JS 什么时候将 {} 解释为空块而不是空对象? [英] When does JS interpret {} as an empty block instead of an empty object?

问题描述

我正在阅读这个:

声明:堵塞变量语句空语句表达式语句……很多其他的东西……

语句可以是块、变量语句、空语句、表达式语句或许多其他东西，这是一种非常奇特的说法.请注意，第一个选项有一个阻止":

块:{ 语句列表(选择)}声明清单:陈述StatementList 语句

这又是一种奇特的说法，表示一个块是一个 {，可选地后跟一堆语句，后跟一个 }.

这就是您在示例中看到的内容:在 JavaScript 解析器认为您拥有的可能是对象字面量之前(在 ExpressionStatement 下的某处定义，Statement"的第四件事可能是)，它首先认为你有一个块".

如果你愿意，你可以在 JavaScript 引擎的源代码中看到它:

• 在 V8 中，Chrome 的 JavaScript 引擎，我们进入Parser::ParseStatement.首先检查是我们是否重新在 { 上，如果是，则解析为块.
• 在 Firefox 的 JavaScript 引擎 SpiderMonkey 中，我们从 Parser::statement 再次看到 第一次检查针对 { 并将其解析为块语句.

关于您的第二个问题，关于这个问题已详细介绍.用一句话总结:Node.js 将您的输入视为表达式(因此它不能是块")，而 Firebug/Chrome 开发工具将其视为语句".

I was reading the answer to this question (about the "wat" video) and it said:

3. {}+[]
   This is interpreted as an empty block of code, unary plus and empty array. First part does nothing, array is converted to a comma-separated string of it's elements (empty string for empty array), then to a number (empty string is converted to 0), hence 0.

I am currently learning JS from "The Definitive Guide" so I try to really understand things like that.

My question is, when does JS decide to interpret {} as an empty block of code, instead of an empty object?

Also, there are some inconsistencies between Node.js and Firebug which I would like to understand.

Firebug:

Node.js:

解决方案

Let's look at the language grammar, shall we? Section 12, Statements:

Statement :
    Block
    VariableStatement
    EmptyStatement
    ExpressionStatement
    ...lots of other stuff...

That's a very fancy way of saying that a statement can be a block, a variable statement, an empty statement, an expression statement, or lots of other stuff. Notice that the first option there is a 'Block':

Block :
    { StatementList(opt) }

StatementList :
    Statement
    StatementList Statement

Which is again, a fancy way of saying that a block is a {, optionally followed by a bunch of statements, followed by a }.

And that's what you see in your example: Before the JavaScript parser thinks that what you have could be an object literal (which is defined somewhere under ExpressionStatement, the 4th thing a 'Statement' could be), it first thinks that you have a 'Block'.

Edit: If you want, you can see it live in a JavaScript engine's source code:

• In V8, Chrome's JavaScript engine, we go into Parser::ParseStatement. The first thing it checks is whether we're on a {, and if it does, parses as block.
• In SpiderMonkey, Firefox's JavaScript engine, we go from Parser::statement to again see the first check being against a { and parsing it as a block statement.

Regarding your second question, that's been covered to great detail on this question. To summarise in a sentence: Node.js treats your input as if it were an expression (thus it can't be a 'Block'), while Firebug/Chrome dev tools treat it like a 'Statement'.

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