INTRODUCTION TO ASYNCHRONOUS PROGRAMMING
The duality between synchronous and asynchronous behavior is a fundamental concept in computer science—especially in a single-threaded event loop model such as JavaScript. Asynchronous behavior is borne out of the need to optimize for higher computational throughput in the face of high-latency operations. If it is feasible to run other instructions while a computation is completing and still maintain a stable system, then it is pragmatic to do so.
Importantly, an asynchronous operation is not necessarily a computationally intensive or high-latency operation. It can be used anywhere it doesn't make sense to block a thread of execution to wait for the asynchronous behavior to occur.
Synchronous vs. Asynchronous JavaScript
Synchronous behavior is analogous to sequential processor instructions in memory. Each instruction is executed strictly in the order in which it appears, and each is also capable of immediately retrieving information...
