Nonblocking JavaScript

First of all, let's look at what really happens when we do things asynchronously. Whenever we invoke a function in JavaScript, it creates a new stack frame (execution object). Every inner call gets into this frame. Here the frames are pushed and popped from the top of the call stack in the LIFO (last in, first out) manner. In other words, in the code, we call the foo function and then the bar function; however, during execution, foo calls the baz function. In this case, in the call stack, we have the following sequence: foo, baz, and only then bar. So bar is called after the stack frame of foo is empty. If any of the functions perform a CPU-intensive task, all the successive calls wait for it to finish. However, JavaScript engines have Event Queues (or task queues).

If we subscribe a function to a DOM event or pass a callback to a timer (setTimeout or setInterval) or through any Web I/O APIs (XHR, IndexedDB, and FileSystem), it ends up in a corresponding queue...