And how is that relevant? I've heard the argument "Your backend developers can work on the front end too" -- but its a tired argument. Any developer who can't already do both has no place working for a company producing web-based software.

For example, you don't have to write validating logic for inputs twice.

That somebody can do both things should, in my opinion not mean that he has do both things.

There's a huge army of teams who aren't competent to do both, judging by the number of sites that fail completely if the client isn't trusting and running javascript. Node.js seems like a good solution for unobtrusive enhancement, making reliable semantic resources that may still optimize out many server round trips with mobile code.

In a perfect world you would be correct. But answer me this:

How many Java backend engineers know how to unit test javascript, how to make modular templates (e.g. writing their own tags, or nesting/tiling their JSP's), how to systematically minify their code in production and use CDN's ... or even how to use any design patterns in Javascript?

You can use the same code at the front end and at the back end.

Ideally, would you write your web application using one language, or ten?

And that is how it's relevant. Any developer who can't grasp this concept has no place working for a company producing web-based software.

I would choose to write it in however many languages fit the various pieces best. To choose otherwise would be ill-advised.

By "however many languages fit the various pieces best," did you mean the most or the fewest?

I mean I wouldn't make technical decisions dogmatically.

I think everyone here will back you up on that, but don't infer that zinxq is advocating choice by technical dogma. I read their post as simply asking for an explanation for node.js's accelerating popularity; of course there are many factors, but its ability to reduce lexical context switching is obviously a major component. To argue otherwise would be ill-advised.

Sure you can! http://code.google.com/webtoolkit/

(But who wants to is a separate question.)

I stand corrected.