilation_level of ADVANCED_OPTIMIZATIONS offers better compression rates than compilation with SIMPLE_OPTIMIZATIONS or WHITESPACE_ONLY. Compilation with ADVANCED_OPTIMIZATIONS achieves extra compression by being more aggressive in the ways that it transforms code and renames symbols. However, this more aggressive approach means that you must take greater care when you use ADVANCED_OPTIMIZATIONS to ensure that the output code works the same way as the input code.

This tutorial illustrates what the ADVANCED_OPTIMIZATIONS compilation level does and what you can do to make sure your code works after compilation with ADVANCED_OPTIMIZATIONS. It also introduces the concept of the extern: a symbol that is defined in code external to the code processed by the compiler.

Before reading this tutorial you should be familiar with the process of compiling JavaScript with one of the Closure Compiler tools (the compiler service UI, the compiler service API, or the compiler application).

  1. Even Better Compression
  3. What to Watch Out for When Using ADVANCED_OPTIMIZATIONS
    1. Removal of Code You Want to Keep
    2. Inconsistent Property Names
    3. Compiling Two Portions of Code Separately
    4. Broken References between Compiled and Uncompiled Code
  4. Declaring Externs
    1. Do Not Use Externs Instead of Exports!
    2. How to Declare Externs with the Closure Compiler Application
    3. How to Declare Externs with the Closure Compiler Service API

Even Better Compression

With the default compilation level of SIMPLE_OPTIMIZATIONS, the Closure Compiler makes JavaScript smaller by renaming local variables. There are symbols other than local variables that can be shortened, however, and there are ways to shrink code other than renaming symbols. Compilation with ADVANCED_OPTIMIZATIONS exploits the full range of code-shrinking possibilities.

Compare the outputs for SIMPLE_OPTIMIZATIONS and ADVANCED_OPTIMIZATIONS for the following code:

function unusedFunction(note) {

function displayNoteTitle(note) {

var flowerNote = {};
flowerNote['title'] = "Flowers";

Compilation with SIMPLE_OPTIMIZATIONS shortens the code to this:

function unusedFunction(a){alert(a.text)}function displayNoteTitle(a){alert(a.title)}var flowerNote={};flowerNote.title="Flowers";displayNoteTitle(flowerNote);

Compilation with ADVANCED_OPTIMIZATIONS shortens the code even further to this:

var a={};a.title="Flowers";alert(a.title);

Both of these scripts produce an alert reading "Flowers", but the second script is much smaller.

The ADVANCED_OPTIMIZATIONS level goes beyond simple shortening of variable names in several ways, including:

  • more aggressive renaming:

    Compilation with SIMPLE_OPTIMIZATIONS only renames the note parameters of the displayNoteTitle() and unusedFunction() functions, because these are the only variables in the script that are local to a function. ADVANCED_OPTIMIZATIONS also renames the global variable flowerNote.

  • dead code removal:

    Compilation with ADVANCED_OPTIMIZATIONS removes the function unusedFunction() entirely, because it is never called in the code.

  • function inlining:

    Compilation with ADVANCED_OPTIMIZATIONS replaces the call to displayNoteTitle() with the single alert() that composes the function's body. This replacement of a function call with the function's body is known as "inlining". If the function were longer or more complicated, inlining it might change the behavior of the code, but the Closure Compiler determines that in this case inlining is safe and saves space. Compilation with ADVANCED_OPTIMIZATIONS also inlines constants and some variables when it determines that it can do so safely.

This list is just a sample of the size-reducing transformations that ADVANCED_OPTIMIZATIONS compilation can perform.


The Closure Compiler service UI, service API, and application all have different methods for setting the compilation_level to ADVANCED_OPTIMIZATIONS.

How to Enable ADVANCED_OPTIMIZATIONS in the Closure Compiler service UI

To enable ADVANCED_OPTIMIZATIONS for the Closure Compiler service UI, click the "Advanced" radio button.

How to Enable ADVANCED_OPTIMIZATIONS in the Closure Compiler service API

To enable ADVANCED_OPTIMIZATIONS for the Closure Compiler service API, include a request parameter named compilation_level with a value of ADVANCED_OPTIMIZATIONS, as in the following python program:


import httplib, urllib, sys

params = urllib.urlencode([
    ('code_url', sys.argv[1]),
    ('compilation_level', 'ADVANCED_OPTIMIZATIONS'),
    ('output_format', 'text'),
    ('output_info', 'compiled_code'),

headers = { "Content-type": "application/x-www-form-urlencoded" }
conn = httplib.HTTPSConnection('')
conn.request('POST', '/compile', params, headers)
response = conn.getresponse()
data =
print data

How to Enable ADVANCED_OPTIMIZATIONS in the Closure Compiler application

To enable ADVANCED_OPTIMIZATIONS for the Closure Compiler application, include the command line flag --compilation_level ADVANCED_OPTIMIZATIONS, as in the following command:

java -jar compiler.jar --compilation_level ADVANCED_OPTIMIZATIONS --js hello.js

What to Watch Out for When Using ADVANCED_OPTIMIZATIONS

Below are listed some common unintended effects of ADVANCED_OPTIMIZATIONS, and steps you can take to avoid them.

