Get the current image source of a picture element

You can access the currentSrc of the img element within a picture element to get the applied image.

In the demo below (if you're on a desktop with your browser full-width) the currentSrc will be the http://placehold.it/600x100 image.

window.onload = function() {
  const picture = document.getElementById('pictureEl');
  const currentSource = document.getElementById('currentSource');
  currentSource.innerText = picture.querySelector('img').currentSrc;
}

<picture id="pictureEl">
 <source srcset="http://placehold.it/600x100?text=Source" media="(min-width: 300px)">
 <img src="http://placehold.it/300x100?text=Img">
</picture>
<div id="currentSource"></div>

Use this to get source of image.

function getCurrentSrc(element, cb){
    var getSrc;
    if(!window.HTMLPictureElement){
        if(window.respimage){
            respimage({elements: [element]});
        } else if(window.picturefill){
            picturefill({elements: [element]});
        }
        cb(element.src);
        return;
    }

    getSrc = function(){
        element.removeEventListener('load', getSrc);
        element.removeEventListener('error', getSrc);
        cb(element.currentSrc);
    };

    element.addEventListener('load', getSrc);
    element.addEventListener('error', getSrc);
    if(element.complete){
        getSrc();
    }
}