Introduction

Originally devised to enhance web pages in Netscape 2.0, JavaScript is now faced with being a single-threaded language in a multimedia, multitasking, multicore world. Yet JavaScript has not only persevered since 1995, it’s thrived. One after the other, potential rivals in the browser—Flash, Silverlight, and Java applets, to name a few—have come and (more or less) gone.

Meanwhile, when a programmer named Ryan Dahl wanted to build a new framework for event-driven servers, he searched the far reaches of computer science for a language that was both dynamic and single-threaded before realizing that the answer was right in front of him. And so, Node.js was born, and JavaScript became a force to be reckoned with in the server world.

How did this happen? As recently as 2001, Paul Graham wrote the following in his essay “The Other Road Ahead”:^[1]

I would not even use JavaScript, if I were you… Most of the JavaScript I see on the Web isn’t necessary, and much of it breaks.

Today, Graham is the lead partner at Y Combinator, the investment group behind Dropbox, Heroku, and hundreds of other start-ups—nearly all of which use JavaScript. As he put it in a revised version of the essay, “JavaScript now works.”

When did JavaScript become a respectable language? Some say the turning point was Gmail (2004), which showed the world that with a heavy dose of Ajax you could run a first-class email client in the browser. Others say that it was jQuery (2006), which abstracted the rival browser APIs of the time to create a de facto standard. (As of 2011, 48 percent of the top 17,000 websites use jQuery.^[2])

Whatever the reason, JavaScript is here to stay. Apple got behind JavaScript with WebKit and Safari. Microsoft is getting behind JavaScript with Metro. Even Adobe is getting behind JavaScript with tools to generate HTML5 instead of Flash. What began as a humble browser feature has become arguably the most important programming language in the world.

Thanks to the ubiquity of web browsers, JavaScript has come closer than any other language to fulfilling Java’s old promise of “write once, run anywhere.” In 2007, Jeff Atwood coined Atwood’s law:

Any application that can be written in JavaScript will eventually be written in JavaScript.^[3]