Removal of Code You Want to Keep

If you compile just the function below with ADVANCED_OPTIMIZATIONS, Closure Compiler produces empty output:

function displayNoteTitle(note) {

Because the function is never called in the JavaScript that you pass to the compiler, Closure Compiler assumes that this code is not needed!

In many cases this behavior is exactly what you want. For example, if you compile your code together with a large library, Closure Compiler can determine which functions from that library you actually use and discard the ones that you don't use.

If, however, you find that Closure Compiler is removing functions you want to keep, there are two ways to prevent this:

  • Move your function calls into the code processed by Closure Compiler.
  • Export the symbols you want to keep.

The next sections discuss each option in more detail.

Solution: Move Your Function Calls into the Code Processed by the Closure Compiler

You may encounter unwanted code removal if you only compile part of your code with Closure Compiler. For example, you might have a library file that contains only function definitions, and an HTML file that includes the library and that contains the code that calls those functions. In this case, if you compile the library file with ADVANCED_OPTIMIZATIONS, Closure Compiler removes all of your library functions.

The simplest solution to this problem is to compile your functions together with the portion of your program that calls those functions. For example, Closure Compiler will not remove displayNoteTitle() when it compiles the following program:

function displayNoteTitle(note) {
displayNoteTitle({'myTitle': 'Flowers'});

The displayNoteTitle() function isn't removed in this case because Closure Compiler sees that it is called.

In other words, you can prevent unwanted code removal by including your program's entry point in the code that you pass to Closure Compiler. The entry point of a program is the place in the code where the program begins executing. For example, in the flower note program from the previous section, the last three lines are executed as soon as the JavaScript is loaded in the browser. This is the entry point for this program. To determine what code you need to keep, Closure Compiler starts at this entry point and traces the control flow of the program forward from there.

Solution: Export the Symbols You Want to Keep

What if you want to compress only a set of function definitions to create a small, reusable library? How do you prevent Closure Compiler from removing all the functions?

The best solution in this case is to export your functions, as the following example illustrates:

function displayNoteTitle(note) {
// Store the function in a global property referenced by a string:
window['displayNoteTitle'] = displayNoteTitle;

Compilation with ADVANCED_OPTIMIZATIONS compresses this to:

function a(b){alert(b.myTitle)}window.displayNoteTitle=a;

Note that the compressed code still contains a function with a single alert statement. This function is the old displayNoteTitle() function, renamed to a(). The function can still be called by its old name, however, because its old name has been added as a property of the global object. For example, if code external to this compiled code makes the following call, the call still correctly produces an alert:

displayNoteTitle({'myTitle': 'Flowers'});

This call works because the window.displayNoteTitle property now has a function as its value, just as the native window.alert property has the alert() function as its value.

Furthermore, Closure Compiler does not remove the definition of the function (now renamed to a()), because Closure Compiler sees that the function is used in code that gets executed. It's used in the assignment statement window['displayNoteTitle'] = displayNoteTitle;

You can export constructors and prototype properties in the same way. For example:

MyClass = function(name) {
  this.myName = name;
MyClass.prototype.myMethod = function() {
window['MyClass'] = MyClass; // <-- Constructor
MyClass.prototype['myMethod'] = MyClass.prototype.myMethod;

If putting together these export statements seems too tedious, you can use a function to do the exporting for you, using the Closure Library functions goog.exportSymbol() and goog.exportProperty().

Inconsistent Property Names

Closure Compiler compilation never changes string literals in your code, no matter what compilation level you use. This means that compilation with ADVANCED_OPTIMIZATIONS treats properties differently depending on whether your code accesses them with a string. If you mix string references to a property with dot-syntax references, Closure Compiler renames some of the references to that property but not others. As a result, your code will probably not run correctly.

For example, take the following code:

function displayNoteTitle(note) {
var flowerNote = {};
flowerNote.myTitle = 'Flowers';


The last two statements in this source code do exactly the same thing. However, when you compress the code with ADVANCED_OPTIMIZATIONS, you get this:

var a={};a.a="Flowers";alert(a.a);alert(a.myTitle);

The last statement in the compressed code produces an error. The direct reference to the myTitle property has been renamed to a, but the quoted reference to myTitle within the displayNoteTitle function has not been renamed. As a result, the last statement refers to a myTitle property that is no longer there.

