Getting Started with the Closure Compiler Application

  • The Closure Compiler is a Java command-line utility for compressing, optimizing, and checking JavaScript for errors.

  • To use the Closure Compiler application, you need Java Runtime Environment version 7, the compiler JAR file, and a JavaScript file to compile.

  • Compiling a JavaScript file with the Closure Compiler reduces its size by stripping comments and whitespace and shortening variable names.

  • The compiled JavaScript code retains its functionality and can be included in an HTML file to be run in a browser.

The Hello World of the Closure Compiler Application

The Closure Compiler application is a Java command-line utility that compresses, optimizes, and looks for mistakes in your JavaScript. To try out the Closure Compiler application with a simple JavaScript program, follow the steps below.

To work through this exercise you need the Java Runtime Environment version 7.

  1. Download the Closure Compiler package

    Create a working directory called closure-compiler.

    Download the most recently released JAR file from the Maven repository, and save it in closure-compiler.

  2. Create a JavaScript file

    Create a file named hello.js containing the following JavaScript: 

    // A simple function.
function hello(longName) {
  alert('Hello, ' + longName);
}
hello('New User');

    Save this file in the closure-compiler directory.

  3. Compile the JavaScript file

    Run the following command from the closure-compiler directory: 

    java -jar compiler.jar --js hello.js --js_output_file hello-compiled.js

    This command creates a new file called hello-compiled.js, which contains the following JavaScript: 

    function hello(a){alert("Hello, "+a)}hello("New User");

    Note that the compiler has stripped comments, whitespace and an unnecessary semi-colon. The compiler has also replaced the parameter name longName with the shorter name a. The result is a much smaller JavaScript file.

    To confirm that the compiled JavaScript code still works correctly, include hello-compiled.js in an HTML file like this one: 

    <html>
<head><title>Hello World</title></head>
<body>
<script src="hello-compiled.js"></script>
</body>
</html>

    Load the HTML file in a browser, and you should see a friendly greeting!

Next Steps

This example illustrates only the most simple optimizations performed by the Closure Compiler. To learn more about the compiler's capabilities, read Advanced Compilation and Externs.

To learn more about other flags and options for the Closure Compiler, execute the jar with the --help flag: 

java -jar compiler.jar --help