JavaScript execution and page rendering are done in the same execution thread, which means that while your code is executing the browser will not be redrawing the page. (Though even if it was redrawing the page with each iteration of the for loop it would all be so fast that you wouldn't really have time to see the individual numbers.)
What you want to do instead is use the setTimeout() or setInterval() functions (both methods of the window object). The first allows you to specify a function that will be executed once after a set number of milliseconds; the second allows you to specify a function that will be executed repeatedly at the interval specified. Using these, there will be "spaces" in between your code execution in which the browser will get a chance to redraw the page.
So, try this:
function launch() {
var inc = 0,
max = 9999;
delay = 100; // 100 milliseconds
function timeoutLoop() {
document.getElementById('result').innerHTML = inc;
if (++inc < max)
setTimeout(timeoutLoop, delay);
}
setTimeout(timeoutLoop, delay);
}
Notice that the function timeoutLoop() kind of calls itself via setTimeout() - this is a very common technique.
Both setTimeout() and setInterval() return an ID that is essentially a reference to the timer that has been set which you can use with clearTimeout() and clearInterval() to cancel any queued execution that hasn't happened yet, so another way to implement your function is as follows:
function launch() {
var inc = 0,
max = 9999;
delay = 100; // 100 milliseconds
var iID = setInterval(function() {
document.getElementById('result').innerHTML = inc;
if (++inc >= max)
clearInterval(iID);
},
delay);
}
Obviously you can vary the delay as required. And note that in both cases the inc variable needs to be defined outside the function being executed by the timer, but thanks to the magic of closures we can define that within launch(): we don't need global variables.
