Node processes JavaScript instructions using a single thread. Within your JavaScript program no two operations will ever execute at exactly the same moment, as might happen in a multithreaded environment. Understanding this fact is essential to understanding how a Node program, or process, is designed and runs.
This does not mean that only one thread is being used on the machine hosting this a Node process. Simply writing a callback does not magically create parallelism! Recall Chapter 1, Understanding the Node Environment, and our discussion about the process object—Node's "single thread" simplicity is in fact an abstraction created for the benefit of developers. It is nevertheless crucial to remember that there are many threads running in the background managing I/O (and other things), and these threads unpredictably insert instructions, originally packaged as callbacks, into the single JavaScript thread for processing.
Node executes instructions one by one until...