Mastering JavaScript

Server-Side JavaScript
This fragment uses two important concepts. First, we are using named functions and
using them as callbacks. Second, we are not nesting these asynchronous functions. If
you are accessing closure variables within the inner functions, the preceding would
be a bit different implementation. In such cases, using inline anonymous functions is
even more preferable.
Callbacks are most frequently used in Node. They are usually preferred to define
logic for one-off responses. When you need to respond to repeating events, Node
provides another mechanism for this. Before going further, we need to understand
the function of timers and events in Node.
Timers
Timers are used to schedule the execution of a particular callback after a specific
delay. There are two primary methods to set up such delayed execution: setTimeout
and setInterval. The setTimeout() function is used to schedule the execution of
a specific callback after a delay, while setInterval is used to schedule the repeated
execution of a callback. The setTimeout function is useful to perform tasks that need
to be scheduled such as housekeeping. Consider the following example:
setTimeout(function() {
console.log("This is just one time delay");
},1000);
var count=0;
var t = setInterval(function() {
count++;
console.log(count);
if (count> 5){
clearInteval(t);
}
}, 2000 );
First, we are using setTimeout() to execute a callback (the anonymous function)
after a delay of 1,000 ms. This is just a one-time schedule for this callback. We
scheduled the repeated execution of the callback using setInterval(). Note that we
are assigning the value returned by setInterval() in a variable t—we can use this
reference in clearInterval() to clear this schedule.
[ 210 ]
www.it-ebooks.info