https://web4all.de/files/theme/bootstrap/js/responsive-bg.js?v=7b05eedb

1class ResponsiveBackgroundImage { 
2
3    constructor(element) { 
4        this.element = element; 
5        this.img = element.querySelector('img'); 
6        this.src = ''; 
7
8        this.img.addEventListener('load', () => { 
9            this.update(); 
10        }); 
11
12        if (this.img.complete) { 
13            this.update(); 
14        } 
15    } 
16
17    update() { 
18        let src = typeof this.img.currentSrc !== 'undefined' ? this.img.currentSrc : this.img.src; 
19        if (this.src !== src) { 
20            this.src = src; 
21            this.element.style.backgroundImage = 'url("' + this.src + '")'; 
22
23        } 
24    } 
25} 
26
27let elements = document.querySelectorAll('.responsive-background-image');   
28for (let i=0; i<elements.length; i++) {   
29  new ResponsiveBackgroundImage(elements[i]); 
30}