Writing asynchronous code

Most of us have surely written asynchronous code without paying much attention. Actually, in simple situations we do not need to make great assumptions and our code follows the simplicity of the context. But when the complexity grows, writing, managing, and understanding the asynchronous code may become a messy business. Let's take a look at how we usually write asynchronous code and which issues may come out.

Using events properties

The most common situation where we write asynchronous code is when we need to manage events. Consider the interaction of the user with the GUI—a click on a button activates the execution of a handler that we attached to it.

One of the ways to attach a handler to an event is using the event property. For example, an HTML button has an onclick property, we can attach our handler to:

var btn = document.getElementById("myBtn"); 
btn.onclick = function() { 
  console.log("The button was clicked!"); 
}; 

When the click event occurs, the function...