Why does alert appear before background changes?

Question

So, I've been trying stuff lately and got this piece of code in my script:

document.body.bgColor = "red";
alert("hello");

But in Chrome, the alert dialog pops up first and only after I close it does the background of my body changes. In Firefox, I get the expected behaviour with body background changing to red followed by the popup.

I know we shouldn't rely on alerts and similar browser controls but can anyone tell me if this is happening because the behaviour is not in the standards or if it's because my understanding of synchronous execution of the above code is wrong?

Answer 1

The rendering process has a lifecycle of it's own and does not block the javascript thread. They both work independently.

The solution is to "pause" the JavaScript execution to let the rendering threads catch up. This can be done via a simple setTimeout set to 0

document.body.style.backgroundColor = "red";

setTimeout(function() {
  alert("hey");
}, 0)

Note that bgColor has been deprecated since 2003 with the DOM Level 2 Spec. The current way to set the background color of an element is via element.style.backgroundColor.