section: develop

Get Started with the Closure Library

This get started guide is designed to teach you some fundamental Closure development concepts in the context of creating your first Closure application. This guide also provides the steps common to a Closure development workflow.

Download the Closure library

Execute the following command from the command line to download the Closure Library from the Git repository:

git clone https://github.com/google/closure-library

A directory named closure-library is created. This directory contains the Closure Library code.

Create Closure JavaScript

Create a hello.js JavaScript file in path/to/myapp/ with the following contents:

/**
 * @fileoverview Closure getting started tutorial code example.
 */
goog.require('goog.dom');

/**
 * Appends an `h1` tag to the body with the message "Hello world!".
 * @export
 */
function sayHi() {
  var newHeader = goog.dom.createDom(goog.dom.TagName.H1, {'style': 'background-color:#EEE'},
    'Hello world!');
  goog.dom.appendChild(document.body, newHeader);
}

The hello.js JavaScript calls two Closure Library functions: goog.dom.createDom() and goog.dom.appendChild(). These functions are defined in the Closure Library's closure/goog/dom/dom.js file. The next steps explains how the correct files are loaded to provide access these functions.

Create an HTML file

Create a hello.html file in path/to/myapp/ with the following contents::

<html>
  <head>
    <script src="closure/goog/base.js"></script>
    <script src="hello.js"></script>
  </head>
  <body onload="sayHi()">
  </body>
</html>

The first thing this HTML file does is to load base.js which defines the function goog.require and other core functionality. Then the HTML file loads hello.js (from step 2) which calls goog.require() (goog.require('goog.dom')) to load dom.js (where goog.dom.createDom() and goog.dom.appendChild() are defined). Typically a single goog.require() statement will load a JavaScript file and all of its transitive dependencies.

The Closure Library loads required files by dynamically adding a script tag to the document for each needed Closure Library file.

Test your app with a web server

Use a simple HTTP server, such as the node.js or python simple http server to test your application.

For example, start a simple server in your project directory with:

python -m SimpleHTTPServer 8000

And visit localhost:8000/path/to/myapp/hello.html.

Compile your app

Use the Closure compiler to compile your application when your app is ready for production. The Closure Compiler (also known as JSCompiler) creates a single-file version of the application library. This file includes all the dependencies your application needs, but shortens identifier names where possible to minimize the size of the code and the time it takes to transmit over the network. In addition, the Closure Compiler performs type checking, removes dead code, and applies optimizations.

To compile your application:

Download and install the latest release of the Closure compiler from the Getting Started section of the compiler's Git Hub page.

Execute the closure compiler on your JavaScript file:

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

Note: You can compile multiple JavaScript files at once using the Closure Compiler. For example:

java -jar compiler.jar --js_output_file=out.js in1.js in2.js in3.js ...

Reference the compiled file in your HTML file without using base.js to load the dependencies individually (hello_compiled.js includes hello.js as well as all of its requirements, in compiled form):
```
<html>
<head>
<script src="/hello_compiled_dev.js"></script>
</head>
<body onload="sayHi()">
</body>
</html>
```

Refer to Getting Started with the Closure Compiler Application for more information on the Closure Compiler.

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