javascript - 关于函数重载的问题

Question

在《javascript忍者秘籍》上看到一段代码，用于函数重载。

function addMethod(object, name, fn) {
  var old = object[name];                        //#1
  object[name] = function(){
    if (fn.length == arguments.length)           //#2
      return fn.apply(this, arguments)           //#2
    else if (typeof old == 'function')           //#3
      return old.apply(this, arguments);         //#3
  };
}

示例是这样的：

var ninja = {};
addMethod(ninja,'whatever',function(){/*do something*/});
addMethod(ninja,'whatever',function(a){/*do something else*/});
addMethod(ninja,'whatever',function(a,b){/*yet something else*/});

但是有点看不懂addMethod方法，疑问如下：

• if (fn.length == arguments.length)这个arguments是谁的arugments?调用addMethod时的？还是object[name] = function()这个匿名函数？

菜鸟一枚，若拍砖请轻拍

Answer 1

你只需要加两句话做个试验就清楚了

function addMethod(object, name, fn) {
    var _addMethodArguments = arguments;
    var old = object[name];                        //#1
    object[name] = function() {
        console.log("is arguments of addMethod():", _addMethodArguments === arguments);
        if (fn.length == arguments.length)           //#2
            return fn.apply(this, arguments);           //#2
        else if (typeof old == "function")           //#3
            return old.apply(this, arguments);         //#3
    };
}


var ninja = {};
addMethod(ninja, "whatever", function() {/*do something*/ });

ninja.whatever();