Matteo Caprari home

Learning Closure: managing dependencies and compiling

posted on 04 Dec 2009

Google's closure library offers semantics and tools to manage dependencies between modules. This is specially useful when building single-page javascript applications.

To find out how smoothly this works, I'll build a simple javascript application that given a string, displays its MD5 hash. We'll

If you have used jQuery you'll find that this code is verbose and painfully similar to java.

To ease your mind, think that structured libraries like Closure and YUI tend to promote more readable and maintainable code (at the expense of coolness).

Closure and YUI are a better fit when the goal is to build a complex application.

Try this at home

See the demo of the sample application discussed below. You can browse browse or download all source code discussed here, including all 3rd party libraries. I welcome all comments and corrections.

Project setup

Create an empty directory (i called mine goog-dependencies-example), enter it and download all the stuff

You should now have a structure like this:
  |-- closure-templates\
  |-- closure-library\
  |-- jshash-2.2\

Publish this directory on a web server.

During development, I use lighttpd with this minimal configuration file to quickly publish just a directory. Save the file in the directory you want to publish, run lighttpd -D -f lhttpd-minimal.txt and browse to http://localhost:3030/. Lighttpd is available on ubuntu, macos (via macports) and windows (via cygwin).

UI: template

We need a ui component with a textarea to input some text, a button to execute and and a textarea to see the results. Using closure template is definitely overkill here, but it's useful to demonstrate how to load templates a dependencies. Create the file
{namespace net.caprazzi.md5.ui.template}

 * md5 ui template
 * To retrieve its contents, invoke the function
 * net.caprazzi.md5.template.ui.main()
{template .main}
<div class="md5-ui">
	<textarea class="md5-input"></textarea><br/>
	<button class="md5-action">Hash It!</button><br/>
	<textarea class="md5-output"></textarea><br/>

And execute this command to compile it to a javascript file:

$ java -jar closure-templates/SoyToJsSrcCompiler.jar \
	--shouldProvideRequireSoyNamespaces \
	--outputPathFormat js/md5.ui.template.js
// This file was automatically generated from
// Please don't edit this file by hand.



net.caprazzi.md5.ui.template.main = function(opt_data, opt_sb) {
  var output = opt_sb || new soy.StringBuilder();
  output.append('<div class="md5-ui">
	<textarea class="md5-input"></textarea><br/>
	<button class="md5-action">Hash It!</button><br/>
	<textarea class="md5-output"></textarea><br/></div>');
  if (!opt_sb) return output.toString();

UI: javascript

Now let's write some code that hashes the contents of the first textarea when the button is clicked. Name the file js/md5.ui.js.

// NOTE the inclusion of our template

// NOTE the inclusion of jshash
// as defined in third_party_deps.js


net.caprazzi.md5.Ui = function(parent) {
	this.parent = parent;

net.caprazzi.md5.Ui.prototype.render = function() {
	var html = net.caprazzi.md5.ui.template.main();
	this.parent.innerHTML = html;
	this.input = goog.dom.$$('textarea', 'md5-input', this.parent)[0];
	this.button = goog.dom.$$('button', 'md5-action', this.parent)[0];
	this.output = goog.dom.$$('textarea', 'md5-output', this.parent)[0];

	var self = this;,,
			function() { self.onButton_()});

net.caprazzi.md5.Ui.prototype.onButton_ = function() {
	this.output.value = hex_md5(this.input.value);

Application entry point

This file exposes the function main(), that will be executed at page load. It only requires the two modules that uses directly



net.caprazzi.md5.application.main = function() {
	var container = document.getElementById('md5-ui-container');
	var ui = new net.caprazzi.md5.Ui(container);

deps.js: the dependencies file is a script that comes with closure library. It parses the source files in search of goog.require() and goog.provide() declarations. It then uses those declarations to build a dependency tree.

The dependency tree is stored in a file as a list of goog.addDependency calls. Each calls describes a file and the modules it provieds and requires. This line declares that md5.component.js will provide 'net.caprazzi.md5' and require 'net.caprazzi.md5.template' and others:

    ['net.caprazzi.md5.template', '', 'goog.dom', 'jshash']);

Run this command in the root of the project to generate the deps.js all the dependecies (including google's). This may take a while.

