异步跨域加载js

var ET = function(){
  this.config = {
    url      : "",
    charset  : "utf-8",
    callback :function(){}
  }
}

ET.merge = function(d, s){
  for(var k in s){d[k] = s[k]}
	return d;
}

ET.prototype = {
  getScript : function(config){
    var _script, _callback;

    this.config = ET.merge(this.config, config);

    _script = document.createElement("script");
    _script.setAttribute("type", "text/javascript");
    _script.setAttribute("charset", this.config.charset);
    _script.setAttribute("src", this.config.url);

    document.body.appendChild(_script);
    
    _callback = this.config.callback;
    _script.onload = _script.onreadystatechange = function(callback){
      if(!this.readyState || this.readyState=='loaded' || this.readyState=='complete'){
        _callback();
      }
    }
  }
}