JSFiddle - React, Tailwind, and code Playground

HTML

<img data-src='abcd.img'/>

<img data-src='abcd2.img'/>

<img src='abcd3.img'/>

JavaScript

var imgEl = document.getElementsByTagName('img');
for (var i=0; i<imgEl.length; i++) {
    if(imgEl[i].getAttribute('data-src')) {
       imgEl[i].setAttribute('src',imgEl[i].getAttribute('data-src'));
       imgEl[i].removeAttribute('data-src');
    }
}

console.log(document.body.innerHTML);