loadScript.js

function loadScript(url, callback) {
  var script = document.createElement("script")
  script.type = "text/javascript";
  if (script.readyState) {
    script.onreadystatechange = function () {
      if (script.readyState === "loaded" || script.readyState === "complete") {
        script.onreadystatechange = null;
        callback();
      }
    };
  } else {
    script.onload = function () {
      callback();
    };
  }
  script.src = url;
  document.getElementsByTagName("head")[0].appendChild(script);
}

loadScript("myScript.js", function() {
  // Use the loaded script here
);