Google Closure Compiler compiles a typed superset of modern JavaScript to a target version of JavaScript, applying type checking, dead code elimination, minification and other improvements.
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.
The Closure Compiler has three optimization modes: whitespace only, simple optimizations, and advanced optimizations. The first two are safe for all JavaScript. The latter one imposes restrictions on JavaScript usage, but it is where Closure Compiler shines and the valuable type checking, dead code removal, and better minification.