$ python closure-library/closure/bin/ \
    -p closure-library/closure \
    -p closure-library/third_party \
    -p closure-templates \
    -p js \
    -i js /md5.* \
    -o deps \
> deps.js

The option '-o deps' tells to output a dependency tree. The other options specify the source directories and files. Spend a moment to review the contents of the resulting file.

Managing the third party library 'md5.js'

As I explained above, calcdeps uses special declarations in the javascript files to make sense of the dependencies. We know that by 'jshash.md5' i mean 'jshash-2.2/md5.js' but calcdeps could only figure this out if the file included a correct goog.require statement.

At this point we may just add the line to the file and go on with our lives. But we all agree that editing 3rd party libraries is not a good practice.

I maintain a text file 'extradeps.txt' where each line associates a module to a file, in this case the contents are

jshash.md5 jshash-2.2/md5-min.js

Then a simple python script parses that file and generates the missing addDependency() statements:

import sys
for mod, path in [  line.strip().split(' ') for line in file(sys.argv[1]) ]:
	print "goog.addDependency('%s',['%s'],[]);" % (path, mod)

Execute the script to see what the output looks like. Then remember to concatenate its output to deps.js each time your rebuild:

$ python extradeps.txt >> deps.js

in dev: index_dev.html

The last piece of the puzzle is the one that holds everything together: the html

		<title>Hash me, hash me</title>
		<script src="closure-library/closure/goog/base.js"></script>
		<script src="deps.js"></script>
	<body onload="net.caprazzi.md5.application.main();">
		<h3>Hash me, hash me</h3>
		<div id="md5-ui-container"></div>

Note the two global directives before the inclusion of base.js:

  • CLOSURE_NO_DEPS stops closure from loading its own deps.js
  • CLOSURE_BASE_PATH tells closure file loader how to build the script urls.

Note that the html file only imports closure's base.js and deps.js then invokes directly code that comes from md5.ui.js

Navigate to index.html, the application should be working.

Use firebug or another network monitor, to see how the browser is downloading many different files to satisfy all the dependencies. This is the perfect behaviour for development, because separate files are easier to debug.

going live: the single js file and index_live.html

Loading many small javascript files is an evil practice in production. We definitely want to have all our javascript files concatenated in one big blob.

I previously used with the option '-o deps' to build a dependency tree. Calcdeps can also concatenate all your dependancies right away, using the option '-o script'. Unfortunately if you try to run it against our code, it will terminate with an exception (

Exception: Missing provider for (jshash.md5)
. Clearly, again, the dependency script doesn't know which files provides that module

Python to the rescue again: we'll reuse the extradeps.txt file with a new python script that takes all the external deps and puts them in a file along with the correct goog.provide() statements.

import sys
mods = [  line.strip().split(' ') for line in file(sys.argv[1]) ];
for mod, path in mods:
	print "goog.provide('%s');" % (mod)
for mod, path in mods:
	for line in file(path):
		print line

Running this script produces a calcdeps-friendly javascript file:

$ mkdir tmp/
$ python extradeps.txt > tmp/extra.js

We can now tell calcdeps to look into tmp/ to find what's needed to build our big target file:

$ python closure-library/closure/bin/ \
	-p closure-library \
	-p tmp \
	-p js \
	-p closure-templates \
 	-i js/md5.application.js \
    -o script \
> md5_application.js

Now we can produce the final artifact and go live with index_live.html:

		<title>Hash me, hash me</title>
		<script src="md5_application.js"></script>
	<body onload="net.caprazzi.md5.application.main();">
		<h3>Hash me, hash me</h3>
		<div id="md5-ui-container"></div>


I've done this exercise to understand if the closure's dependency management would be usable in a production environment.

There was some initial misunderstanding between calcdeps and me and I had to look at base.js to understand what was going on and to find out about the global directives that influence the loader.

The syntax is neat and serves the double purpose of managing namespaces and dependencies.

My impression is that at least the goog.require/provide syntax and the script can be used in production with confidence.

Closure dosen't allow to specify css files as dependencies, while it's possible with YUI3 (that sports a cool dependency management mechanism of his own).

As an exercise you can think about adding a compilation step using google closure compiler.

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