const obj = {
  say: function(name) {
    console.log("say hello " + name);
  }
};

const addSay = () => {
  console.log("addSay");
};
var oldMethod = obj["say"];
obj["say"] = function(arg) {
  addSay.call(this, arg);
  return oldMethod.call.apply(
    oldMethod,
    [this].concat(Array.prototype.slice.call(arguments))
  );
};

obj.say("allen");

代码如上，其中这块代码看不懂，有懂的么

obj["say"] = function(arg) {
  addSay.call(this, arg);
  return oldMethod.call.apply(
    oldMethod,
    [this].concat(Array.prototype.slice.call(arguments))
  );
};