The single-threaded, event-loop based concurrency model of JavaScript deals with processing of events using so-called “asynchronous non-blocking I/O model.” Unlike computer languages such as Java, where events are handled using additional threads and processed in parallel with the main execution thread, JavaScript code is executed sequentially. In order to prevent blocking the main thread on I/O-bound operations, JavaScript uses a callback mechanism where asynchronous operations specify a callback – the function to be executed when the result of an asynchronous operation is ready; while the code control flow continues executing.
Whenever we want to use the result of a callback to make another asynchronous call, we need to nest callbacks. Since I/O operations can result in errors, we need to handle errors for each callback before processing the success result. This necessity to do error handling and having to embed callbacks makes the callback code difficult to read. Sometimes this is referred to as “JavaScript callback hell.”