Why alert() dialog shows first regardless of order in the code?

Question

In the code sample below, I expect the text in div to change first.

But the text changes only after I click ok in the alert dialog.

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Answer 1

Most browsers render changes to the DOM only between JavaScript tasks. (JavaScript code is run as discrete tasks in an event loop, aka jobs in a job queue.) alert and its cousins confirm and prompt are archaic stop-the-world functions that completely block the event loop and UI. So although your change to the DOM has been made, the browser hasn't had a chance to render that change visually before the alert stops the world.

If you really, really need to use alert, do so after a timeout:

setTimeout(function() {
    alert(/*...*/);
}, 100);

var x = 0;

function counter() {
  x++;

  document.getElementById('aDiv').innerHTML = x;
  setTimeout(function() {
    alert(x);
  }, 100);
}

<div id="aDiv">
0
</div>
<input type="button" value="+1" onclick="counter()">

A delay of 0 is sufficient to give most browsers a chance to do the rendering, but I've generally had to use 100 with Firefox when I've needed to do this. (That may relate only to slightly-older versions of Firefox, though; with the current Firefox I see the DOM change in your snippet even without setTimeout. That's at least newish behavior as I write this in May 2018.)

Answer 2

Modern browsers are set up such that once they encounter alert() browser will essentially pause the rendering of HTML until the alert() is done.

Answer 3

The browser do not update the DOM immediately. You can add a delay before calling the alert() function:

setTimeout(function(){ alert(x) }, 10);