There are a number of things that you can't do with JavaScript. Printing directly to a printer without displaying the print dialog box first is one of them. JavaScript can't tell which printer your visitor wants to print to or even whether they have a printer at all. There are a number of very good reasons for this and it makes perfect sense that JavaScript not be able to write directly to the printer.
Print Directly To Printer
From Stephen Chapman, About.com GuideMarch 24, 2011
Explore JavaScript
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If i want to create a web based KIOSK where I call a webpage to print a ticket?
that is a very good reason to allow the print directly to printer
– it might be a good reason but programming languages don’t care if you have a good reason or not for doing something. Javascript can’t write directly to the printer no matter how good your reason is –