Javascript Development with Closure Tools

This is an introductory guide to developing and building Javascript for MediaDrop.

Beginning with v0.9, all new javascript development will be based on Closure Tools instead of the Mootools framework. Currently we’re using two main components of Closure Tools:

Closure Library

“The Closure Library is a broad, well-tested, modular, and cross-browser JavaScript library. You can pull just what you need from a large set of reusable UI widgets and controls, and from lower-level utilities for DOM manipulation, server communication, animation, data structures, unit testing, rich-text editing, and more.”

Closure Compiler

“The Closure Compiler is a tool for making JavaScript download and run faster. It is a true compiler for JavaScript. Instead of compiling from a source language to machine code, it compiles from JavaScript to better JavaScript. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what’s left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls.”

Getting Closure Tools

First off, you’ll need to get Closure Library. Checkout the official SVN repository into your root MediaDrop directory:

$ cd MediaDrop
$ svn checkout http://closure-library.googlecode.com/svn/trunk/ closure-library

Next, you’ll need to download the latest version of Closure Compiler and extract it into place:

cd batch-scripts/closure/compiler
wget http://closure-compiler.googlecode.com/files/compiler-latest.tar.gz
tar -xzvf compiler-latest.tar.gz

Note

The shell scripts detailed below require the library and compiler to be accessible at these locations. If you’d like to store them elsewhere, you’ll have to edit mediadrop/config/middleware.py and the shell scripts at batch-scripts/closure/.

Development Workflow

Ordinarily all MediaDrop javascript that is loaded is pre-compiled. This is ideal for file size and run-time performance on the client, but as a side effect, it obfuscates the code. This makes it extremely difficult to trace compiled code, so it’s important for developers to be able to run the uncompiled source code when necessary.

MediaDrop comes with a javascript debug mode built-in. When enabled, all javascript is loaded in its original source form. Simply append ?debug=true to any URL to enable debug mode for that request.

It’s important to compile your code regularly throughout your development process. The compiler is capable of quickly finding many bugs in your code and it often provides more precise error messages than a browser might. Don’t think of it as a white-space remover: it’s a very useful tool and you should run it regularly.

Compiling your code can sometimes introduce bugs that aren’t present in uncompiled code. This is because the compiler imposes certain restrictions on your Javascript. You should familiarize yourself with the optimizations the compiler makes and their ramifications.

Working in Debug Mode

MediaDrop contains a special debug mode which triggers uncompiled source files be loaded instead of a single pre-compiled and obfuscated file. Enable it by appending ?debug=true to any MediaDrop URL.

In debug mode, there are three files that are initially loaded. Any additional files that they depend on are loaded at run time, which we’ll explain in more detail below.

The initial files are:

goog/base.js

This is the core of Closure Library. Among other things, it defines the dependency management functions goog.require() and goog.provide().

mcore/deps.js

This file is auto-generated and informs the dependency manager in what files dependencies can be found. This allows the correct script file to be loaded when one of your source files calls goog.require('mcore.players.Html5Player'); (which happens to be mcore/players/html5.js in this example).

mcore/base.js

This file contains our actual application code. It exposes the API that you can call from within the page as needed. It can depend on other javascript files which will be loaded at run-time when in debug mode.

Additional dependencies are loaded dynamically thanks to these two core functions:

goog.provide(string name)

Indicate that the file which makes this call defines the given name. This does not actually do anything at run time, but is parsed by the build script that generates mcore/deps.js. (It is also parsed by the compiler build script.)

goog.require(string name)

Load the file which has indicated it provides the given name. In debug mode this creates a new <script> tag which points to the correct source file, as defined by goog/deps.js or mcore/deps.js.

The build script that generates mcore/deps.js should be run any time you add, modify or move a goog.provide() call. Do so by running this shell script:

$ batch-scripts/closure/writedeps.sh

Compiling Your Javascript

MediaDrop and Closure Library both share a very verbose coding style that spans a large number of source files; Closure Compiler concatenates all relevant source files, strips out dead code and optimizes everything that remains.

MediaDrop makes use of the most advanced optimizations offered by the compiler, which imposes some restrictions that you should be aware of. If compiling your code introduces a bug, review the compiler documentation.

MediaDrop makes use of all the strict type checking offered by the compiler. You should enter complete JSDoc type annotations as much as possible, to improve the utility of these checks. This has already proven useful and will no doubt become even more useful as our codebase increases in size.

Our build script is configured to complain as loudly as possible about a great number of things. Please do not ignore these warnings. Learn about the problems they indicate and fix them.

Run the compiler by invoking this shell script:

$ batch-scripts/closure/jscompile.sh

This produces a single file: mcore-compiled.js.

Conventions and Guidelines

MediaDrop attempts to adhere the conventions of the Closure Library. This includes their coding style and design principles as much as possible.

The best way to get up to speed on Closure development is to read Closure: The Definitive Guide by Michael Bolin. It is the missing narrative documentation for all of Closure Tools, and also proves insightful into more general aspects of Javascript.

We make extensive use of goog.ui.Component. It provides a consistent and structured life cycle for UI elements. Components can render DOM elements onto the page or they can decorate DOM elements that have been included in the initial page load, preferably both. You should familiarize yourself with the component architecture.

We follow the Google Javascript Style Guide. The key points are:

• Two spaces instead of tabs.
• Line length should not exceed 79 characters.
• If an expression wraps to two lines, the second line should be indented twice (for a total of 4 spaces). This includes long argument lists but excludes function bodies as well as multi-line object and array literals. More info.
• Built-in prototypes should never be modified. This was the reason for our move from the Mootools framework; it’s convenient at times, but it causes all kinds of interoperability problems with other libraries.

You can optionally use the Closure Linter for finding style and convention errors and fixing them:

# Install the python scripts into your MediaDrop virtualenv
$ source mediadrop_env/bin/activate
$ easy_install http://closure-linter.googlecode.com/files/closure_linter-latest.tar.gz

# Lint one file:
$ gjslint --strict mcore/fx.js

# Try fixing some of the common errors with this utility:
$ fixjsstyle --strict mcore/fx.js