Introduction

Asynchronous tasks allow the execution of the main thread of a program to proceed even while waiting for data, an event, or the result of another process, and achieve snappier UIs as well as allowing some types of multitasking.

Unlike other languages that can have many concurrent threads executing, JavaScript typically runs in a single thread. So far, you have already learned in detail about how JavaScript's single-threaded model is enabled by the event loop and the associated event queue. Under the hood, the browser or the Node.js runtime has background threads that listen for events or issue service calls, and when a new event is captured or a service call responds, it is pushed into the event queue. JavaScript continually scans the event queue and triggers the handlers for those events when available. Event handlers are most commonly callback methods, but there are other types as well, such as Promises, which you will learn about in this chapter.

Some threads...