Function.prototype.before = function (beforeFn) {
  var self = this; //保存原函数的引用
  return function () { //返回包含了新函数和原函数的代理函数
    beforeFn.apply(this,arguments); //执行新函数,且保证this不被劫持
    return self.apply(this,arguments); //执行原函数,并返回原函数的执行结果,并保证this不被劫持
  }
};
Function.prototype.after = function (afterFn) {
  var self = this;
  return function () {
    var ret = self.apply(this,arguments);
    afterFn.apply(this,arguments);
    return ret;
  }
}; 

 var before = function (fn, beforeFn) {
  return function () {
    beforeFn.apply(this,arguments);
    return fn.apply(this,arguments);
  }
};
var after = function (fn, afterFn) {
  return function () {
    var ret = fn.apply(this,arguments);
    afterFn.apply(this,arguments);
    return ret;
  }
}; 

 var ajax = function (type, url, param) {
  console.log(param); //发送ajax请求代码略...
};
var beforeFn = function (type, url, param) {
  param.Token = 'Token';
};
ajax = ajax.before(beforeFn);
ajax('get','http://...com/userinfo',{name:'SuFa'});
//{ name: 'SuFa', Token: 'Token' } 

 //验证输入函数
var validata = function () {
  if(username.value === ''){
    alert('用户名不能为空');
    return false;
  }
  if(password.value === ''){
    alert('密码不能为空');
    return false;
  }
};
//表单提交函数
var formSubmit = function () {
  var param = {
    username: username.value,
    password: password.value
  };
  ajax('http://xxx.com/login',param);
};

formSubmit = formSubmit.before(validata);
submitBtn.onclick = function(){
  formSubmit();
};