Javascript 第 Class 个函数

Question

在阅读有关 Javascript First Class Functions here at SO 时，我遇到了这个 link： What is meant 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。特别是表达式 men(like(women)) and/or women(like(men))。这是如何产生最终结果的...

提前致谢

Answer 1

只要记住你的 "order of operations" 高中代数：

括号
指数
乘法
师
加法
减法

括号总是先求值。并且，在表达式具有嵌套括号集的情况下，内部集先于外部集计算。

因此，与：

men(like(women))

like 函数首先使用传递给它的 women 变量的值调用。

无论那个函数 returns 是传递给 men 函数的参数。像这样：

men( result of calling like(women) )

而且，women(like(men))

是同一个思路，只是颠倒了先调用的那个。

所以，让我们一步一个脚印。

首先认识到 men、women 非常重要和 alien 函数实际上都是相同的。他们只是调用无论你传递给他们什么，结果 return （在这种情况下永远是另一个功能）。你可以推断出这一点，因为他们都做的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）。

因此，让我们少关注这些函数，多关注 like 函数，因为这是 return 与其调用者不同的函数。

阅读内联评论以获取解释：

// 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!"

Javascript 第 Class 个函数

Javascript First Class Functions

javascript

first-class-functions