Mastering JavaScript Design Patterns - Second Edition

Promise pattern

I said earlier that JavaScript is single threaded. This is not entirely accurate. There is a single event loop in JavaScript. Blocking this event loop with a long running process is considered to be bad form. Nothing else can happen while your greedy algorithm is taking up all the CPU cycles.

When you launch an asynchronous function in JavaScript, such as fetching data from a remote server, then much of this activity happens in a different thread. The success or failure handler functions are executed in the main event thread. This is part of the reason that success handlers are written as functions: it allows them to be easily passed back and forth between contexts.

Thus there are activities which truly do happen in an asynchronous, ...