CoffeeScript 存在运算符和 this [英] CoffeeScript Existential Operator and this

问题描述

我注意到 CoffeeScript 编译器有些奇怪，想知道这是否是正确的行为.如果它是正确的，我很好奇为什么会有区别..

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..

鉴于以下 CoffeeScript:

Given the following CoffeeScript:

if @myVar?
  alert myVar

我期待它像这样编译成 JavaScript:

I was expecting it to compile to JavaScript like this:

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

但是 CoffeeScript 编译器的输出是这样的:

But instead what the CoffeeScript compiler outputs is this:

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

如果我不引用这个(或任何其他父对象)，CoffeeScript 将按照我的预期编译.

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

这是正确的行为吗?如果是这样，为什么它在使用时会有所不同?

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

补充一点说明.这不仅仅发生在这个，而是对象的任何其他属性.例如，如果我将上面的 CoffeeScript 替换为下面的内容，它仍然只能使用！= null"进行编译...

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

推荐答案

情况:

myVar = 10
if myVar?
  alert myVar

Coffeescript 编译器能够看到 myVar 确实定义在第一行，所以它可以省略 typeof myVar !== "undefined" 检查.

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);
}

但在这种情况下:

if myVar?
  alert myVar

编译器不能保证 myVar 确实被定义了，所以需要额外检查:

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

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

所以，答案是:Coffeescript 编译器试图聪明地产生高效的代码.

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

编辑Coffeescript 处理属性的方式也是正确的:如果属性未定义，this.prop 将返回 undefined.!= 会将其转换为 null.这就是我们不需要额外检查的原因.简而言之:

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:

• 访问未定义的变量抛出异常——需要检查typeof
• 访问未定义的属性返回 undefined -- != 就足够了

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

这篇关于CoffeeScript 存在运算符和 this的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！