Friday, November 6, 2009

Google's new Closure JavaScript optimizer

I'm pretty excited to see the release, today, of Google's Closure JavaScript compiler.

Closure goes way beyond a simple JavaScript minifier. It can do things like unwind function calls, and replace them with the body of the function (inlining). It also changes local variable names to single characters.

You can either download the compiler locally, or use their web service (though the UI or via a REST API). Here a sample of how aggressively Closure can reduce your code size:

function Foo(string)


In Simple optimization mode this yields:

function Foo(a) {

In Advanced mode this compresses to:


I'm still learning how to use Closure optimally for some of my code. For example, in Advanced mode, my JavaScript Namespace code is pretty severely compressed. First, Simple optimization yields:

While Advanced Optimization saves a few hundred more bytes, but mangles some variable names that should be left alone as external method names:

There is a tutorial on how to annotate your code to make sure that Advanced optimization does not break your code by applying variable renaming too aggressively.

To fix my Namespace code I add these lines:

// Export names
var p = Namespace.prototype;
p['Extend'] = p.Extend;
p['Define'] = p.Define;
p['Import'] = p.Import;
p['SGlobalName'] = p.SGlobalName;

which then add the following lines to my function in optimized form to restore the "exports" from my class library:

d = f.prototype;
d.Extend = d.d;
d.Define = d.c;
d.Import = d.f;
d.SGlobalName = d.g 

With all these fixes, I'm able to get a clean compile of the Namespace library that compresses down to: