SrcSet debug utility to print info on the currently loaded image

srcset-debug-util.js

/**
 * Use this to help determine which images are being used
 * at which breakpoint. Will print lots of useful information
 * on shown resolution, actual resolution, DPR and more
 * Usage: 
 *      var elems = document.querySelectorAll('.c-hero-banner__background img')
 *      responsiveImageDebugOutput(elems);
 *
 * Author: Carl-Erik Kopseng 
 * Source: https://gist.github.com/fatso83/55fc446df3f3965ecd66e8307a5dc0e6
 */
function responsiveImageDebugOutput(img) {

    if (!img) {
        throw new TypeError("Expected an image node. Got none.");
    }

    if(Array.isArray(img)) {
        addToAllElements(img);
        return;
    }

    if(arguments.length > 1) {
        addToAllElements([].slice.apply(arguments));
        return;
    }

    function addToAllElements(elems){
        elems.forEach( function(el) {
            responsiveImageDebugOutput(el);
        });
    }

    var evalPrint = function (arg1, arg2, argN) {
        var len = arguments.length;
        var s = "";
        while (len--) {
            var arg = arguments[len];
            var val = eval(arg);

            if(val) {
                s += '[' + arg + "=" + val + ']  ';
            }
        }
        console.log(s);
    }

    var listener = function () {

        //Old browser
        if (typeof img.currentSrc === "undefined") evalPrint('img.src');

        //Modern browser
        else evalPrint('img.currentSrc');
        evalPrint('img.id');
        evalPrint('img.width', 'img.height');
        evalPrint('img.naturalWidth', 'img.naturalHeight');
        evalPrint('window.devicePixelRatio');
        var xDim = img.naturalWidth * window.devicePixelRatio;
        var yDim = img.naturalHeight * window.devicePixelRatio;
        console.log('Resolution: ' + xDim + 'x' + yDim);
    };

    // run the listener function if the image had already loaded 
    // before the listener had been set up
    if (img.complete) listener();

    img.addEventListener('load', listener);
}

// var elem = document.querySelector('.c-hero-banner__background img')
// responsiveImageDebugOutput(elem);
// var elems = document.querySelectorAll('.c-hero-banner__background img')
// responsiveImageDebugOutput(elems);