CoffeeScript Existential操作符和这 [英] CoffeeScript Existential Operator and this

问题描述

我注意到一些有点奇怪的CoffeeScript编译器，并想知道这是否正确的行为或不。如果是正确的，我很好奇为什么会有差别。

鉴于以下CoffeeScript：

 如果@myVar？ 
 alert myVar 
  

我期望它可以像这样编译成JavaScript：

  if（typeof this.myVar！==undefined&& this.myVar！== null）{
 alert （myVar）; 
} 
  

但是CoffeeScript编译器输出的是：

  if（this.myVar！= null）{
 alert（myVar）; 
} 
  

如果我不引用此（或任何其他父对象） CoffeeScript按我的预期编译。

这是正确的行为吗？

编辑：

若要进一步说明。这不会发生只有这一点，但对象的任何其他属性。例如，如果我用下面的代码替换上面的CoffeeScript，它仍然只会编译！= null...

  if myVar.myProp？ 
 alert myVar 
  

解决方案

  myVar = 10 
如果myVar？ 
 alert myVar 
  

Coffeescript编译器能够看到 myVar 真的在第一行定义，所以可以省略 typeof myVar！==undefined检查。

  if（myVar！== null）{
 alert（myVar）; 
} 
  

但在这种情况下：

 如果myVar？ 
 alert myVar 
  

编译器不能保证 myVar 实际上是定义的，所以需要额外的检查：

  if（typeof myVar！==undefined& ;& myVar！== null）{
 alert（myVar）; 
} 
  



因此，答案是：Coffeescript编译器试图聪明地生成高效的代码

EDIT
Coffeescript处理属性的方式也是正确的： this.prop 将返回 undefined 。 ！= 将它转换为null。这就是为什么我们不需要额外的检查。

简单来说：

• 访问未定义的变量throws exception - - 需要检查 typeof


• 访问undefined属性返回 undefined - just ！= 够了

I noticed something a little odd with the CoffeeScript compilier and was wondering if this was correct behavior or not. If it is correct I'm curious why there is a difference..

Given the following CoffeeScript:

if @myVar?
  alert myVar

I was expecting it to compile to JavaScript like this:

if (typeof this.myVar !== "undefined" && this.myVar !== null) {
  alert(myVar);
}

But instead what the CoffeeScript compiler outputs is this:

if (this.myVar != null) {
  alert(myVar);
}

If I don't reference this (or any other parent object), the CoffeeScript compiles as I would expect.

Is this the correct behavior? If so, why does it work different when using this?

Edit:

To add a little more clarification. This doesn't happen with only this, but any other properties of objects. For instance, if I were replace the above CoffeeScript with what's below it still would compile with only "!= null"...

if myVar.myProp?
  alert myVar

解决方案

In the case of:

myVar = 10
if myVar?
  alert myVar

Coffeescript compiler is able to see that myVar is really defined in the first line, so it can omit typeof myVar !== "undefined" check.

if (myVar !== null) {
  alert(myVar);
}

But in this case:

if myVar?
  alert myVar

compiler can't guarantee that myVar is actually defined, so extra check is required:

if (typeof myVar !== "undefined" && myVar !== null) {
  alert(myVar);
}

So, the answer is: Coffeescript compiler tries to be smart to produce efficient code.

EDIT The way Coffeescript deals with properties is also correct: this.prop will return undefined if property is not defined. != will convert it to null. That is why we don't need additional check.
In few words:

• accessing undefined variable throws exception -- need to check typeof
• accessing undefined property returns undefined -- just != is enough

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