Вот так
function lazy_load_ads() {
var ll = 0;
window.addEventListener("scroll",load_ads);
function load_ads() {
if (ll === 0 && window.pageYOffset > 150) {
ll = 1;
fetch_ad('.selector', 'your_ad_code_here.html');
}
}
function fetch_ad(elementClass, appendData) {
var xhr = new XMLHttpRequest();
var element = document.querySelector(elementClass);
if (element === null)
return;
xhr.onreadystatechange = function () {
if (xhr.readyState == 4 && xhr.status == 200) {
element.innerHTML = xhr.responseText;
var scripts =
Array.prototype.slice.call(element.getElementsByTagName("script"));
for (var i = 0; i < scripts.length; i++) {
if (scripts[i].src != "") {
var tag = document.createElement("script");
tag.src = scripts[i].src;
document.getElementsByTagName("head")[0].appendChild(tag);
} else {
eval(scripts[i].innerHTML);
}
}
}
};
xhr.open("GET", appendData, true); // true for asynchronous
xhr.overrideMimeType('application/javascript');
xhr.send(null);
}
}
