How to detect if a JS is packed already

Hey guys.. I am writing a Windows application in C# that minifies CSS files and packs JS files as a batch job. One hurdle for the application is, what if the user selects a JavaScript file that has already been packed? It will end up increasing the file size, defeating my purpose entirely!

Is opening the file a开发者_高级运维nd looking for the string eval(function(p,a,c,k,e,d) enough? My guess is no, as there are other JS packing methods out there. Help me out!

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One might suggest that you compare the size of the pre and post packed JS and return/use the smaller of the two.

UPDATE based on question in comment by GPX on Sep 30 at 1:02

The following is a very simple way to tell. There may be different, or more accurate, ways of determining this, but this should get you going in the right direction:

var unpackedJs = File.ReadAllText(...)
var unpackedSize = jsContent.Length;
var packedJs = ... // Your Packaging routine
File.WriteAllText(pathToFile, unpackedSize < packedJs.Length ? unpackedJs : packedJs)

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I would check file size and lines of code (e.g.: average line length). These two information should be enough to know if the code is sufficiently compact.

Try this demo.

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I direct you to a post that suggests packing is bad.

http://ejohn.org/blog/library-loading-speed/

Rather use minification. Google Closure compiler can do this via a REST web service. Only use a .min.js extension for minified (not packed).

Gzip will do a better job and will be uncompressed by the browser. Its best to switch on zip compression on the server which will zip a minified file down further.

Of course this raises the question 'How can I tell if my Javascript is already minified!'

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When you create/save a minified file, use the standard file name convention of "Filename.min.js". Then when they select the file, you can check for that as a reliable indicator.

I do not think it is wise to go overboard on the dummy-proofing. If a user (who is a developer, at that), is dumb enough to double-pack a file, they should experience problems. I know you should give them the benefit of the doubt, but in this case it does not seem worth the overhead.

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If you're using a safe minimization routine, your output should be the same as the input. I would not recommend the routine you mention. MS's Ajax Minifier is a good tool and even provides dll's to use in your project. This would make your concern a non-issue.

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I would suggest adding a '.min' prefix to the extension of the packed file, something like 'script.min.js'. Then just check the file name.

Other than that, I would suggest checking how long the lines are, and how many spaces are used. Minified/packed JS typically has almost no spaces (typically in strings) and very long lines.