Detect Click into Iframe using JavaScript

This is small solution that works in all browsers even IE8:

var monitor = setInterval(function(){
    var elem = document.activeElement;
    if(elem && elem.tagName == 'IFRAME'){
        clearInterval(monitor);
        alert('clicked!');
    }
}, 100);

You can test it here: http://jsfiddle.net/oqjgzsm0/

Based on Mohammed Radwan's answer I came up with the following jQuery solution. Basically what it does is keep track of what iFrame people are hovering. Then if the window blurs that most likely means the user clicked the iframe banner.

the iframe should be put in a div with an id, to make sure you know which iframe the user clicked on:

<div class='banner' bannerid='yyy'>
    <iframe src='http://somedomain.com/whatever.html'></iframe>
<div>

so:

$(document).ready( function() {
    var overiFrame = -1;
    $('iframe').hover( function() {
        overiFrame = $(this).closest('.banner').attr('bannerid');
    }, function() {
        overiFrame = -1
    });

... this keeps overiFrame at -1 when no iFrames are hovered, or the 'bannerid' set in the wrapping div when an iframe is hovered. All you have to do is check if 'overiFrame' is set when the window blurs, like so: ...

    $(window).blur( function() {
        if( overiFrame != -1 )
            $.post('log.php', {id:overiFrame}); /* example, do your stats here */
    });
});

Very elegant solution with a minor downside: if a user presses ALT-F4 when hovering the mouse over an iFrame it will log it as a click. This only happened in FireFox though, IE, Chrome and Safari didn't register it.

Thanks again Mohammed, very useful solution!

Is something like this possible?

No. All you can do is detect the mouse going into the iframe, and potentially (though not reliably) when it comes back out (ie. trying to work out the difference between the pointer passing over the ad on its way somewhere else versus lingering on the ad).

I imagine a scenario where there is an invisible div on top of the iframe and the the div will just then pass the click event to the iframe.

Nope, there is no way to fake a click event.

By catching the mousedown you'd prevent the original click from getting to the iframe. If you could determine when the mouse button was about to be pressed you could try to get the invisible div out of the way so that the click would go through... but there is also no event that fires just before a mousedown.

You could try to guess, for example by looking to see if the pointer has come to rest, guessing a click might be about to come. But it's totally unreliable, and if you fail you've just lost yourself a click-through.

This is certainly possible. This works in Chrome, Firefox, and IE 11 (and probably others).

const message = document.getElementById("message");

// main document must be focused in order for window blur to fire when the iframe is interacted with. 
// There's still an issue that if user interacts outside of the page and then click iframe first without clicking page, the following logic won't run. But since the OP is only concerned about first click this shouldn't be a problem.
window.focus()

window.addEventListener("blur", () => {
  setTimeout(() => {
    if (document.activeElement.tagName === "IFRAME") {
      message.textContent = "clicked " + Date.now();
      console.log("clicked");
    }
  });
}, { once: true });

<div id="message"></div>
<iframe width="50%" height="300" src="//example.com"></iframe>

Caveat: This only detects the first click. As I understand, that is all you want.