Mastering JavaScript

Chapter 9
What we changed was to wrap the asynchronous code in a getSiteStatus()
function, pass a function named callback() as a parameter to this function, and
execute this function on the last line of getSiteStatus(). The showStatusCode()
callback function simply wraps around console.log() that we called earlier. The
difference, however, is in the way the asynchronous execution works. The most
important idea to understand while learning how to program with callbacks is that
functions are first-class objects that can be stored in variables and passed around
with different names. Giving simple and descriptive names to your variables is
important in making your code readable by others. Now that the callback function
is called once the HTTP call is completed, the value of the status_code variable
will have a correct value. There are genuine circumstances where you want an
asynchronous task executed only after another asynchronous task is completed.
Consider this scenario:
http.createServer(function (req, res) {
getURL(url, function (err, res) {
getURLContent(res.data, function(err,res) {
...
});
});
});
As you can see, we are nesting one asynchronous function in another. This kind of
nesting can result in code that is difficult to read and manage. This style of callback
is sometimes known as callback hell. To avoid such a scenario, if you have code
that has to wait for some other asynchronous code to finish, then you express that
dependency by putting your code in functions that get passed around as callbacks.
Another important idea is to name your functions instead of relying on anonymous
functions as callbacks. We can restructure the preceding example into a more
readable one as follows:
var urlContentProcessor = function(data){
...
}
var urlResponseProcessor = function(data){
getURLContent(data,urlContentProcessor);
}
var createServer = function(req,res){
getURL(url,urlResponseProcessor);
};
http.createServer(createServer);
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