How do I tell javascript to immediately update the DOM?

Question

I am trying to load a 'loading' message to the user before a time-intensive function is called in javascript.

HTML:

<p id='foo'>Foo</p>​

Javascript:

var foo = document.getElementById('foo');

function tellViewerLoading() {
  // Tell the user that loading is occuring.
  foo.innerHTML = 'loading...';   
}

function someActionThatTakesALongTime() {
  // Do some action that takes a long time.
  var i = 0;
  while(i < 100000) {i++; console.log(i);};
}

function domUpdateDelayExperiment() {
  tellViewerLoading();
  someActionThatTakesALongTime();
}

domUpdateDelayExperiment();

JSFiddle: http://jsfiddle.net/johnhoffman/xDRVF/

What I want to happen is for the DOM to be updated immediately after tellViewerLoading() is called.

Instead, what happens is that the DOM seems to be updated after someActionThatTakesALongTime() finishes running. At that point, it is useless to display a loading message.

How do I tell javascript to immediately update the DOM after tellViewerLoading() is called?

Answer 1

Spawn the long-time running function with setTimeout:

function domUpdateDelayExperiment() {
  tellViewerLoading();
  setTimeout(someActionThatTakesALongTime, 50);
}

Explanation: the tellViewerLoading() function updates the DOM but the browser won't reflect changes on screen until domUpdateDelayExperiment() returns. By delaying someActionThatTakesALongTime by means of setTimeout() we let the browser to reflect DOM changes. The timeout is arbitrary, but its minimum value may be important in some browsers. A value of 50 ms is fair enough.