When a 'blur' event occurs, how can I find out which element focus went *to*?

Hmm... In Firefox, you can use explicitOriginalTarget to pull the element that was clicked on. I expected toElement to do the same for IE, but it does not appear to work... However, you can pull the newly-focused element from the document:

function showBlur(ev)
{
   var target = ev.explicitOriginalTarget||document.activeElement;
   document.getElementById("focused").value = 
      target ? target.id||target.tagName||target : '';
}

...

<button id="btn1" onblur="showBlur(event)">Button 1</button>
<button id="btn2" onblur="showBlur(event)">Button 2</button>
<button id="btn3" onblur="showBlur(event)">Button 3</button>
<input id="focused" type="text" disabled="disabled" />

Caveat: This technique does not work for focus changes caused by tabbing through fields with the keyboard, and does not work at all in Chrome or Safari. The big problem with using activeElement (except in IE) is that it is not consistently updated until after the blur event has been processed, and may have no valid value at all during processing! This can be mitigated with a variation on the technique Michiel ended up using:

function showBlur(ev)
{
  // Use timeout to delay examination of activeElement until after blur/focus 
  // events have been processed.
  setTimeout(function()
  {
    var target = document.activeElement;
    document.getElementById("focused").value = 
      target ? target.id||target.tagName||target : '';
  }, 1);
}

This should work in most modern browsers (tested in Chrome, IE, and Firefox), with the caveat that Chrome does not set focus on buttons that are clicked (vs. tabbed to).

I solved it eventually with a timeout on the onblur event (thanks to the advice of a friend who is not StackOverflow):

<input id="myInput" onblur="setTimeout(function() {alert(clickSrc);},200);"></input>
<span onclick="clickSrc='mySpan';" id="mySpan">Hello World</span>

Works both in FF and IE.

Works in Google Chrome v66.x, Mozilla v59.x and Microsoft Edge... Solution with jQuery.

I test in Internet Explorer 9 and not supported.

$("#YourElement").blur(function(e){
     var InputTarget =  $(e.relatedTarget).attr("id"); // GET ID Element
     console.log(InputTarget);
     if(target == "YourId") { // If you want validate or make a action to specfic element
          ... // your code
     }
});

Comment your test in others internet explorer versions.

Can you reverse what you're checking and when? That is if you remeber what was blurred last:

<input id="myInput" onblur="lastBlurred=this;"></input>

and then in the onClick for your span, call function() with both objects:

<span id="mySpan" onClick="function(lastBlurred, this);">Hello World</span>

Your function could then decide whether or not to trigger the Ajax.AutoCompleter control. The function has the clicked object and the blurred object. The onBlur has already happened so it won't make the suggestions disappear.

I am also trying to make Autocompleter ignore blurring if a specific element clicked and have a working solution, but for only Firefox due to explicitOriginalTarget

Autocompleter.Base.prototype.onBlur = Autocompleter.Base.prototype.onBlur.wrap( 
        function(origfunc, ev) {
            if ($(this.options.ignoreBlurEventElement)) {
                var newTargetElement = (ev.explicitOriginalTarget.nodeType == 3 ? ev.explicitOriginalTarget.parentNode : ev.explicitOriginalTarget);
                if (!newTargetElement.descendantOf($(this.options.ignoreBlurEventElement))) {
                    return origfunc(ev);
                }
            }
        }
    );

This code wraps default onBlur method of Autocompleter and checks if ignoreBlurEventElement parameters is set. if it is set, it checks everytime to see if clicked element is ignoreBlurEventElement or not. If it is, Autocompleter does not cal onBlur, else it calls onBlur. The only problem with this is that it only works in Firefox because explicitOriginalTarget property is Mozilla specific . Now I am trying to find a different way than using explicitOriginalTarget. The solution you have mentioned requires you to add onclick behaviour manually to the element. If I can't manage to solve explicitOriginalTarget issue, I guess I will follow your solution.

i think it's not possibe, with IE you can try to use window.event.toElement, but it dosn't work with firefox!

Edit: A hacky way to do it would be to create a variable that keeps track of focus for every element you care about. So, if you care that 'myInput' lost focus, set a variable to it on focus.

<script type="text/javascript">
   var lastFocusedElement;
</script>
<input id="myInput" onFocus="lastFocusedElement=this;" />

Original Answer: You can pass 'this' to the function.

<input id="myInput" onblur="function(this){
   var theId = this.id; // will be 'myInput'
}" />

I suggest using global variables blurfrom and blurto. Then, configure all elements you care about to assign their position in the DOM to the variable blurfrom when they lose focus. Additionally, configure them so that gaining focus sets the variable blurto to their position in the DOM. Then, you could use another function altogether to analyze the blurfrom and blurto data.

keep in mind, that the solution with explicitOriginalTarget does not work for text-input-to-text-input jumps.

try to replace buttons with the following text-inputs and you will see the difference:

<input id="btn1" onblur="showBlur(event)" value="text1">
<input id="btn2" onblur="showBlur(event)" value="text2">
<input id="btn3" onblur="showBlur(event)" value="text3">

I do not like using timeout when coding javascript so I would do it the opposite way of Michiel Borkent. (Did not try the code behind but you should get the idea).

<input id="myInput" onblur="blured = this.id;"></input>
<span onfocus = "sortOfCallback(this.id)" id="mySpan">Hello World</span>

In the head something like that

<head>
    <script type="text/javascript">
        function sortOfCallback(id){
            bluredElement = document.getElementById(blured);
            // Do whatever you want on the blured element with the id of the focus element


        }

    </script>
</head>

This way:

<script type="text/javascript">
    function yourFunction(element) {
        alert(element);
    }
</script>
<input id="myinput" onblur="yourFunction(this)">

Or if you attach the listener via JavaScript (jQuery in this example):

var input = $('#myinput').blur(function() {
    alert(this);
});

Edit: sorry. I misread the question.