Solution: Be Consistent in Your Property Names

This solution is pretty simple. Whenever possible, use dot-syntax property names rather than quoted strings. Use quoted string property names only when you don't want Closure Compiler to rename a property at all. For example, to export a property you must use a quoted string. However, for properties used only within your compiled code, use dot syntax.

Compiling Two Portions of Code Separately

If you split your application into different modules of code, you might want to compile the modules separately. However, if two chunks of code interact at all, you may have trouble compiling them separately. Even if you succeed, the output of the two Closure Compiler runs will not be compatible.

For example, assume that an application is divided into two parts: a part that retrieves data, and a part that displays data.

Here's the code for retrieving the data:

function getData() {
  // In an actual project, this data would be retrieved from the server.
  return {title: 'Flower Care', text: 'Flowers need water.'};

Here's code for displaying the data:

var displayElement = document.getElementById('display');
function displayData(parent, data) {
  var textElement = document.createTextNode(data.text);
displayData(displayElement, getData());

If you try to compile these two chunks of code separately, you will encounter several problems. First, the Closure Compiler removes the getData() function, for the reasons described in Removal of Code You Want to Keep. Second, the Closure Compiler produces a fatal error when processing the code that displays the data.

input:6: ERROR - variable getData is undefined
displayData(displayElement, getData());

Because the compiler does not have access to the getData() function when it compiles the code that displays the data, it treats getData as undefined.

Solution: Compile All Code for a Page Together

To ensure proper compilation, compile all the code for a page together in a single compilation run. The Closure Compiler can accept multiple JavaScript files and JavaScript strings as input, so you can pass library code and other code together in a single compilation request.

Note: This approach won't work if you need to mix compiled and uncompiled code. See Broken References between Compiled and Uncompiled Code for tips on handling this situation.

Broken References between Compiled and Uncompiled Code

Symbol renaming in ADVANCED_OPTIMIZATIONS will break communication between code processed by the Closure Compiler and any other code. Compilation renames the functions defined in your source code. Any external code that calls your functions will break after you compile, because it still refers to the old function name. Similarly, references in compiled code to externally defined symbols may be altered by Closure Compiler.

Keep in mind that "uncompiled code" includes any code passed to the eval() function as a string. Closure Compiler never alters string literals in code, so Closure Compiler does not change strings passed to eval() statements.

Be aware that these are actually very different problems: maintaining compiled-to-external communication, and maintaining external-to-compiled communication. These separate problems have different solutions, and to get the most out of Closure Compiler it's important to use the right solution for the right situation.

Solutions for these two problems are discussed further in the next sections.

Solution for Calling in from External Code to Compiled Code: Exports

If you have JavaScript code that you reuse as a library, you may want to use Closure Compiler to shrink only the library while still allowing uncompiled code to call functions in the library.

The solution in this situation is identical to the solution for unwanted code removal described in Export the Symbols You Want to Keep. Exporting symbols not only keeps those symbols from being removed, but also makes them available to external code.

Solution for Calling out from Compiled Code to External Code: Externs

If you use third-party JavaScript libraries or APIs like the OpenSocial API or the Google Maps API, you need to be sure that Closure Compiler doesn't rename the symbols you use that are defined in those external libraries. For example, if your code calls the OpenSocial JavaScript function opensocial.newDataRequest(), you do not want Closure Compiler to transform this call into a.b(). Your code must use the same names that the external file uses.

Closure Compiler provides a mechanism for declaring that a name is defined in external code and so should not be renamed. This mechanism is called the extern. The compiler assumes that externs will exist in the environment in which the compiled JavaScript will be interpreted.

Declaring Externs

The following JavaScript file contains code that requires an externs declaration:

 * A simple script for adding a list of notes to a page. The list displays
 * the text of each note under its title.

 * Creates the DOM structure for a note and adds it to the document.
function makeNoteDom(noteTitle, noteContent, noteContainer) {
  // Create DOM structure to represent the note.
  var headerElement = textDiv(noteTitle);
  var contentElement = textDiv(noteContent);

  var newNote = document.createElement('div');

  // Add the note's DOM structure to the document.

 * Iterates over a list of note data objects and creates a DOM
function makeNotes(data, noteContainer) {
  for (var i = 0; i < data.length; i++) {
    makeNoteDom(data[i].title, data[i].content, noteContainer);

function main() {
  var noteData = [
      {title: 'Note 1', content: 'Content of Note 1'},
      {title: 'Note 2', content: 'Content of Note 2'}];
  var noteListElement = document.getElementById('notes');
  makeNotes(noteData, noteListElement);


Let's assume that the textDiv() function is defined in a separate file called textops.js that is maintained by a third party. The function looks like this:

function textDiv(text) {
  var divElement = document.createElement('div');
  var textElement = document.createTextNode(text);
  return divElement;

You don't want Closure Compiler to rename textDiv, because it is defined externally, so you declare an extern.

