I’d use Promises for state-management and Promise.all to check when all scripts are loaded.

function load(src) {
  return new Promise((resolve, reject) => {
    const script = document.createElement('script');
    script.onload = resolve;
    script.onerror = reject;
    script.async = true;
    script.src = src;
    document.head.appendChild(script);
  });
}

Promise.all(scripts.map(load)).then(callback);

In my app.js I do something like this:

load(src) {
  script = document.createElement('script');
  script.src = src;
  script.async = true;

  document.head.appendChild(script);
}

for (var i=0; scripts.length; i++) {
  load(scripts[i])
}

callback();

all the scripts are fetched with async flag. I want to execute callback only when all the scripts are loaded, how can I do it?