Many developers have considered JavaScript to be a second-rate programming language, good for stitching together HTML content, but not for serious application development. That may have been the case a few years ago, but with the introduction of HTML5 and prevalence of JavaScript frameworks, JavaScript should be considered an essential skill for all developers.
In the last few years, several notable JavaScript frameworks have been released that changed the playing field for JavaScript developers, including jQuery, JSON and Node.js. The Node.js framework allows developers to create high performance web apps that are able to respond to a high volume of requests–and quickly enough for high level web server applications. Combine that with the advances that Google has pushed forward in the industry, and web browsers are now able to be host to browser-based applications that rival native apps.