How do you make asynchronous HTTP request

Browsers provide an XMLHttpRequest object which can be used to make asynchronous HTTP requests from JavaScript by passing the 3rd parameter as true.

function httpGetAsync(theUrl, callback) {
var xmlHttpReq = new XMLHttpRequest();
xmlHttpReq.onreadystatechange = function () {
if (xmlHttpReq.readyState == 4 && xmlHttpReq.status == 200)
callback(xmlHttpReq.responseText);
};
xmlHttpReq.open("GET", theUrl, true); // true for asynchronous
xmlHttpReq.send(null);
}

Today this is considered deprecated, because an async fetch call (in browsers later than 2016) is simpler and more robust.