函数作为对象键与块中标记函数之间的语法冲突 [英] Javascript Conflicting Syntax Between Function As Object Key And Labeled Function in Block

问题描述

假设您有一个同时支持标有标签的函数声明和块语句，浏览器确定以下内容的标准方法/方法是:对象是具有名为L的属性的对象，该属性为函数F，还是包含标记为L的函数F的块:

Assuming you have a browser that supports both labeled function declarations and block statements, what is the standard way/method for browsers to determine if the following is an object with a property named L that is function F, or a block that contains function F labeled as L:

{
    L: function F(){}
}

例如

为了展示我的意思，以下是上述代码的两个不同副本，将其修改为将其展示为数组和函数:

E.g.

To expose what I mean, here are two different copies of the above code modified to expose it as an array and as a function:

document.body.textContent = typeof( () => {
    L: function F(){}
} )

在上面的代码中，浏览器识别出箭头功能符号并确定它是一个块语句.但是，

In the above code, the browser recognizes the arrow function notation and determines that it is a block statement. However,

document.body.textContent = typeof {
    L: function F(){}
}

上面的代码使浏览器认为它是写为对象文字的对象，其索引L为函数F

The above code makes the browser think that it is an object written out as an object literal with index L being function F

推荐答案

您可以将问题简化为:浏览器如何知道{是否启动块，以及何时启动对象文字?

You can reduce the question to: How does the browser know whether { starts a block and when does it start an object literal?

其答案是，JS引擎将{视为在语句位置出现的块的开始，而将{视为在表达式位置出现的对象文字的开始.

And the answer to that is that JS engines will treat { as the start of a block if it appears in a statement position and as the start of an object literal if it is in an expression position.

这就是为什么当{}出现在语句位置但您想要一个对象时，必须在{}周围加上括号(())的原因.

That's the reason why you have to add parenthesis (()) around {} when they appear in a statement position but you want an object instead.

带标签的函数声明的引入根本不会改变情况，因为情况已经模棱两可了:

The introduction of labeled function declarations doesn't change the circumstances at all because the situation was already ambiguous:

{
  foo: 42
}

---

再次看一下规格，实际上指出了这种歧义:

---

Looking at the spec again, this ambiguity is actually pointed out:

ExpressionStatement 不能以U + 007B(左卷曲的括号)开头，因为它可能与(语法也反映了这一点)

(and the grammar reflects that too)

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