javascript request dynamic html without using "window.open()"

I try to get extract some data from a webpage which is dynamic (js changes content at window.onload). A single http get request is thus not enough to get access to the data needed. Is there a way to do this without the need to render the webpage in a window/tab? I have 300 such data requests to make and don/t want 300 tabs to open.

Right now I have this:

var w = window.open(url, '_blank');
//$.get(url)
// cannot be used because the data changes dynamically after beeing loaded (via js)
var data
setTimeout(function () {
  var html = w.document.documentElement 
  data = $("smthg", html)
  w.close()
}, 500)

Note that I need to delay the data extraction until the "dynamic" content is present.

edit: The data is on a 3rd party webpage.