In both cases, you declare the extern by writing JavaScript that defines the symbol you want preserved, and then giving that JavaScript to the Closure Compiler. In this example, we want to prevent renaming of the textDiv() function, so we write JavaScript that declares the function, giving it an empty body:

function textDiv(text){};

If we give this JavaScript to Closure Compiler as an extern definition, Closure Compiler does not include it in the output JavaScript. The sole purpose of this JavaScript is to tell Closure Compiler, "Our code uses a function named textDiv() that's defined somewhere you don't know about, so don't rename any calls to textDiv()."

Sending our JavaScript to Closure Compiler with the extern declaration function textDiv(text){}; produces the following output. Note that we have given the Closure Compiler service a formatting parameter of pretty_print to make it easier to see how Closure Compiler uses the externs declaration.

for(var a = document.getElementById("notes"), b = [{title:"Note 1", content:"Content of Note 1"}, {title:"Note 2", content:"Content of Note 2"}], c = 0;c < b.length;c++) {
  var d = textDiv(b[c].title), e = textDiv(b[c].content), f = document.createElement("div");

Closure Compiler has changed the original program dramatically, but the textDiv calls are still there with the original function name. This code will therefore interoperate with the third party textops.js file that defines textDiv().

Both the Closure Compiler application and the Closure Compiler service API allow extern declarations. The Closure Compiler service UI does not provide an interface element for specifying externs files.

Do Not Use Externs Instead of Exports!

Externs are such a handy way to protect symbols from renaming that you might be tempted to use them instead of exports. If you want to expose an API from a compiled file so that external code can call functions in the file, you might think that you can just declare your API functions in an externs file.

Don't do this! Always use the exporting technique described in Export the Symbols You Want to Keep to expose functions in your compiled files.

Exporting allows better compression than externs. Closure Compiler never renames symbols declared in externs, no matter how many times they appear in your code. This means that a single extern declaration can create many long names in your compiled file. Exporting, in contrast, only includes the full name of the exported symbol once. This one instance of the full name sets up an alias, so that the symbol can be shortened everywhere else it appears.

Note: Only use externs to declare symbols in external code. Never use them to protect symbols defined in the compiled code.

How to Declare Externs with the Closure Compiler Application

Pass the names of externs files to the Closure Compiler application using a separate --externs command-line flag for each extern file, as in the following command:

java -jar compiler.jar --compilation_level ADVANCED_OPTIMIZATIONS \
  --js makeallnotes.js --externs extern1.js --externs extern2.js

How to Declare Externs with the Closure Compiler Service API

There are two ways to send an extern declaration to the Closure Compiler service:

  • Pass JavaScript to the Closure Compiler service in the js_externs parameter.
  • Pass the URL of a JavaScript file to the Closure Compiler service in the externs_url parameter.

The only difference between using js_externs and using externs_url is how the JavaScript gets communicated to the Closure Compiler service.

These two methods are described in more detail in the next sections.

Declaring Externs by Passing JavaScript to the Closure Compiler Service in the js_externs Parameter

If you don't have many externs to declare, you can include the JavaScript for the declarations as the value of a js_externs parameter in your Closure Compiler service request, as in this example:


import httplib, urllib, sys

# Define the parameters for the POST request and encode them in
# a URL-safe format.

params = urllib.urlencode([
    ('code_url', sys.argv[1]),
    ('compilation_level', 'ADVANCED_OPTIMIZATIONS'),
    ('output_format', 'text'),
    ('output_info', 'compiled_code'),
    ('js_externs', 'function textDiv(text){}'), # <-- New parameter!
    ('formatting', 'pretty_print')

# Always use the following value for the Content-type header.
headers = { "Content-type": "application/x-www-form-urlencoded" }
conn = httplib.HTTPSConnection('')
conn.request('POST', '/compile', params, headers)
response = conn.getresponse()
data =
print data

You can include multiple js_externs parameters in your request. You can also include multiple extern declarations in a single parameter value by separating them with a semi-colon.

Declaring Externs by Passing the URL of a JavaScript File to the Closure Compiler Service in the externs_url Parameter

If you have many externs to declare, passing them all in js_externs parameters may be unwieldy. In this case:

  • Store all of your externs declarations in a separate JavaScript file.
  • Upload this file to a webserver.
  • Pass the URL of this file to the Closure Compiler service as the value of the externs_url parameter.

For example:

params = urllib.urlencode([
    ('code_url', sys.argv[1]),
    ('compilation_level', 'ADVANCED_OPTIMIZATIONS'),
    ('output_format', 'text'),
    ('output_info', 'compiled_code'),
    # Different parameter:
    ('externs_url', ''),
    ('formatting', 'pretty_print')

You can specify multiple externs_url parameters. The Closure Compiler service concatenates all extern declarations from js_externs JavaScript and externs_url files.