javascript

Chapter 18: Advanced Techniques
Firefox ’ s implementation of timers actually allows you to determine how far behind a timer has slipped.
It does so by passing in the differential between the time that it was executed and the interval specified.
Here is an example:
//works in Firefox only
setTimeout(function(diff){
if (diff > 0) {
//call is late
} else if (diff < 0){
//call is early
} else {
//call is on time
}
}, 250);
When the execution of one set of code is complete, the JavaScript process yields for a short amount of
time so that other processes on the page can be executed. Since the JavaScript process blocks other page
processes, these small breaks are necessary to prevent the user interface from locking (which can still
happen during long - running code). Setting a timer ensures that there will be at least one process break
before the timer code is executed.
Most browsers don ’ t distinguish intervals that are less than 10 milliseconds. Any
timers set between 1 and 10 milliseconds tend to be treated the same way. Chrome
has a very precise timer mechanism that is accurate to within 2 milliseconds.
Repeating Timers
Timers created using setInterval() ensure regular injection of timer code into the queue. The problem
with this approach is that the timer code may not finish execution before the code is added to the queue
again. The result would be that the timer code is run multiple times in a row, with no amount of time
between them. Fortunately, JavaScript engines are smart enough to avoid this issue. When using
setInterval() , timer code is added to the queue only if there are no other instances of the timer code
already in the queue. This ensures that the time between additions of the timer code to the queue is, at a
minimum, the specified interval.
The downside to this regulation of repeating timers is twofold: (1) intervals may be skipped, and (2)
intervals may be smaller than expected between multiple timer-code executions. Suppose you have a
situation where an onclick event handler sets a repeating timer using setInterval() at any interval
of 200 milliseconds. If the event handler takes a little over 300 milliseconds to complete, and the timer
code takes about the same amount of time, you ’ ll end up with both a skipped interval and timer code
running back - to - back. See Figure 18 - 3 .
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