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在 SO 阅读 Javascript First Class Functions 时，我遇到了这个链接： What is mean by 'first class object'？ 其中我发现了这个有趣的例子：

var men = function (objectOfAdmiration) {
  return objectOfAdmiration();
};
men.isSweetHeart = true;

var women = function (objectOfAdmiration) {
  return objectOfAdmiration();
};
women.isSweetHeart = true;

var aliens = function (objectOfAdmiration) {
  return objectOfAdmiration();
};

function like(obj){
  if (obj.isSweetHeart) {
      return function (){ return "Holy TRUE!"}; 
  }
  else {
      return function (){ return "Holy CRAP!"};
  }
}

alert("Men like women is " + men(like(women))); // -> "Holly TRUE!"
alert("Women like men is " + women(like(men))); // -> "Holly TRUE!"

alert("Men like aliens is " + men(like(aliens))); // -> "Holly CRAP!"
alert("Aliens like women is " + aliens(like(women))); // -> "Holly TRUE!"

我需要有人来解释这段代码，这里的执行方式。尤其是表达式men(like(women))和​​/或women(like(men))。这是如何产生最终结果的......

提前致谢

4

1 回答 1

2

只要记住你高中代数的“运算顺序”：

  • 插入语
  • 指数
  • 乘法
  • 分配
  • 添加
  • 减法

括号总是首先被评估。并且，在具有嵌套括号集的表达式的情况下，内部集合在外部集合之前被评估。

所以，有：

men(like(women))

该like函数首先使用women传递给它的变量的值来调用。

无论该函数返回什么，都是传递给该men函数的参数。像这样：

men( result of calling like(women) )

和，women(like(men))

是相同的想法，只是颠倒首先调用哪个。

所以，让我们一步一步来。

  • 首先认识到men,women 和alien函数实际上都是相同的，这一点非常重要。他们只需调用您传递给他们的任何内容并返回该结果（在这种情况下将始终是另一个函数）。您可以推断出这一点，因为他们所做的只是在输入参数的末尾加上一组括号。因此，他们每个人都期望传递一个函数（因为这是您唯一可以调用的东西）。

所以，让我们少关注那些函数，多关注like函数，因为它是向调用者返回不同函数的函数。

阅读内联注释以获取解释：

// Function expression that sets men to hold a function 
var men = function (objectOfAdmiration) {
  // This function is expecting a function to be passed to it
  // because all it does is invoke what is passed and only 
  // functions can be invoked.
  return objectOfAdmiration();
};

// Treat men like an object now and give it an isSweetHeart property with a value of true
men.isSweetHeart = true;

// Function expression that sets women to hold a function 
var women = function (objectOfAdmiration) {
  // This function is expecting a function to be passed to it
  // because all it does is invoke what is passed and only 
  // functions can be invoked.
  return objectOfAdmiration();
};

// Treat men like an object now and give it an isSweetHeart property with a value of true
women.isSweetHeart = true;

// Function expression that sets alients to hold a function 
var aliens = function (objectOfAdmiration) {
  // This function is expecting a function to be passed to it
  // because all it does is invoke what is passed and only 
  // functions can be invoked.
  return objectOfAdmiration();
};

// Function declaration for like.
// This function is expecting an object that has an isSweetHeart property.
// Since we have multiple types of objects that support that property, this
// function is polymorphic
function like(obj){
  if (obj.isSweetHeart) {
      // If the men or women objects are passed, this will be returned
      return function (){ return "Holy TRUE!"}; 
  }
  else {
      // Anything that doesn't have the isSweetHeart property or does
      // but that property has a "falsey" value will end up here
      return function (){ return "Holy CRAP!"};
  }
}

// Invoke like(woman)...
// since the woman object does have an isSweetHeart property and that
// property has a value of true, the function that will return "Holy TRUE!"
// when it is invoked is returned.

// That function is then passed to the men function, which recieves it under
// the parameter name of "objectOfAdmiration".

// Invoking that function outputs "Holy TRUE!"
alert("Men like women is " + men(like(women))); // -> "Holy TRUE!"

// This line does just about the same as above, but in reverse and
// because the men and women functions are the same, the same output
// is produced.
alert("Women like men is " + women(like(men))); // -> "Holy TRUE!"

// Same general procedure as before, but since aliens do not even 
// have an isSweetHeart property, a different function is returned
// for further processing.
alert("Men like aliens is " + men(like(aliens))); // -> "Holy CRAP!"

// This is essentially the same as the men(like(women)) function call
// except that the result of like(women) is passed to aliens, which 
// Simply invokes that function.
alert("Aliens like women is " + aliens(like(women))); // -> "Holy TRUE!"

于 2017-07-27T19:46:59.090 回答