Requesting http stuff.
Source
const req = new XMLHttpRequest();
req.open("GET",'/json/cats.json',true);
req.send();
req.onload = function(){
const json = JSON.parse(req.responseText);
document.getElementsByClassName('message')[0].innerHTML = JSON.stringify(json);
};equivalent:
fetch('/json/cats.json')
.then(response => response.json())
.then(data => {
document.getElementById('message').innerHTML = JSON.stringify(data);
})