什么时候JS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：

 声明：
阻止
 VariableStatement 
 EmptyStatement 
 ExpressionStatement 
 ......很多其他东西...... 
  

这是一种非常奇特的方式，可以说语句可以是块，变量语句，空语句，表达式语句，或许多其他的东西。请注意，第一个选项是阻止：

 阻止：
 {StatementList（opt）} 
 
 StatementList：
语句
 StatementList语句
  

这是再一次，一种奇特的方式，说一个块是 {，可选地后跟一堆语句，然后是} 。

这就是你在你的例子中看到的：在JavaScript解析器认为你拥有的东西之前可能是一个对象文字（在<$ c下的某个地方定义） $ c> ExpressionStatement ，'声明'可能是第四件事），它首先认为你有一个'阻止'。

编辑：如果你愿意，你可以在JavaScript引擎的源代码中看到它：

• 在V8，Chrome的JavaScript引擎中，我们进入< a href =https://github.com/v8/v8/blob/3969195bb2e224db3bf60d3a1e9bf17ee1be88b8/src/parser.cc#L1572 =nofollow noreferrer> Parser :: Parse声明 。 它首先检查是我们是否'依赖于 {，如果是，则解析为阻止。


• 在SpiderMonkey中，Firefox的JavaScript引擎，我们来自< a href =http://hg.mozilla.org/mozilla-central/file/78a4540b0a9c/js/src/frontend/Parser.cpp#l5413\"rel =nofollow noreferrer> Parser ::语句 再次查看第一次检查是针对 {并将其解析为块语句。

关于你的第二个问题，关于这个问题已经详细介绍了这个问题。 。总结一句话：Node.js将您的输入视为表达式（因此它不能是阻止），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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