こばのなにかひどいなにかを動作確認しないで書き換えてみた

load.js

var load = new (function () {
  var count = 0;
  var next = function () {}
  var callback = function () {
    count --;
    if (count <= 0) {
      next();
    }
  }
  self = this;
  this.load = function (src) {
    count ++;
    var scriptElement = document.createElement('script');
    scriptElement.type = 'text/javascript';
    scriptElement.onload = callback;
    scriptElement.src = src;
    (document.body?document.body:document.getElementsByTagName("head")[0]).appendChild(scriptElement);
    return self;
  }
  this.then = function (callback) {
    next = callback;
  }
})().load;