Javascript 中函数的 length 属性

每个函数都有一个 length属性 （函数名.length）， 表示期望接收的函数的个数（而不是实际接收的参数个数）

它与arguments不同。 arguments.length 是表示函数实际接收的参数个数。

试想一下 利用这个功能 可以不可以实现根据函数参数个数的重载呢。如何解决正常情况下如果定义重名函数则会把前面的覆盖的问题？

function addMethod(object, name, fn){
  // Save a reference to the old method
  var old = object[ name ]; 

  // Overwrite the method with our new one
  object[ name ] = function(){
    // Check the number of incoming arguments,
    // compared to our overloaded function
    if ( fn.length == arguments.length )
      // If there was a match, run the function
      return fn.apply( this, arguments ); 

    // Otherwise, fallback to the old method
    else if ( typeof old === "function" )
      return old.apply( this, arguments );
  };
} 

function Ninjas(){
  var ninjas = [ "Dean Edwards", "Sam Stephenson", "Alex Russell" ];
  addMethod(this, "find", function(){
    return ninjas;
  });
  addMethod(this, "find", function(name){
    var ret = [];
    for ( var i = 0; i < ninjas.length; i++ )
      if ( ninjas[i].indexOf(name) == 0 )
        ret.push( ninjas[i] );
    return ret;
  });
  addMethod(this, "find", function(first, last){
    var ret = [];
    for ( var i = 0; i < ninjas.length; i++ )
      if ( ninjas[i] == (first + " " + last) )
        ret.push( ninjas[i] );
    return ret;
  });
} 

测试代码

var ninjas = new Ninjas();
assert( ninjas.find().length == 3, "Finds all ninjas" );
assert( ninjas.find("Sam").length == 1, "Finds ninjas by first name" );
assert( ninjas.find("Dean", "Edwards").length == 1, "Finds ninjas by first and last name" );
assert( ninjas.find("Alex", "X", "Russell") == null, "Does nothing" );

参考：http://ejohn.org/apps/learn/#90