如何在JavaScript中创建一个CoffeeScript风格存在操作符？ [英] How to create a CoffeeScript style existential operator in JavaScript?

问题描述

CoffeeScript将用户？.id 转换为

  if user！==undefined&& user！== null）{
 user.id; 
} 
  

是否可以创建一个JavaScript函数 exists 会做类似的事情吗？即

  exists（user）.id 
  

会导致 user.id 或 null

如果一个函数接受另一个参数，即 exists（user，'id'）很好。

解决方案

不，你不能生成这样的函数。问题是：

  any_function（undeclared_variable）
   undeclared_variable 没有在任何地方声明，则p> 
 
 
会产生一个ReferenceError。例如，如果你运行这个独立的代码：
  function f（）{} 
 f（pancakes）; 
  
你会得到一个ReferenceError，因为 pancakes 没有声明任何地方。演示： http://jsfiddle.net/ambiguous/wSZaL/  
 
 
 
但是，  typeof 运算符可用于未声明的内容，因此：
  console.log（typeof pancakes）; 
  
只会在控制台中记录 undefined  。演示： http://jsfiddle.net/ambiguous/et2Nv/  
 
 
 
如果你不介意可能的ReferenceErrors那么你已经在你的问题中有必要的功能：
 函数存在（obj，key）{
 if（typeof obj！==undefined&& obj！== null）
 return obj [key]; 
 return null; //也许你想要undefined而不是
} 
  
需要能够在未声明的变量上使用 typeof ，您可以将其简化为：
  function exists（obj，key）{
 if（obj！= null）
 return obj [key]; 
 return null; 
} 
  
请注意！= ， undefined == null 为true，即使 undefined === null  
CoffeeScript turns user?.id into
if (typeof user !== "undefined" && user !== null) {
   user.id;
}
Is it possible to create a JavaScript function exists that would do something similar? i.e.
exists(user).id
would result in either user.id or null

It would be easier if a function accepts another parameter, i.e. exists(user, 'id'), but that wouldn't look as nice.
解决方案
No, you can't produce such a function. The problem is that this:
any_function(undeclared_variable)
will produce a ReferenceError if undeclared_variable was not declared anywhere. For example, if you run this stand alone code:
function f() { }
f(pancakes);
you'll get a ReferenceError because pancakes was not declared anywhere. Demo: http://jsfiddle.net/ambiguous/wSZaL/

However, the typeof operator can be used on something that has not been declared so this:
console.log(typeof pancakes);
will simply log an undefined in the console. Demo: http://jsfiddle.net/ambiguous/et2Nv/

If you don't mind possible ReferenceErrors then you already have the necessary function in your question:
function exists(obj, key) {
    if (typeof obj !== "undefined" && obj !== null)
        return obj[key];
    return null; // Maybe you'd want undefined instead
}
or, since you don't need to be able to use typeof on undeclared variables here, you can simplify it down to:
function exists(obj, key) {
    if(obj != null)
      return obj[key];
    return null;
}
Note the change to !=, undefined == null is true even though undefined === null is not.

                        
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