Test compression rate via following tools: Closure Compiler (2010/01/04) YUI Compressor 2.4.2 (2009/06/10) Microsoft Ajax Minifier 1.1 (2009/11/15) Source file: jQuery 1.3.2 regular , jQuery 1.3.2 minified jQuery 1.3.2 regular jQuery 1.3.2 minified Original 124,995 57,254 Closure Compiler (default) 55,334 55,334 Closure Compiler (advanced) 49,647 49,647 YUI Compressor 57,524 56,705 Microsoft Ajax Minifier (default) 72,346 56,061 Microsoft Ajax Minifier (hypercrunching) 53,573 53,570 由上表可知， Closure Compiler 不管source是否有經過其他方式壓縮，都能做出正確地壓縮，達成最小壓縮；贏家為： Closure Compiler ！ Conclusion: Closure Compiler can provide minimized javascript, it's the best choice! 2011 Update: Please take a look at UglifyJS – 更強大的 JS Compressor