Closure Tools is set of opensource (release info) developer tools by Google which are used while building Gmail, Google Maps, Google Docs  & many others.

The tools include a JavaScript compiler, library & templating system.

Closure Compiler

JavaScript Compiler

It compiles JavaScript into compact, high-performance code. The compiler removes dead code and rewrites and minimizes what's left so that it downloads and runs quickly.

It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls.

The compiler also offers a Firebug extension, named Inspector, for better debugging.

Closure Library

It is a broad, well-tested, modular, & cross-browser JavaScript library.

Only what you need can be pulled from a large set of reusable UI widgets, controls, lower-level utilities for DOM manipulation, server communication, animation, data structures, unit testing, rich-text editing, & more.

Closure Templates

They simplifty the task of dynamically generating HTML.

In contrast to traditional templating systems, in which you use one big template per page, you can think of Closure Templates as small components that you compose to form your user interface.

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