Communicating with the Closure Compiler Service API


The Closure Compiler service API provides programmatic access to Closure Compiler JavaScript compilation through a Web-based API. While the Closure Compiler UI gives you an easy way to use the compiler service through a simple form on a web page, copying output from this web page is not the most efficient way to work. With the Closure Compiler service API, you gain the versatility to build your own tools and create your own work flow.

This tutorial walks you through the process of sending JavaScript to the Closure Compiler service and getting the Closure Compiler output back. The example uses the most basic level of Closure Compiler compilation, which simply strips comments and whitespace from your JavaScript.

This tutorial assumes that you have basic familiarity with JavaScript and HTTP. While it uses a Python script to submit JavaScript to the Closure Compiler service, you don't need to know Python to follow the example.

How to Communciate with the API

  1. Determine the Request Parameters

    You interact with the Closure Compiler service by making HTTP POST requests to the Closure Compiler server. With every request you must send at least the following parameters:

    js_code or code_url

    The value of this parameter indicates the JavaScript that you want to compile. You must include at least one of these parameters, and you can include both. The js_code parameter must be a string that contains JavaScript, such as alert('hello'). The code_url parameter must contain the URL of a JavaScript .js file that's available via HTTP.


    The value of this parameter indicates the degree of compression and optimization to apply to your JavaScript. There are three possible compilation levels: WHITESPACE_ONLY, SIMPLE_OPTIMIZATIONS, and ADVANCED_OPTIMIZATIONS. This example use WHITESPACE_ONLY compilation, which just strips comments and whitespace.

    The compilation_level parameter defaults to a value of SIMPLE_OPTIMIZATIONS.

    language and language_out

    The value of language specifies what version of ECMAScript the Closure Compiler will parse. If this is unspecified, the default is ECMASCRIPT3, but you may want to specify ECMASCRIPT5 or even ECMASCRIPT6. This example uses ECMASCRIPT5

    The optional parameter language_out may be specified, if so, it will transpile source into the specified version of ECMAScript.

    For more information on supported levels, see the API Reference.


    The value of this parameter indicates the kind of information that you want from the compiler. There are four possible kinds of output: compiled_code, warnings, errors, and statistics. This example uses the value compiled_code, which tells the Closure Compiler service to output the compressed version of the JavaScript it receives in the request.


    The format for the Closure Compiler service's output. There are three possible output formats: text, json, or xml. This example uses the value text, which outputs raw text.

    The output_format parameter defaults to a value of text.

    For more information about these required parameters and additional optional parameters, see the API Reference.

    The example in this introductory tutorial just sends one line of raw JavaScript to the Closure Compiler service, so it uses js_code instead of code_url. It uses a compilation_level of WHITESPACE_ONLY, asks for raw text output with an output_format of text, and asks for an output_info type of compiled_code.

  2. Make a Post Request to the Closure Compiler Service

    To get output from the Closure Compiler service, send the parameters you chose in Step 1 in a POST request to the Closure Compiler service API URL. One way to do this is with a simple HTML form like the one in the Hello World of the Closure Compiler Service API.

    To use a form like this during development, however, you would have to copy the output out of the browser and paste it into a .js file. If, instead, you write a small program to send the request to the Closure Compiler service, you can write the Closure Compiler output straight to a file. For example, the following python script sends the request to the Closure Compiler service and writes out the response:

    import httplib, urllib, sys
    # Define the parameters for the POST request and encode them in
    # a URL-safe format.
    params = urllib.urlencode([
        ('js_code', sys.argv[1]),
        ('compilation_level', 'WHITESPACE_ONLY'),
        ('language', 'ECMASCRIPT5'),
        ('output_format', 'text'),
        ('output_info', 'compiled_code'),
    # Always use the following value for the Content-type header.
    headers = { "Content-type": "application/x-www-form-urlencoded" }
    conn = httplib.HTTPConnection('closure-compiler.appspot.com')
    conn.request('POST', '/compile', params, headers)
    response = conn.getresponse()
    data = response.read()
    print data

    Note: To reproduce this example, Windows users may need to install Python. See the Python Windows FAQ for instructions on installing and using Python under Windows.

    This script optimizes JavaScript passed to it as a command line argument. Paste the above code into a file called compile.py, change the permissions of the file to make it executable, and execute the following command:

    $ python compile.py 'alert("hello");// This comment should be stripped'

    This command prints out the compressed code from the Closure Compiler response:


    Because this example uses basic compilation, the compiler doesn't do anything other than strip off the comment.

    Here are a few things to note about this script:

    • The parameters are passed to the request method of the HTTPConnection as a URL-encoded string. After the call to urllib.urlencode, the params variable contains the following string:
      If you write your own script, the script should post URL-encoded content like this.
    • The request must always have a Content-type header of application/x-www-form-urlencoded
  3. Next Steps

    To learn to use the service to achieve better compression in a more realistic development scenario, continue on to Compressing Files with the API.