Learning JavaScript, 3rd Edition

Chapter 14. Asynchronous Programming

We got a hint of asynchronous programming in Chapter 1 when we responded to user interaction. Recall that user interaction is naturally asynchronous: you can’t control when a user clicks, touches, speaks, or types. But user input isn’t the only reason for asynchronous execution: the very nature of JavaScript makes it necessary for many things.

When a JavaScript application runs, it runs single-threaded. That is, JavaScript only ever does one thing at a time. Most modern computers are capable of doing multiple things at once (assuming they have multiple cores), and even computers with a single core are so fast that they can simulate doing multiple things at once by doing a tiny bit of task A, then a tiny bit of task B, then a tiny bit of task C, and so on until all the tasks are done (this is called preemptive multitasking). From the user’s perspective, tasks A, B, and C ran simultaneously, whether or not the tasks were actually running simultaneously on multiple cores.

So JavaScript’s single-threaded nature might strike you as a limiting factor, but it actually frees you from having to worry about some very thorny problems that are present in multithreaded programming. This freedom comes at a cost, though: it means that to write smoothly running software, you have to think asynchronously, and not just for user input. Thinking this way can be difficult at first, especially if you’re coming from a language where execution is normally synchronous. ...