Flow control
JavaScript is a single-threaded asynchronous language, meaning that it doesn't really fork in the same way you would expect a multi-threaded language like C++ to, and thus a function blocking the main thread causes everything to grind to a halt. Luckily, the asynchronous part of that means functions generally won't do this, and functions can be written so that the next one can fire before the first one finishes.
On the one hand, this is one of the most interesting and powerful aspects of JavaScript as a language. On the other, it makes things somewhat more difficult to reason about, and adds a degree of complexity to organizing one's code. While you can do pretty much anything without getting too bogged down in the JavaScript event model, the one place where the asynchronous nature of JavaScript is particularly visible is when you make a request -- no matter how fast broadband Internet gets, so long as it conforms to the standard model of physics, there will always be some degree...
